[HybridWebView] fix trimmer warnings on Android (#24744)

jonathanpeppers · web-flow · commit 70fbb835d74b · 2024-09-20T13:54:13.000+01:00
Context: #23769 Context: dotnet/android#9300 026e046 introduced `HybridWebView`, which unfortunately introduces trimmer warnings in the `dotnet new maui` project template: > dotnet new maui > dotnet build -f net9.0-android -c Release -p:TrimMode=Full ... hellomaui succeeded with 1 warning(s) (7.9s) /_/src/Core/src/Handlers/HybridWebView/HybridWebViewHandler.Android.cs(53,5): Trim analysis warning IL2026: Microsoft.Maui.Handlers.HybridWebViewHandler.HybridWebViewJavaScriptInterface.SendMessage(String): Using member 'Java.Interop.ExportAttribute.ExportAttribute(String)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. [ExportAttribute] uses dynamic features. This is due to usage of `Java.Interop.ExportAttribute`: private sealed class HybridWebViewJavaScriptInterface : Java.Lang.Object { //... [JavascriptInterface] [Export("sendMessage")] public void SendMessage(string message) `Java.Interop.ExportAttribute` makes heavy usage of unbounded System.Reflection, System.Reflection.Emit, etc. for it to be able to work. It brings in an additional assembly `Mono.Android.Export.dll` as well. It is inherently trimming unsafe, but how did it get through these tests? https://github.com/dotnet/maui/blob/08ff1246383ed4fdaef84a40d5b2ae8e6096bb56/src/TestUtils/src/Microsoft.Maui.IntegrationTests/AndroidTemplateTests.cs#L50-L61 This slipped through, unfortunately, as we had missed solving all the trimmer warnings in `Mono.Android.Export.dll`! dotnet/android#9300 aims to fix that. After dotnet/android#9300, new trimming warnings specific to .NET MAUI will surface such as the one above. But we can easily replace `[Export]` by: * Define a Java interface & create a binding for it in C#, we already have `maui.aar` setup for this. * We can simply implement the interface in C# and remove `[Export]`. Lastly, I fixed some of the defaults in `Metadata.xml`, it didn't look like we were automatically making Java interfaces `internal`. It looks like we probably made `ImageLoaderCallback` public by mistake.
diff --git a/src/Core/AndroidNative/maui/src/main/java/com/microsoft/maui/HybridJavaScriptInterface.java b/src/Core/AndroidNative/maui/src/main/java/com/microsoft/maui/HybridJavaScriptInterface.java
@@ -0,0 +1,7 @@
+package com.microsoft.maui;
+
+import androidx.annotation.NonNull;
+
+public interface HybridJavaScriptInterface {
+    void sendMessage(@NonNull String message);
+}
diff --git a/src/Core/src/Handlers/HybridWebView/HybridWebViewHandler.Android.cs b/src/Core/src/Handlers/HybridWebView/HybridWebViewHandler.Android.cs
@@ -35,7 +35,7 @@ protected override AWebView CreatePlatformView()
 			return platformView;
 		}
 
-		private sealed class HybridWebViewJavaScriptInterface : Java.Lang.Object
+		private sealed class HybridWebViewJavaScriptInterface : Java.Lang.Object, IHybridJavaScriptInterface
 		{
 			private readonly WeakReference<HybridWebViewHandler> _hybridWebViewHandler;
 
@@ -47,7 +47,6 @@ public HybridWebViewJavaScriptInterface(HybridWebViewHandler hybridWebViewHandle
 			private HybridWebViewHandler? Handler => _hybridWebViewHandler is not null && _hybridWebViewHandler.TryGetTarget(out var h) ? h : null;
 
 			[JavascriptInterface]
-			[Export("sendMessage")]
 			public void SendMessage(string message)
 			{
 				Handler?.VirtualView?.MessageReceived(message);
diff --git a/src/Core/src/Transforms/Metadata.xml b/src/Core/src/Transforms/Metadata.xml
@@ -2,13 +2,15 @@
   <!-- Normalize the namespace for .NET -->
   <attr path="//package[@name='com.microsoft.maui']" name="managedName">Microsoft.Maui</attr>
 
-  <!-- Make all classes internal by default -->
+  <!-- Make all types internal by default -->
   <attr path="//class[@visibility='public']" name="visibility">internal</attr>
-  <!-- Public classes -->
+  <attr path="//interface[@visibility='public']" name="visibility">internal</attr>
+  <!-- Public types -->
   <attr path="//class[@name='PlatformAppCompatTextView']" name="visibility">public</attr>
   <attr path="//class[@name='PlatformContentViewGroup']" name="visibility">public</attr>
   <attr path="//class[@name='PlatformWrapperView']" name="visibility">public</attr>
   <attr path="//class[@name='MauiViewGroup']" name="visibility">public</attr>
+  <attr path="//interface[@name='ImageLoaderCallback']" name="visibility">public</attr>
   <!-- Internal methods & constructors -->
   <attr path="//class[@name='PlatformAppCompatTextView']/method" name="visibility">internal</attr>
   <attr path="//class[@name='PlatformAppCompatTextView']/constructor" name="visibility">internal</attr>
diff --git a/src/Core/src/maui.aar b/src/Core/src/maui.aar

Original file line number	Diff line number	Diff line change
`@@ -35,7 +35,7 @@ protected override AWebView CreatePlatformView()`
`35`	`35`	`return platformView;`
`36`	`36`	`}`
`37`	`37`
`38`		`- private sealed class HybridWebViewJavaScriptInterface : Java.Lang.Object`
	`38`	`+ private sealed class HybridWebViewJavaScriptInterface : Java.Lang.Object, IHybridJavaScriptInterface`
`39`	`39`	`{`
`40`	`40`	`private readonly WeakReference<HybridWebViewHandler> _hybridWebViewHandler;`
`41`	`41`
`@@ -47,7 +47,6 @@ public HybridWebViewJavaScriptInterface(HybridWebViewHandler hybridWebViewHandle`
`47`	`47`	`private HybridWebViewHandler? Handler => _hybridWebViewHandler is not null && _hybridWebViewHandler.TryGetTarget(out var h) ? h : null;`
`48`	`48`
`49`	`49`	`[JavascriptInterface]`
`50`		`- [Export("sendMessage")]`
`51`	`50`	`public void SendMessage(string message)`
`52`	`51`	`{`
`53`	`52`	`Handler?.VirtualView?.MessageReceived(message);`