Taoffi's blog

If you are a Windows C# programmer, you may know about the DescriptionAttribute and its help in describing an object, a property, method or any other member of a class in a human way.

You may also use [Description], to associate a Label to a property and later use it in the UI.

As Xamarin.Forms does not (yet) offer this simple useful attribute, I decided to write one.

The basic code

Our Description attribute class is quite simple:

[AttributeUsage(validOn: AttributeTargets.All, AllowMultiple = false, Inherited = false)]
public class DescriptionAttribute : Attribute
{
    public string _description;

    public DescriptionAttribute(string description)
    {
         _description = description;
    }

    public string Description
    {
        get { return _description; }
        set { _description = value; }
    }
} 

With this in place, we can now write things like:

[Description("This is my object")]
public class MyClass
{
    [Description("This is my constructor")]
    public MyClass()
    {
    }

    [Description("My property")]
    public string Property1
    {
     … 
    }

Reading back the descriptions

Well, but for this to be useful, we must do something to get back these descriptions when needed.

First: how to read the descriptions assigned to objects and members?

Some extension helpers can simplify this work (remember: we are in portable (PCL) library, and also using System.Reflection):

public static string Description(this Type objectType)
{
    if(objectType == null)
        return null;

    TypeInfo typeInfo = objectType.GetTypeInfo();
    var attrib = typeInfo.GetCustomAttribute<DescriptionAttribute>();
    return attrib == null ? null : attrib.Description;
}

To get the description of MyClass, The above extension method now allows us to write:

string    classDescription    = typeof(MyClass).Description();

Another method may even make that simpler: get the description of an object's instance of a given class:

public static string Description(this object obj)
{
    if(obj == null)
        return null;

    return obj.GetType().Description();
}

This allows us to write: myObject.Description(); to get the description of myObject's class.

Reading back the description of class members

Reading a given member (property / method…) description, we have to look into the MemberInfo object of that given member to find the Description attribute:

public static string MemberDescription(this MemberInfo member)
{
    if(member == null)
        return null;

    var attrib = member.GetCustomAttribute<DescriptionAttribute>();
    return attrib == null ? null : attrib.Description;
}

How to get a member info? Not quite handy!...

Let us simplify a little more. The following code (though it may be somehow difficult to read… see System.Linq.Expressions) will allow a much easier syntax.

public static string PropertyDescription<TMember>( Expression<Func<TMember>> memberExpression)
{
    if (memberExpression == null)
        return null;

    var expression = memberExpression.Body as MemberExpression;
    var member = expression == null ? null : expression.Member;

    if (member == null)
        return null;

    return member.MemberDescription();
}

The above method analyses its Expression parameter to extract the MemberInfo for us and calls the original method to extract the description of the member.

To use this method to extract the description of 'Property1' , we can write:

string propertyLabel = PropertyDescription(()=> Property1);

What about Enums?

As you know, we often give short (occasionally cryptic!) names for enum members. Displaying these names in a clear human-readable manner to the user for selecting a value is usually a challenge.

The Description attribute can help us assign these labels. Reading them back is somehow another challenge. The reason is that enum members are fields (in contrast to member properties and methods we saw before)

Actually, with enums, we have two challenges: find the description of each element, but also be able to retrieve the value of a selected description.

For the first challenge, find the description of an element, let us try this extension:

public static string EnumDescription(this Enum value)
{
    if(value == null)
        return null;

    var type = value.GetType();
    TypeInfo typeInfo = type.GetTypeInfo();
    string typeName = Enum.GetName(type, value);

    if (string.IsNullOrEmpty(typeName))
        return null;

    var field = typeInfo.DeclaredFields.FirstOrDefault(f => f.Name == typeName);

    if(field == null)
        return typeName;

    var attrib = field.GetCustomAttribute<DescriptionAttribute>();
    return attrib == null ? typeName : attrib.Description;
}

Sample usage:

public enum Flowers
{
    [Description("African lily")]
    Agapanthus,

    [Description("Alpine thistle")]
    Eryngium,

    [Description("Amazon lily")]
    Eucharis,
};

string africanLilly = Flowers.Agapanthus.EnumDescription(); //"African lily"
string alpineThistle = Flowers.Eryngium.EnumDescription(); //"Alpine thistle"

Find the enum value by its description

The second task is to retrieve the enum value by its description. For instance when the user selects one of the displayed descriptions.

public static int EnumValue(this Enum value, string selectedOption)
{
    var values = Enum.GetValues(value.GetType());

    foreach(Enum v in values)
    {
        string description = v.EnumDescription();

        if(description == selectedOption)
            return (int) v;
    }

    return 0; // arbitrary default value
}

Now we can write:

Flowers africanLilly = Flowers.EnumValue("African lily");    // Agapanthus
Flowers amazonLilly = Flowers.EnumValue("Amazon lily");    // Eucharis

After some months spent on other project types, so glad to find back Xamarin.Forms and to see how it evolved (matured) with the integration in Visual Studio 2017.

The new Xamarin.Forms project template is awesome and pedagogic. You can start building an entire professional solution around the basic objects delivered within that template!

This 2017 spring (well: it is autumn elsewhere!) other interesting projects also blossomed. Don't miss Grial 2.0 of UXDivers. A great ergonomic framework for Xamarin.Forms.

