Custom native control wrappers for UX markup

This tutorial takes you through how to create a custom wrapper for a native Android or iOS control using Uno and foreign code, and bootstrapping it so it can be used from UX Markup. As an example, we'll show how to wrap a slider control.

This tutorial assumes you are familiar with the native language of the platform you are working with, that is, Java for Android and Objective-C for iOS.

Implementing LeafView

There are two types of views we can implement. ParentView allows you to wrap a view that contains other views (i.e. panels), while LeafView wraps a view that does not contain other views (e.g. buttons, switches, pickers). In this example, we'll focus on LeafView as this is what you'll most commonly need.

We start out by creating a class which defines our slider with some boilerplate code for handling properties. We also declare a pair of interfaces that we will use to communicate between native and our slider class.

namespace Native
{
	using Uno;
	using Uno.UX;

	using Fuse.Controls;

	public interface ISliderHost
	{
		void OnValueChanged(float newValue);
	}

	public interface ISlider
	{
		float Value { set; }
	}

	public class MySlider : Control, ISliderHost
	{
		float _value;
		[UXOriginSetter("SetValue")]
		public float Value
		{
			get { return _value; }
			set { SetValue(value, this); }
		}

		static readonly Selector _valueName = "Value";
		public void SetValue(float newValue, IPropertyListener origin)
		{
			if (_value != newValue)
			{
				_value = newValue;
				OnPropertyChanged(_valueName, origin);
			}
			if (origin != null)
			{
				var ns = NativeSlider;
				if (ns != null)
					ns.Value = newValue;
			}
		}

		void ISliderHost.OnValueChanged(float newValue)
		{
			SetValue(newValue, null);
		}

		ISlider NativeSlider
		{
			get { return NativeView as ISlider; }
		}
	}

}

We move on to creating a wrapper for the native view, by extending the LeafView class(es). We need to make separate versions for iOS and Android, preferably in separate files. If you only want to support one platform, that's fine. We can then simply skip the steps for the other platform.

There are two versions of the LeafView class for iOS and Android respectively, in the namespaces Fuse.Controls.Native.iOS and Fuse.Controls.Native.Android.

Remember that to access foreign code (i.e. the Foreign attribute), we must also add using Uno.Compiler.ExportTargetInterop;

Implementing LeafView for iOS

We create our LeafView class in Uno with foreign Objective-C code:

namespace Native.iOS
{
	using Uno;
	using Uno.UX;
	using Uno.Compiler.ExportTargetInterop;

	using Fuse.Controls.Native.iOS;

	[Require("Source.Include", "UIKit/UIKit.h")]
	extern(iOS) public class MySlider: LeafView, ISlider
	{
		[UXConstructor]
		public MySlider([UXParameter("Host")]ISliderHost host) : base(Create()) { }

		[Foreign(Language.ObjC)]
		static ObjC.Object Create()
		@{
			::UISlider* slider = [[::UISlider alloc] init];
			[slider setMinimumValue:   0.0f];
			[slider setMaximumValue: 100.0f];
			return slider;
		@}

		public float Value
		{
			get { return GetValue(Handle); }
			set { SetValue(Handle, value); }
		}

		[Foreign(Language.ObjC)]
		static float GetValue(ObjC.Object handle)
		@{
			::UISlider* slider = (::UISlider*)handle;
			return [slider value];
		@}

		[Foreign(Language.ObjC)]
		static void SetValue(ObjC.Object handle, float value)
		@{
			::UISlider* slider = (::UISlider*)handle;
			[slider setValue:value animated:false];
		@}

		void OnValueChanged()
		{
			// TODO: implement value changed callback
		}
	}
}

The above example only implements setting the Value, it does not hook onto the UIControlEventValueChanged to feed events back to Fuse when the user interacts with the control. A complete useful wrapper would need that.

Note how the LeafView constructor expects to get an ObjC.Object as argument. We feed it by creating a static method where we instantiate the UISlider and call that as argument to the base constructor.

To get access to the ObjC.Object representing the UISlider from within your class later, we can use the Handle property, as seen in how we implement the Value property above.

