How to include Android binding dependencies and avoid ClassNotFoundExceptions

Handling transitive dependencies is common challenge with .NET for Android and Android Native Library Interop binding projects when attempting to bind a Maven artifact. This story is further complicated by the fact that .NET for Android binds and redistributes AndroidX and other support libraries via NuGet packages, and MAUI projects depend on a handful of these libraries.

Starting in .NET 9 there are a few build items that can be used to help validate your Java dependency tree which are outlined in the [Java Dependency Verification](https://learn.microsoft.com/en-us/dotnet/android/binding-libs/advanced-concepts/java-dependency-verification) and [Resolving Java Dependencies](https://learn.microsoft.com/en-us/dotnet/android/binding-libs/advanced-concepts/resolving-java-dependencies) docs.

We can look at the Facebook sample in this repo for an example on how to use these items with Native Library Interop. Here's a quick overview of how the various pieces fit together.

1. In your `build.gradle.kts` file, declare the Maven artifact(s) you are interested in binding: https://github.com/CommunityToolkit/Maui.NativeLibraryInterop/blob/e91dadd35f0c4ceb333034c3fd7b3c6ceebf4c94/facebook/android/native/mauifacebook/build.gradle.kts#L31
2. In your application project, add the same Maven artifact(s) as an `@(AndroidMavenLibrary)` item. Set the `%(Bind)` metadata to false; we do not want to bind this artifact again. We are only including this reference to opt-in to the Java dependency verification system mentioned above to figure out which artifact dependencies we will need to bring into the app:
```xml

```
3. Build your application project, and you should see a list of unfulfilled dependencies:
```log
error XA4241: Java dependency 'com.facebook.android:facebook-core:17.0.2' is not satisfied.
error XA4241: Java dependency 'com.facebook.android:facebook-common:17.0.2' is not satisfied.
error XA4241: Java dependency 'com.facebook.android:facebook-login:17.0.2' is not satisfied.
error XA4241: Java dependency 'com.facebook.android:facebook-share:17.0.2' is not satisfied.
error XA4241: Java dependency 'com.facebook.android:facebook-applinks:17.0.2' is not satisfied.
error XA4241: Java dependency 'com.facebook.android:facebook-messenger:17.0.2' is not satisfied.
error XA4241: Java dependency 'com.facebook.android:facebook-gamingservices:17.0.2' is not satisfied.
error XA4242: Java dependency 'org.jetbrains.kotlin:kotlin-stdlib:1.5.10' is not satisfied. Microsoft maintains the NuGet package 'Xamarin.Kotlin.StdLib' that could fulfill this dependency.
```
4. It's recommended that you include all of the dependencies listed in this output in your application project. However, given the scope of what you are trying to bind, you can try to reason about which of these dependencies you could safely exclude. In the case of the Facebookj sample, I replaced the `facebook-android-sdk` dependency with the first dependency listed above (`facebook-core`) and rebuilt:
https://github.com/CommunityToolkit/Maui.NativeLibraryInterop/blob/e91dadd35f0c4ceb333034c3fd7b3c6ceebf4c94/facebook/sample/Sample.csproj#L65
```
error XA4241: Java dependency 'com.facebook.android:facebook-bolts:17.0.2' is not satisfied.
error XA4242: Java dependency 'androidx.annotation:annotation:1.1.0' is not satisfied. Microsoft maintains the NuGet package 'Xamarin.AndroidX.Annotation' that could fulfill this dependency.
error XA4242: Java dependency 'androidx.legacy:legacy-support-core-utils:1.0.0' is not satisfied. Microsoft maintains the NuGet package 'Xamarin.AndroidX.Legacy.Support.Core.Utils' that could fulfill this dependency.
error XA4242: Java dependency 'com.android.installreferrer:installreferrer:1.0' is not satisfied. Microsoft maintains the NuGet package 'Xamarin.Google.Android.InstallReferrer' that could fulfill this dependency.
error XA4242: Java dependency 'androidx.core:core-ktx:1.3.2' is not satisfied. Microsoft maintains the NuGet package 'Xamarin.AndroidX.Core.Core.Ktx' that could fulfill this dependency.
error XA4242: Java dependency 'org.jetbrains.kotlin:kotlin-stdlib:1.5.10' is not satisfied. Microsoft maintains the NuGet package 'Xamarin.Kotlin.StdLib' that could fulfill this dependency.
```
5. After adding the full set of dependencies you should be able to deploy and run the application on an emulator or device. If you are attempting to only include a subset you may encounter a crash at runtime and likely need to return to the list from step 3 and start adding back any dependencies that were excluded.

The live Facebook sample code demonstrates the end result of this iterative process, and contains a trimmed down list of dependencies that were initially presented. These were the only dependencies required to light up the small code path I was trying to bind. This solution is not perfect and can still lead to unexpected behavior due to the complexity and size of some Maven artifacts.

We should continue to build out better documentation around these challenges and potential solutions. We're also hoping to continue to make improvements to this area of the .NET for Android binding experience in the future.

Please feel free to also use this issue to provide feedback on how this process interacts with Native Library Interop.

Start with the Facebook sample's build.gradle.kts and Sample.csproj, then read the linked Java Dependency Verification and Resolving Java Dependencies documentation. Build the sample to observe the XA4241 and XA4242 dependency output. Done means documenting how to declare Maven artifacts, evaluate reported dependencies, and validate deployment without ClassNotFoundExceptions.

索引モデルが issue の本文から書いたものです。