Those guys @UXDivers are unique! (Will talk about this later)

The new Xamarin.Forms template base objects

Inside the main portable project, you will find the 'Helpers' folder containing some interesting foundation objects, of which, the ObservableObject class.

ObservableObject is the base class of another delivered sample object 'BaseDataObject' of which derives the 'Item' object… another delivered sample object.

The relationship between those objects are shown in the following class diagram:

That looks to be a great and mature foundation. Deriving your objects from this simple architecture would allow you, among other benefits, to have a reliable property change notification mechanism (See my last year's post about this question)

Great… But!

As often among our developer community, we may agree on an architectural pattern and disagree on parts of its implementation.

In the new Xamarin implementation, a class calls its parent to set a property value. The parent then sets the new value and notifies the property change.

Here is the base class's (ObservableObject) code to set a property value:

protected bool SetProperty<T>(ref T backingStore, T value,
             [CallerMemberName]string propertyName = "",
             Action onChanged = null)
{
     if (EqualityComparer<T>.Default.Equals(backingStore, value))
         return false;

      backingStore = value;
     onChanged?.Invoke();
     OnPropertyChanged(propertyName);
     return true;
 }

A derived class can then use this:

string description = string.Empty;

public string Description
{
     get { return description; }
     set { SetProperty(ref description, value); }
}

In the above example, the property change notification is executed using the caller name (thanks to CompilerService.CallerMemberName attribute) unless the name is explicitly specified.

In so many case, a property change may require notifying the change of one or more of other object properties. A simple example would be: when you change the 'Birth date' of a person object, which requires notifying the change of his or her Age property as well.

In fact, an object's property value is, in most cases, linked to business rules and is better handled at the object's level. The property change notification, as its name suggests, is related to notifying external objects who may be interested by the value change of that property.

Summarizing this need by a SET mechanism can be good but does not seem to be an all-purpose solution.

We still need to complement this with the 'old' notification mechanism (based on Expressions) exposed in the pattern of the abovementioned post (which, itself, derives from other community knowledge as I mentioned!)

[This post corrects and clarifies points mentioned in a previous post]

Let us go back to Xamarin Forms Maps!

In a previous post, I exposed a WinPhone custom renderer for maps.

The object of our renderer is to:

• Display custom pushpin (marker) icon
• Display a custom info window for each place (pushpin / marker)

The custom renderer

Our Droid custom renderer will derive from MapRenderer and, for reasons we will see later, will implement the GoogleMap.IInfoWindowAdapter:

[assembly: ExportRenderer (typeof(iMaps.iCustomMap),
                                    typeof(iMaps.Droid.iCustomMapViewRenderer))]

namespace iMaps.Droid
{
    public class iCustomMapViewRenderer : MapRenderer, GoogleMap.IInfoWindowAdapter

Customizing the marker icon

In android, markers can relatively easily be customized to show a custom icon. Here, we use the custom icon of our Custom pin

iCustomPin    customPin    = pin.BindingContext as iCustomPin;
string    resourceName    = System.IO.Path.GetFileNameWithoutExtension(customPin.IconResource);
int        resourceId        = Context.Resources.GetIdentifier(resourceName, "drawable",
                                        Context.PackageName);

bmp = BitmapDescriptorFactory.FromResource(resourceId);
marker.SetIcon(bmp);

Setting markers and their custom icons will be done when the map is to be rendered (i.e. in the OnElementPropertyChanged override of our renderer). That is best done once when the map will be ready.

To know when the map is ready, we must implement the IOnMapReadyCallback Interface which defines one only method: void OnMapReady(GoogleMap googleMap). With this in place, google maps will call us when the map is ready.

Our renderer will thus now look like this:

public class iCustomMapViewRenderer : MapRenderer, GoogleMap.IInfoWindowAdapter,
IOnMapReadyCallback

Our OnMapReady method will set variables required for drawing the map.

GoogleMap            _map;

public void OnMapReady (GoogleMap googleMap)
{
    _map        = googleMap;
    _map.InfoWindowClick    += Map_InfoWindowClick;

    // required if you wish to handle info window and its content
    _map.SetInfoWindowAdapter (this);
}

OnElementPropertyChanged of our renderer will proceed to putting the pins on the map:

bool                _isDrawingDone;

protected override void OnElementPropertyChanged (object sender,

                                        PropertyChangedEventArgs e)
{
    base.OnElementPropertyChanged (sender, e);

    if(_map == null || _isDrawingDone || e.PropertyName != "VisibleRegion")
        return;

    _map.Clear ();
    _map.MarkerClick            += HandleMarkerClick;
    _map.InfoWindowClick    += Map_InfoWindowClick;

    foreach (var pin in _mapPins)
    {
        var            marker        = new MarkerOptions ();
        iCustomPin    customPin    = pin.BindingContext as iCustomPin;

        marker.SetPosition (new LatLng (pin.Position.Latitude, pin.Position.Longitude));
        marker.SetTitle (pin.Label);

        if(! string.IsNullOrEmpty(pin.Address))
            marker.SetSnippet(pin.Address);

        BitmapDescriptor    bmp        = null;

        string    resourceName    =
                    System.IO.Path.GetFileNameWithoutExtension(customPin.IconResource);
        int        resourceId        = Context.Resources.GetIdentifier(resourceName,
                                                "drawable", Context.PackageName);

        bmp = BitmapDescriptorFactory.FromResource(resourceId);
        marker.SetIcon(bmp);

        _map.AddMarker(marker);
    }

    _isDrawingDone = true;
}

Customizing the popup info window

Customizing the popup info window is a different kettle of fish ('une autre paire de manche' en françaisJ)

Why?