The MySlider class has the extern(iOS) attribute, and will therefore only be available when building for iOS. To make it possible to use this class in UX markup, it must be valid on all platforms. Therefore we must provide a dummy implementation with just the public interface marked with extern(!iOS) within the same namespace:

namespace Native.iOS
{
	extern(!iOS) public class MySlider
	{
		[UXConstructor]
		public MySlider([UXParameter("Host")]ISliderHost host) { }
	}
}

Implementing LeafView for Android

And then, let's write the equivalent Android implementation:

namespace Native.Android
{
	using Uno;
	using Uno.UX;
	using Uno.Compiler.ExportTargetInterop;

	using Fuse.Controls.Native.Android;

	extern(Android) public class MySlider : LeafView, ISlider
	{
		ISliderHost _host;
		[UXConstructor]
		public MySlider([UXParameter("Host")]ISliderHost host) : base(Create())
		{
			_host = host;AddChangedCallback(Handle, OnValueChanged);
		}

		float ISlider.Value
		{
			set { SetValue(Handle, value); }
		}

		[Foreign(Language.Java)]
        static Java.Object Create()
        @{
            android.widget.SeekBar seekBar = new android.widget.SeekBar(@(Activity.Package).@(Activity.Name).GetRootActivity());
            seekBar.setMax(100);
            return seekBar;
        @}

        [Foreign(Language.Java)]
		void AddChangedCallback(Java.Object handle, Action<float> onValueChanged)
		@{
			((android.widget.SeekBar)handle).setOnSeekBarChangeListener(new android.widget.SeekBar.OnSeekBarChangeListener() {
				public void onProgressChanged(android.widget.SeekBar seekBar, int progress, boolean fromUser) {
					onValueChanged.run((float)progress);
				}
				public void onStartTrackingTouch(android.widget.SeekBar seekBar) { }
				public void onStopTrackingTouch(android.widget.SeekBar seekBar) { }
			});
		@}

		[Foreign(Language.Java)]
		void SetValue(Java.Object handle, float value)
		@{
			((android.widget.SeekBar)handle).setProgress((int)value);
		@}

		void OnValueChanged(float newValue)
		{
			_host.OnValueChanged(newValue);
		}
	}
}

The above implementation also shows how to implement an OnSeekBarChangeListener to get events back from android to update the Value proeprty.

As for iOS, we also need a dummy implementation for extern(!Android) to allow this to be used in UX markup:

namespace Native.Android
{
	extern(!Android) public class MySlider
	{
		[UXConstructor]
		public MySlider([UXParameter("Host")]ISliderHost host) { }
	}
}

Creating a UX wrapper Control

The LeafView classes do not inherit Visual, so they can not be used directly in a UX markup tree. We need to create an intermediary Control class that acts as a wrapper. This is nice, because it allows us to distinguish between the public API we expose to UX, and the implementation details of each platform. It also allows us to create a unified wrapper that will work cross-platform.

The Fuse.Controls.Control class contains a neat piece of magic. It allows you to specify multiple templates for different scenarios, and it will instantiate the correct template at runtime based on context:

  • AndroidTemplate - will be used on Android, when inside a NativeViewHost
  • iOSTemplate - will be used on iOS, when inside a NativeViewHost
  • GraphicsTemplate - will be used in graphics contexts (when outside NativeViewHost), and as a fallback on desktop

So in order to make our Slider work on all platforms, we must provide one template for each scenario. We pass this to the Host attibuite to connect Native.MySlider with the native counterpart.

<Native.MySlider ux:Class="NativeSlider">
	<Native.Android.MySlider ux:Template="AndroidAppearance" Host="this" />
	<Native.iOS.MySlider ux:Template="iOSAppearance" Host="this" />
	<Text ux:Template="GraphicsAppearance">
		MySlider is not available in this context.
	</Text>
</Native.MySlider>

This hooks everything up, so if we use <NativeSlider /> in UX markup now, it will use the correct implementation on each platform if we are inside a NativeViewHost. It will display an error Text message in other scenarios.

If you want to, we can certainly provide a proper implementation of a slider as GraphicsTemplate instead of just displaying an error message.

And done

Happy coding. This tutorial is work in progress. If you encounter problems, feedback is appreciated.