橋模式

Bridge 模式是最基本的控制反轉設計模式之一。對於 Xamarin，此模式用於從獨立於平臺的上下文中引用與平臺相關的程式碼。例如：使用來自 Portable Class Library 或 Xamarin Forms 的 Android 的 AlertDialog。這些上下文都不知道 AlertDialog 物件是什麼，因此你必須將其包裝在一個框中供他們使用。

// Define a common interface for the behavior you want in your common project (Forms/Other PCL)
public interface IPlatformReporter
{
    string GetPlatform();
}

// In Android/iOS/Win implement the interface on a class
public class DroidReporter : IPlatformReporter
{
    public string GetPlatform()
    {
        return "Android";
    }
}

public class IosReporter : IPlatformReporter
{
    public string GetPlatform()
    {
        return "iOS";
    }
}

// In your common project (Forms/Other PCL), create a common class to wrap the native implementations
public class PlatformReporter : IPlatformReporter
{
    // A function to get your native implemenation
    public static func<IPlatformReporter> GetReporter;

    // Your native implementation
    private IPlatformReporter _reporter;

    // Constructor accepts native class and stores it
    public PlatformReporter(IPlatformReporter reporter)
    {
        _reporter = GetReporter();
    }

    // Implement interface behavior by deferring to native class
    public string GetPlatform()
    {
        return _reporter.GetPlatform();
    }
}

// In your native code (probably MainActivity/AppDelegate), you just supply a function that returns your native implementation
public class MainActivity : Activity
{
    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        SetContentView(Resource.Layout.activity_main);

        PlatformReporter.GetReporter = () => { return new DroidReporter(); };
    }
}

public partial class AppDelegate : UIApplicationDelegate
{
    UIWindow window;

    public override bool FinishedLaunching(UIApplication app, NSDictionary options)
    {
        window = new UIWindow(UIScreen.MainScreen.Bounds);
        window.RootViewController = new UIViewController();
        window.MakeKeyAndVisible();

        PlatformReporter.GetReporter = () => { return new IosReporter(); };

        return true;
    }
}

// When you want to use your native implementation in your common code, just do as follows:
public void SomeFuncWhoCares()
{
    // Some code here...

    var reporter = new PlatformReporter();
    string platform = reporter.GetPlatform();

    // Some more code here...
}