Simply because the google maps documentation says that the info window is a View, but specify this interesting note:

So, it IS a View… but NOT REALLYJ…

Let us be more specific: it is NOT a view. It is actually a Bitmap.

Anyway, to be able to customize the info window, our renderer must implement the GoogleMap.IInfoWindowAdapter Interface. Which defines two methods:

• View GetInfoContents(Marker marker);
• View GetInfoWindow(Marker marker);

GetInfoWindow: returns the entire popup window. If it returns null, then it is GetInfoContents which will define the contents of the default info window frame of the marker popup.

As the 'pseudo-view' returned in both cases are not really views but bitmaps, our task is to compose a view of the pin information before transforming this to a bitmap.

In our current exercise, we will return the window content. Therefor our GetInfoWindow() will return null.

public Android.Views.View GetInfoWindow(Marker marker)
{
    return null;
}

From Xamarin Forms Xaml à to Native View à to Bitmap: dangerous tour!

As I am not a 'droid boy', I decided to create a Xamarin Forms control (ContentView) and use this in the custom renderer. You may of course decide differently to use a Droid axml control. In both ways, we will have to convert that control's content into Bitmap.

Here is my ContentView Xaml (assumed to be bound to an iCustomPin):

<StackLayout Spacing="4" >
    <Image Source="{Binding ImageFile}"
            WidthRequest="84" HeightRequest="84" HorizontalOptions="StartAndExpand" />
    <Label FontAttributes="Bold" Text="{Binding Name}" />
    <BoxView HeightRequest="1" BackgroundColor="Gray"
                HorizontalOptions="FillAndExpand" VerticalOptions="Start" />
    <Label x:Name="labelText" Text="{Binding MapPinText}" LineBreakMode="WordWrap" />
</StackLayout>

Our GetInfoContents method will have to:

• Identify the clicked custom pin
• Set the binding context of the above ContentView to that pin
• Transform that XF control to a native Droid ViewGroup
• Transform the resulting droid native ViewGroup into a bitmap to be returned to Google maps.

The following code illustrates these steps:

iMapPinInfoCtrl        xamInfoPanel    = new iMapPinInfoCtrl();

public Android.Views.View GetInfoContents(Marker marker)
{
    iCustomMap     myMap         = this.Element as iCustomMap;
    iCustomPin     customPin     = myMap.GetPinAtPosition(
                new Position(marker.Position.Latitude, marker.Position.Longitude));
    double            infoWidth        = this.Control.Width * 0.70,
                        infoHeight        = 380.0 / 2.8;
    SamplePlace    place                = customPin.DataObject as SamplePlace;

    xamInfoPanel.BindingContext = place;

    // get the droid native control
    ViewGroup    viewGrp        = DroidXFUtilities.ConvertFormsToNative(
                xamInfoPanel.Content, new Rectangle(0, 0, infoWidth, infoHeight));

    // transform the native control into a bitmap
    Bitmap        bmp            = DroidXFUtilities.ViewGroupToBitmap(viewGrp,
                                                    this.Context,
                                                    (int)infoWidth, (int)infoHeight,
                                                    true);

    _pinImage.SetImageBitmap(bmp);

    return _pinImage;
}

To Native ViewGroup

An interesting post from Michael Ridland helped solve this task!

Get the native droid ViewGroup of XF View code snippet:

public static ViewGroup ConvertFormsToNative(Xamarin.Forms.View view,
                                            Rectangle size)
{
    var        vRenderer        = Platform.CreateRenderer(view);
    var        viewGroup        = vRenderer.ViewGroup;

    vRenderer.Tracker.UpdateLayout();

    var        layoutParams    = new ViewGroup.LayoutParams((int)size.Width, (int)size.Height);

    viewGroup.LayoutParameters        = layoutParams;
    viewGroup.DrawingCacheEnabled    = true;
    view.Layout(size);
    viewGroup.Layout(0, 0, (int)size.Width, (int)size.Height);
    return viewGroup;
}

Native ViewGroup to Droid.Graphics.Bitmap

I must first confess that this task was hard for meJ.

I read some articles about the subject, of which this interesting one "Converting Views to Bitmap Images in Android"… but, in the practical exercise, couldn't grasp how to get precise measures of rendered elements!

The task steps is to:

• Create a Linear Layout
• Create a Bitmap and put it into a Canvas
• Loop through the ViewGroup's elements (Views) and add them to the Layout
• Draw the Layout into the Canvas
• Return the Bitmap (which will contain the rendered layout's views)

Getting the precise measures of views is not simple. Notably for images and labels with text wrap attribute.

