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Xamarin.Forms Shell custom renderers

  • Стаття

One of the advantages of Xamarin.Forms Shell applications is that their appearance and behavior is highly customizable through the properties and methods that the various Shell classes expose. However, it's also possible to create a Shell custom renderer when more extensive platform-specific customizations are required. As with other custom renderers, a Shell custom renderer can be added to just one platform project to customize appearance and behavior, while allowing the default behavior on the other platform; or a different Shell custom renderer can be added to each platform project to customize appearance and behavior on both iOS and Android.

Shell applications are rendered using the ShellRenderer class on iOS and Android. On iOS, the ShellRenderer class can be found in the Xamarin.Forms.Platform.iOS namespace. On Android, the ShellRenderer class can be found in the Xamarin.Forms.Platform.Android namespace.

The process for creating a Shell custom renderer is as follows:

  1. Subclass the Shell class. This will already be accomplished in your Shell application.
  2. Consume the subclassed Shell class. This will already be accomplished in your Shell application.
  3. Create a custom renderer class that derives from the ShellRenderer class, on the required platforms.

Create a custom renderer class

The process for creating a Shell custom renderer class is as follows:

  1. Create a subclass of the ShellRenderer class.
  2. Override the required methods to perform the required customization.
  3. Add an ExportRendererAttribute to the ShellRenderer subclass, to specify that it will be used to render the Shell application. This attribute is used to register the custom renderer with Xamarin.Forms.

Note

It's optional to provide a Shell custom renderer in each platform project. If a custom renderer isn't registered, then the default ShellRenderer class will be used.

The ShellRenderer class exposes the following overridable methods:

iOS Android UWP
SetElementSize
CreateFlyoutRenderer
CreateNavBarAppearanceTracker
CreatePageRendererTracker
CreateShellFlyoutContentRenderer
CreateShellItemRenderer
CreateShellItemTransition
CreateShellSearchResultsRenderer
CreateShellSectionRenderer
CreateTabBarAppearanceTracker
Dispose
OnCurrentItemChanged
OnElementPropertyChanged
OnElementSet
UpdateBackgroundColor		 CreateFragmentForPage
CreateShellFlyoutContentRenderer
CreateShellFlyoutRenderer
CreateShellItemRenderer
CreateShellSectionRenderer
CreateTrackerForToolbar
CreateToolbarAppearanceTracker
CreateTabLayoutAppearanceTracker
CreateBottomNavViewAppearanceTracker
OnElementPropertyChanged
OnElementSet
SwitchFragment
Dispose		 CreateShellFlyoutTemplateSelector
CreateShellHeaderRenderer
CreateShellItemRenderer
CreateShellSectionRenderer
OnElementPropertyChanged
OnElementSet
UpdateFlyoutBackdropColor
UpdateFlyoutBackgroundColor

The FlyoutItem and TabBar classes are aliases for the ShellItem class, and the Tab class is an alias for the ShellSection class. Therefore, the CreateShellItemRenderer method should be overridden when creating a custom renderer for FlyoutItem objects, and the CreateShellSectionRenderer method should be overridden when creating a custom renderer for Tab objects.

Important

There are additional Shell renderer classes, such as ShellSectionRenderer and ShellItemRenderer, on iOS, Android, and UWP. However, these additional renderer classes are created by overrides in the ShellRenderer class. Therefore, customizing the behavior of these additional renderer classes can be achieved by subclassing them, and creating an instance of the subclass in the appropriate override in the subclassed ShellRenderer class.

iOS example

The following code example shows a subclassed ShellRenderer, for iOS, that sets a background image on the navigation bar of the Shell application:

using UIKit;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly: ExportRenderer(typeof(Xaminals.AppShell), typeof(Xaminals.iOS.MyShellRenderer))]
namespace Xaminals.iOS
{
    public class MyShellRenderer : ShellRenderer
    {
        protected override IShellSectionRenderer CreateShellSectionRenderer(ShellSection shellSection)
        {
            var renderer = base.CreateShellSectionRenderer(shellSection);
            if (renderer != null)
            {
                (renderer as ShellSectionRenderer).NavigationBar.SetBackgroundImage(UIImage.FromFile("monkey.png"), UIBarMetrics.Default);
            }
            return renderer;
        }
    }
}

The MyShellRenderer class overrides the CreateShellSectionRenderer method, and retrieves the renderer created by the base class. It then modifies the renderer by setting a background image on the navigation bar, before returning the renderer.

Android example

The following code example shows a subclassed ShellRenderer, for Android, that sets a background image on the navigation bar of the Shell application:

using Android.Content;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

[assembly: ExportRenderer(typeof(Xaminals.AppShell), typeof(Xaminals.Droid.MyShellRenderer))]
namespace Xaminals.Droid
{
    public class MyShellRenderer : ShellRenderer
    {
        public MyShellRenderer(Context context) : base(context)
        {
        }

        protected override IShellToolbarAppearanceTracker CreateToolbarAppearanceTracker()
        {
            return new MyShellToolbarAppearanceTracker(this);
        }
    }
}

The MyShellRenderer class overrides the CreateToolbarAppearanceTracker method, and returns an instance of the MyShellToolbarAppearanceTracker class. The MyShellToolbarAppearanceTracker class, which derives from the ShellToolbarAppearanceTracker class, is shown in the following example:

using AndroidX.AppCompat.Widget;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

namespace Xaminals.Droid
{
    public class MyShellToolbarAppearanceTracker : ShellToolbarAppearanceTracker
    {
        public MyShellToolbarAppearanceTracker(IShellContext context) : base(context)
        {
        }

        public override void SetAppearance(Toolbar toolbar, IShellToolbarTracker toolbarTracker, ShellAppearance appearance)
        {
            base.SetAppearance(toolbar, toolbarTracker, appearance);
            toolbar.SetBackgroundResource(Resource.Drawable.monkey);
        }
    }
}

The MyShellToolbarAppearanceTracker class overrides the SetAppearance method, and modifies the toolbar by setting a background image on it.

Important

It's only necessary to add the ExportRendererAttribute to a custom renderer that derives from the ShellRenderer class. Additional subclassed Shell renderer classes are created by the subclassed ShellRenderer class.