Here is a simplified snippet of my code (assuming that no images are part of the ViewGroup's elements). The solution's source code has more details about handling images. But you should be able to do much better if you are an android expertJ

public static Android.Graphics.Bitmap ViewGroupToBitmap(ViewGroup viewGroup,
                                             Context context,
                                             int width, int height)
{
    int        viewCount = viewGroup == null ? 0 : viewGroup.ChildCount;
    Android.Widget.LinearLayout layout        = new Android.Widget.LinearLayout(context);
    Bitmap    bmpLayout    = Bitmap.CreateBitmap(width, height, Bitmap.Config.Argb8888);
    Android.Graphics.Color    white        = Android.Graphics.Color.Argb(0xff, 0xff, 0xff, 0xff);

    layout.DrawingCacheEnabled            = true;
    layout.SetBackgroundColor(white);

    Canvas    canvas    = new Canvas(bmpLayout);

    // add the sub views contained in this view group
    for (int ndx = 0; ndx < viewCount; ndx++)
    {
        Android.Views.View view = viewGroup.GetChildAt(0);

        int            wid            = Math.Max(0, view.MeasuredWidth),
                        hi                = Math.Max(0, view.MeasuredHeight);

        viewGroup.RemoveView(view);
        layout.AddView(view, wid, hi);
    }

    layout.Draw(canvas);

    return bmpLayout;
}

The result looks like this:

You may Download the code Here

As XF documentations say, your platform-application (Droid, iOS or WP) should call Forms.Init() at startup.

That should be done:

• In iOS: in the AppDelegate.FinishedLaunching method
  • global::Xamarin.Forms.Forms.Init();
• In Droid: in the MainActivity. OnCreate (Bundle bundle)
  • global::Xamarin.Forms.Forms.Init(this, bundle);
• In Win Phone: in your application's MainPage constructor
  • global::Xamarin.Forms.Forms.Init();

What does Forms.Init do?

On droid

Here is an example of what Forms.Init method does on Android:

public static void Init(Activity activity, Bundle bundle)
{
    // get the calling assembly 
    Assembly callingAssembly = Assembly.GetCallingAssembly(); 

    // Call SetupInit 
    SetupInit(activity, callingAssembly); 
}

Let us continue following the call to SetupInit():

private static void SetupInit(Activity activity, Assembly resourceAssembly) 
{
    // set the Context to current activity 
    Context = activity; 

    // initialize resources for this assembly 
    ResourceManager.Init(resourceAssembly); 

    // set the AccentColor according to OS version 
    if (Build.VERSION.SdkInt <=    BuildVersionCodes.GingerbreadMr1) 
    { 
       Color.Accent = Color.FromHex("#fffeaa0c"); 
    } 
    else 
    { 
       Color.Accent = Color.FromHex("#ff33b5e5"); 
    } 

    // log (if not initialized) 
    if (!IsInitialized) 
    { 
       Log.get_Listeners().Add(new DelegateLogListener( 
 (c, m) => Trace.WriteLine(m, c))); 
    } 

    // set the Device.OS version and platform services (here android) 
       Device.OS = TargetPlatform.Android; 
       Device.PlatformServices = new AndroidPlatformServices(); 

    // recreate the device info from this activity 
    if (Device.info != null) 
    { 
       ((AndroidDeviceInfo) Device.info).Dispose(); 
       Device.info = null; 
    } 

    IDeviceInfoProvider formsActivity = activity as IDeviceInfoProvider; 

    if (formsActivity != null) 
    { 
       Device.Info = new AndroidDeviceInfo(formsActivity); 
    } 

    // recreate the ticker 
    AndroidTicker ticker = Ticker.Default as AndroidTicker; 

    if (ticker != null) 
    { 
       ticker.Dispose(); 
   } 

   Ticker.Default = new AndroidTicker(); 

    // initialize renderers (if not initialized (again)) 
    if (!IsInitialized) 
    { 
       // register renderers for attributes export / cell / image source 
       Type[] typeArray1 = new Type[] 
                   { typeof(ExportRendererAttribute), 
                   typeof(ExportCellAttribute), 
                   typeof(ExportImageSourceHandlerAttribute) 
                   }; 

       // registrer the handlers of these types 
       Registrar.RegisterAll(typeArray1); 
    } 

    // set the device idiom according to screen width dpi 
    Device.Idiom = (Context.Resources.Configuration.SmallestScreenWidthDp >= 600) 
             ? TargetIdiom.Tablet : TargetIdiom.Phone; 
 
    // set search expression default (if not already set) 
    if (ExpressionSearch.Default == null) 
    { 
       ExpressionSearch.Default = new AndroidExpressionSearch(); 
    } 

    // set initialzed flag 
    IsInitialized = true; 
} 

On iOS

public static void Init() 
{ 
    if (!IsInitialized) 
    { 
       // set initialized flag 
       IsInitialized = true; 

       // set the AccentColor 
       Color.Accent = Color.FromRgba(50, 0x4f, 0x85, 0xff); 

       Log.get_Listeners().Add(new DelegateLogListener( 
             // obscure decompilation of an anonymous methodJ 
          <>c.<>9__9_0 
             ?? (<>c.<>9__9_0 = new Action<string, string>      (<>c.<>9.<Init>b__9_0)))); 

       // device os and platform services 
       Device.OS = TargetPlatform.iOS; 
       Device.PlatformServices = new IOSPlatformServices(); 

       Device.Info = new IOSDeviceInfo(); 

       // set the ticker 
       Ticker.Default = new CADisplayLinkTicker(); 

       // register renderers for attributes export / cell / image source 
       Type[] typeArray1 = new Type[] 
                      { typeof(ExportRendererAttribute), 
                      typeof(ExportCellAttribute), 
                      typeof(ExportImageSourceHandlerAttribute) 
                      }; 

       Registrar.RegisterAll(typeArray1); 

       // set device idiom 
       Device.Idiom = (UIDevice.get_CurrentDevice  ().get_UserInterfaceIdiom() == 1L) 
             ? TargetIdiom.Tablet : TargetIdiom.Phone; 

       // set search expression default 
       ExpressionSearch.Default = new iOSExpressionSearch(); 
    } 
}

On Win Phone

public static void Init()
{
    if (!isInitialized)
    {
        // create an event trigger object (why?) 
        // note: constructor initializes an EventNameProperty DependencyProperty 
        new EventTrigger();

        string name = Assembly.GetExecutingAssembly().GetName().Name;
        ResourceDictionary dictionary1 = new ResourceDictionary();

        // load xaml resources 
        dictionary1.set_Source(
                new Uri(string.Format("/{0};component/WPResources.xaml", name),
                UriKind.Relative));

        // add resources to merged dictionaries 
        Application.get_Current().get_Resources().get_MergedDictionaries().Add(dictionary1);

        // set accent( color from resources 
        Color color = (Application.get_Current().get_Resources().get_Item("PhoneAccentBrush") as SolidColorBrush).get_Color();

        byte introduced3 = color.get_R();
        byte introduced4 = color.get_G();
        byte introduced5 = color.get_B();

        Color.Accent = Color.FromRgba((int)introduced3,
                                (int)introduced4,
                                (int)introduced5,
                                (int)color.get_A());

        // log 
        Log.get_Listeners().Add(new DelegateLogListener(<> c.<> 9__3_0
                    ?? (<> c.<> 9__3_0 = new Action<string,
                    string>(<> c.<> 9.< Init > b__3_0))));

        // set device os and platform services 
        Device.OS = TargetPlatform.WinPhone;
        Device.PlatformServices = new WP8PlatformServices();
        Device.Info = new WP8DeviceInfo();

        // register renderers for attributes export / cell / image source 
        Type[] typeArray1 = new Type[]
                    { typeof(ExportRendererAttribute),
                            typeof(ExportCellAttribute),
                            typeof(ExportImageSourceHandlerAttribute)
                    };

        Registrar.RegisterAll(typeArray1);

        // set ticker default 
        Ticker.Default = new WinPhoneTicker();

        // set device idiom 
        Device.Idiom = TargetIdiom.Phone;

        // set search expression default 
        ExpressionSearch.Default = new WinPhoneExpressionSearch();

        // set initialzed flag 
        isInitialized = true;
    }
}

Understanding how your UI code ends up by being shapes and colors on a device screen may help in better usage, better interpretation for encountered issues and for evaluating if you really need a custom renderer for a given control.

Exploring this also explains some of the interesting internal Xamarin forms mechanisms to play its awesome cross-platform game!

To do this, I will here attempt to explore the journey of a simple control: The Label.

For simplicity, I will track the journey to an Android device.

Label hierarchy

Label is a class defined Xamarin.Forms.Core.dll. Its hierarchy is roughly the following:

The Label renderer

At the end of this simple control processing chain, we have the android's Label renderer responsible of drawing it on the screen of each device… The renderer hierarchy (on Android):

The mono component

The ViewRenderer (XF Platform.Android.dll) is defined as:

public abstract class ViewRenderer<TView, TNativeView>

: VisualElementRenderer<TView>,

global::Android.Views.View.IOnFocusChangeListener,

IJavaObject,

IDisposable

It inherits the VisualElementRenderer (abstract) class which is defined as:

public abstract class VisualElementRenderer<TElement>

: FormsViewGroup,

IVisualElementRenderer,

IRegisterable,

IDisposable,

global::Android.Views.View.IOnTouchListener,

IJavaObject,

global::Android.Views.View.IOnClickListener

where TElement : VisualElement

Both of them refer to objects from the Android.Views and Android.Runtime namespaces defined in Mono.Android.dll.

The Label renderer referenced types, inheritance and interface implementations (summary)

Label renderer calls (summary)

The IRegisterable and Registrar link nodes

As you can notice, both ViewRenderer and VisualElementRenderer implement the IRegisterable Interface defined in XF.Core.dll.

IRegistrable is implemented by all Renderers. And is referenced by the Registrar class (we will see later).

Let us follow IRegistrable implemented child tree till the Label Renderer node:

(Incidentally, in he above figure, we again notice our ImageSourceHandler at the very root of the tree… you may look at my notes here)

The key node: XF Registrar

Registrar (Xamarin.Forms.Core.dll) is an internal static class. It exposes (internally) some methods for registering generic handlers of which we find the Renderers.

The method RegisterAll(Type[] attrTypes) of the Registrar class proceeds registration of the input array of types (note: as the class is internal, I could not find – for now – who calls this method: see a sample call at the end of this post).

If we follow the method's decompiled code, we understand that items of the Type array argument are expected to be each decorated with an attribute of type HandlerAttribute (deriving from System.Attribute). Here is a simplified version of the method's code:

internal static void RegisterAll(Type[] attrTypes)
{
    Assembly[] assemblies = Device.GetAssemblies();

    foreach (Assembly assembly2 in assemblies)
    {
        foreach (Type type in attrTypes)
        {
            Attribute[] attributeArray = Enumerable.ToArray<Attribute>(CustomAttributeExtensions.GetCustomAttributes(assembly2, type));
            foreach (HandlerAttribute attribute in attributeArray)
            {
                if (attribute.ShouldRegister())
                {
                    Registered.Register(attribute.HandlerType, attribute.TargetType);
                }
            }
        }
    }
}

HandlerAttribute exposes two members: HandlerType and TargetType, both are of type System.Type

Few attributes derive from this HandlerType attribute. Of which you have… oh… the famous ExportRendererAttribute!

And that is roughly how XF finds the renderer to call for a given control on each platform.

Sample call to Registrar at droid application launch in AppCompat:

In a previous post, I talked about how to manually change the project (.csproj) file to create an App.xaml and App.xaml.cs files in a Xamarin forms project. The reason for this is to be able to declare and use global application resources (styles, assets… etc.) linked to the main application file (App.cs changed to App.xaml.cs). That is, in a way, to repair a guilty XF project template that doesn't create these items by default.

Like in all development processes, when you use xamarin forms, you spend your time coding and building your projects.

In Windows, this essential build process goes through MSBuild which reads and interprets .csproj file's instructions into specific actions to compile and package your applications.

Those .csproj files are XML files containing configurations, variables and instructions… many can be manipulated in Visual Studio or Xamarin Studio. Still, some of these configurations require a deeper dive into MSBuild details. Your project, for instance, may sometimes fail to build for reasons that are difficult to trace without such dive.

Diving into MSBuild is something different from simply reading the .csproj xml instructions.

Understanding the way these instructions would be interpreted and handled by MSBuild is crucial for your investigations success.

MSBuild API to the rescue

Fortunately, MSBuild offers an API that allows you to explore or modify the build engine behavior according to your requirements.

I used this API to build a simple MSBuild browser (Windows WPF application) that allows you to examine the detailed interpretations of MSBuild to a .csproj or .vbproj file.

A project configuration browser

The browser uses the objects defined in the MSBuild Microsoft.Build.Evaluation namespace.

Three main view models of the application are used to represent MSBuild objects:

• iProject: msbuild project view model. Encapsulates the collection of:
• iProjectItemType: project item types (and actions). Each project item type encapsulating a collection of:
• iProjectItem: an item of the related type. This object encapsulates a property bag of the related MSBuild item's properties (For instance: metadata).

PropertyBag object is used to allow future application extensibility. It parses MSBuild objects' properties independently of the version referenced (the current application references MSBuild 14.0.0.0).

The result looks like this:

You can download the binaries Here…

If you are interested in extending features, you can download the source code here.

Xamarin forms offer this class (OnPlatform<T>) of great help to adapt your code (either c# or Xaml) to the current runtime device's platform.

So you may write:

string helloDevice = Device.OnPlatform<string>(
            "Hello iOS", "Hello Droid", "Hello Win Phone");

 At runtime, on iOS, your string would have the related platform value:

In Xaml, you specify the type to which the OnPlatform should act and the values for each platform:

<ContentPage.Padding>
   <OnPlatform x:TypeArguments="Thickness">
      <OnPlatform.iOS>0, 20, 0, 0</OnPlatform.iOS>
      <OnPlatform.Android>0, 0, 0, 0</OnPlatform.Android>
      <OnPlatform.WinPhone>0, 0, 0, 0</OnPlatform.WinPhone>
   </OnPlatform>
</ContentPage.Padding>

The main difficulty in Xaml is that you should know exactly the Type to pass as TypeArgument.

For system known types, you should use the x: prefix like in:

x:TypeArguments="x:String"
x:TypeArguments="x:Double"
x:TypeArguments="x:Int32"

For Xamarin Forms objects properties and types, it is easy to use the Visual Studio Object Browser to obtain the correct information:

Here, for instance, you know that the Padding is of type Xamarin Forms Thickness:

    <OnPlatform x:TypeArguments="Thickness">

I continue about XF maps, where a custom renderer is needed if you want to customize appearance and interaction.

As most Xamarin forms samples about maps expose renderers for Droid and iOS (I did not see any about WP), I will start by exposing a sample for that 'missing' renderer. Another post will follow with some ideas about the renderers for the other platforms.

In this sample, we handle SamplePlace objects (see below) through our custom map and custom pin objects (see previous post)

What we want:

• Display custom pushpins (icons specified for each custom pin)
• When a pushpin is clicked, display a custom information callout about the place.

The way to do this:

The renderer (which derives from MapRenderer) receives our custom map, containing the map pins. The binding context of each Pin is set to the related custom pin. The place data is accessed through the custom pin's DataObject.

The renderer has two levels of duties:

• A 'formal' role inherited from the default renderer: create and display the specific platform map control, position the pins on it and define the interaction with the control's events (map taps / pins taps / callout taps…).
• A customization role: use our specific embedded information to customize pins' appearance and callouts.

The renderer code

Our renderer class:

public class iCustomMapRenderer : ViewRenderer<iCustomMap, Map>

For Xamarin.Forms.Maps to retrieve our renderer at runtime, it should be 'exported'. The code should now be like the following:

// Note: ExportRendererAttribute is defined in Xamarin.Forms namespace
[assembly: ExportRenderer(typeof(iCustomMap), typeof(iCustomMapRenderer))]

namespace iMaps.WinPhone
{
	public class iCustomMapRenderer : ViewRenderer<iCustomMap, Map>
	{
		private Map				_winMap;			// the win phone map control
		private iCustomMap		_xfMap;				// the xamarin forms map we are handling
		private iCustomPin		_selectedPin;	// our current slected pin

		// the callout user control that will display selected place info
		UserControl	_placeInfoCtrl	= new UserControl()
		{
			HorizontalAlignment	= HorizontalAlignment.Stretch,
			VerticalAlignment		= VerticalAlignment.Stretch,
			MinHeight			= 300,
			Background			= whiteBrush,
		};
		…

Here is a global view of the renderer's methods:

In the ElementPropertyChanged (override), we will:

• Create the win phone map control.
• Parse the received pins, and use their specified icons as pushpins.
• Subscribe to each pushpin Tap event to display the callout.
• Subscribe to the map Tap event to hide any displayed callout.

// Center the map on the 1st pin
var			pin	= _xfMap.Pins[0];

// create the win phone map control. set as the native control
_winMap		= new Map
		{
			ZoomLevel = 13,
			Center = new GeoCoordinate(pin.Position.Latitude, pin.Position.Longitude)
		};

this.SetNativeControl(_winMap);

// subscribe to map Tap event (to unselect last pin if any)
_winMap.Tap		+= WinMap_Tap;

// AddCurrentLocationToMap();
// loop through the received pins. display each with its custom pin icon
foreach (var formsPin in _xfMap.Pins)		//.CustomPins)
{
	var			pushPin		= new Pushpin();
	iCustomPin	customPin	= formsPin.BindingContext as iCustomPin;
	SamplePlace	place		= customPin == null ? null 
										: customPin.DataObject as SamplePlace;

	// subscribe to the pushpin tap event (to display the callout)
	pushPin.Tap += PushPin_Tap;

	// set the pushpin position on the map
	var geoCoordinate			= new GeoCoordinate(formsPin.Position.Latitude, formsPin.Position.Longitude);
	pushPin.GeoCoordinate		= geoCoordinate;

	// set the pushpin tag to the custom pin
	pushPin.Tag					= customPin;
	…
	…

The custom pushpin icon

To put a UI element on the map, you create a MapOverlay containing the element on the desired location (geo coordinate) and you add that MapOverlay to a Layer that you put on the map.

A Pushpin (defined in Microsoft.Phone.Maps.Toolkit namespace) is a UI element that exposes a Content property (of type ContentControl) which we can set to our image of choice (the one specified by the custom pin object for instance). In that case, the pushpin would look like this:

We may also directly put our image on the MapOverlay. In that case that would be like this:

The callout control

Let us create a (win phone) xaml UserControl for displaying place object's information. We will use this when a pushpin will be tapped.

<Border x:Name="LayoutBorder" Background="Transparent" Height="auto" Padding="8" >
	<Grid VerticalAlignment="Top" Background="Transparent">
		<Grid.RowDefinitions>
			<RowDefinition Height="24" />
			<RowDefinition />
		</Grid.RowDefinitions>
		<Grid.ColumnDefinitions>
			<ColumnDefinition Width="24" />
			<ColumnDefinition />
		</Grid.ColumnDefinitions>

		<Image Source="{Binding Source, Source={StaticResource imgMapPointer}}" Grid.Column="0" Grid.Row="0" Stretch="Fill" Height="24" Width="24" HorizontalAlignment="Left" VerticalAlignment="Top" />

		<Border Padding="8" Background="White" Grid.Row="0" Grid.Column="1" Grid.RowSpan="2">
			<StackPanel>
				<Image x:Name="imgPlace" Height="128" Width="128" HorizontalAlignment="Left" Source="{Binding Converter={StaticResource placeImageConverter}}" Margin="8" />
				<TextBlock FontWeight="Bold" Text="{Binding Name}" Foreground="Black" />
				<Rectangle Height="1" Fill="Black" HorizontalAlignment="Stretch" />
				<TextBlock Margin="8,0" FontSize="14" Text="{Binding MapPinText}" Foreground="Black" />
			</StackPanel>
		</Border>
	</Grid>
</Border>

At design time, the code above would look like:

At runtime, the Pushpin Tap event handler will set the control's DataContext to the selected SamplePlace object. It will then display its image and information.

The pushpin Tap event handler determines the currently selected place (through the pushpin.Tag) and calls this method to update the control:

void UpdateInfoControlContents(SamplePlace place)
{
	_placeInfoCtrl.ClearValue(UserControl.ContentProperty);

	if(place == null)
		return;

	PlacePushPinInfoCtrl ctrl = new PlacePushPinInfoCtrl() { DataContext = place };

	_placeInfoCtrl.Content = ctrl;
}

There is certainly still much to do for these mechanics to produce something elegant and informative.

That may just be a good start.... Download the code and have more fun!

In a following post I will talk about some ideas for Droid and iOS map renderers (also included in the sample).

This is a follow up for my last post about AppCompat and rounded buttons where your custom renderer would not be called!

In that post, I exposed a 'hack' solution which is better be avoided.

On this page (the same where Kelly Adams presented a decompiled code of the AppCompat launcher), Thomas Omer-Decugis exposed a simple and efficient solution: you create a new class that derives from Button and create (in the Droid project) your custom renderer for this class.

In that case, your renderer is called and can proceed to handling properties like BorderRadius that are ignored by AppCompat.

public class iRoundedButton : Button 

In this screen capture, the first 2 buttons are standard buttons (where border radius is not handled by AppCompat)

The following 2 are defined as: (xmlns:local="clr-namespace:iRoundedButton")

<local:iRoundedButton … 

And where the custom renderer handles the border radius.

The custom renderer itself is the same as in the previous sample, with this minor change:

[assembly: ExportRenderer(typeof(iRoundedButton /*Xamarin.Forms.Button*/), typeof(CustomButtonCompatRenderer))]

Why do we use maps?

Apart from the pleasure (and poesy) of locating things in the global earth space, there is something practical and useful in using maps and geolocation.

I myself could never be able to live in big cities without mapping applications guiding me from a place to another and giving me such vital information like 'where can I park my car' (almost a dilemma in European cities at leastJ)

As we talk about maps, we of course talk about mobility (and vice versa). That puts it in a way where we might think that a mobile app is often linked to mapping features: if your app sells something (it often doesJ), the user will inevitably ask: where is it? How can I go there?... etc.

Xamarin forms maps package

The Xamarin forms maps package delivers a good solution to integrate maps into your XF applications.

A nice sample is delivered that explains how to get started with the package.

To avoid some of startup pitfalls, I compiled my own pitfalls and solutions here.

The package (Xamarin.Forms.Maps.dll) contains a few useful objects:

Xamarin.Forms.Maps - What / Who lives there?

The component exposes three main objects:

• Map object (a View)
• MapSpan object
• Pin object (a BindableObject)

Finally an object: Geocoder for geocoding operations (that I didn't find really useful or simply operational)

More details and Inheritance

A Map (deriving from View) is of:

• MapType (enumeration)
• It has two MapSpan(s) representing the VisibleRegion and the LastMoveToRegion
• And it contains a collection of Pin(s) (IList<Pin>)

A MapSpan is composed of its center (a Position: a structure of Latitude, Longitude), dimensions in terms of Distance (a structure with some factory methods (FromKilometers, FromMiles…) to apply values using known distance units).

The good news is that the Pin is a BindableObject. We will see how to use this to extend some features.

Extending Xamarin Forms Maps

You can simply use the maps package with its provided features and obtain good and useful results for your app's users: display pins on the map, when a pin is clicked, its label is displayed, and you can respond to that label's Tap event to open your selected item's specific form. That can be enough in many cases.

Other more demanding apps may need, for instance, to customize pin appearance and/or the callout for each pins… etc.

The standard Pin object is limited to handling objects' Address (string), Label (string), Position (coordinates) and pin Type (enumeration: Generic, Place, SavedPin or SearchResult). It also exposes a Clicked event to which your app can subscribe.

It does not handle other features that you may need… like specifying a pin icon, or a custom callout.

So, you think: still, we can derive a custom pin object that may provide such requested information.

Unfortunately that is not possible because Pin is a sealed class! (Why is it sealed?... mystery!. I hope the creators have good reason for this choice. For now, I find this a little strange)

Customizing maps: yet another solution

I found some interesting efforts to extend and circumvent the Xamarin maps limitations.

You may have a look at some Here… or Here.

Most efforts concentrate on offering Custom rendering solutions (For Droid and iOS. Few, if any, expose a Win Phone workable renderer).

The fact that we need a custom renderer for maps is inevitable if we opt to using the specific platform mapping features (which seems the reasonable choice). A structured (architectural) approach is required in any case.

What we want to handle in this sample, is a Place object:

It contains information about its location (latitude, longitude, address…) and other properties (Name, description, image…)

What we need in our current sample is:

• Display a custom pushpin according to the type (category) of the Place at the pin location.
• Display a custom callout when the pin would be clicked.
• Raise a Clicked event when the callout would be tapped.

Instead of this:

We would like to have something like this :

As we cannot inherit the Pin class (sealed), let us create an iCustomPin class containing the target map Pin and providing the required customization features (here the pin's icon):

The iCustomPin class will act as a view model for the Pin, thus providing some (gets) of its properties: MapPinLabel, MapPinPosition, MapPinType… etc.

It exposes a DataObject (an 'object', which will contain a Place object in the current sample… Note: That may be refactored to a BindableProperty)

An iCustomMap object (deriving from the Xamarin forms Map) will be in charge of inserting / updating our custom pins. It may also provide a custom Callout control template (ContentView) that can be used for displaying the selected (Clicked) pushpin.

How these objects can be used?

The sample execution sequence can be as follows:

• Data: create a list of sample places, each containing coordinates, description…
• View: Insert a custom map control (iCustomMap object) into the page
• For each ample place, add the corresponding custom pin (iCustomPin) to the custom map:
  • A custom pin contains a 'standard' Pin
  • Set standard Pin's BindingContext to the custom pin. (The custom pin contains the Data Object… which is the Place)
• Rendering: the custom renderer, at each platform, can access the custom pin info (and thus the sample Place) through the Pin's BindingContext… It would then display the pushpins according to the specified icon asset, and eventually displays the place's callout when required.

In a following post, I will expose custom renderers details for Android and WP.

Will also try to polish the sample code a little more before publishing for download. Will keep you posted when readyJ