Can't use TabLayout.IOnTabSelectedListener2

[PureWeen]

Xamarin.Android Version (eg: 6.0):

Tried with 9.0 and 10.0

Operating System & Version (eg: Mac OSX 10.11):

Mac OSX 10.15.3

Support Libraries Version (eg: 23.3.0):

Android X only

Describe your Issue:

If you implement the following interface

TabLayout.IOnTabSelectedListener2

You'll get the following exception

/Users/shane/Projects/AxTabError/obj/Debug/android/src/crc64f10a0ade51b9bb29/MainActivity.java(8,8): Error JAVAC0000:  error: BaseOnTabSelectedListener cannot be inherited with different arguments: <com.google.android.material.tabs.TabLayout.Tab> and <>
public class MainActivity
 (JAVAC0000) (AxTabError) javac

I first noticed this error when updating to Google.Android.Material 1.1.0-rc2 because on that version TabLayout.IOnTabSelectedListener is marked obsolete so I implemented TabLayout.IOnTabSelectedListener2

But this same exception happens on 1.0.0-preview02

Steps to Reproduce (with link to sample solution if possible):

AxTabError.zip

[jonpryor]

The shortened version the Java code that is being compiled is:

interface GenericIface<T extends Runnable> {
    public void m (T value);
}

interface ExtendedRunnable extends Runnable {
}

interface SpecificIface extends GenericIface<ExtendedRunnable> {
}

class NonGenericExtends implements SpecificIface, GenericIface {
    public void m (ExtendedRunnable value) {
    }
}

though in the app's specific case it's:

public class MainActivity
	extends androidx.appcompat.app.AppCompatActivity
	implements
		mono.android.IGCUserPeer,
		com.google.android.material.tabs.TabLayout.OnTabSelectedListener,
		com.google.android.material.tabs.TabLayout.BaseOnTabSelectedListener
{

The problem here is that TabLayout.OnTabSelectedListener extends TabLayout.BaseOnTabSelectedListener<TabLayout.Tab>, and as the Java Callable Wrapper attempts to implement both interfaces, even though TabLayout.OnTabSelectedListener extends TabLayout.BaseOnTabSelectedListener, the result doesn't compile.

We will need to update the Java Callable Wrapper generator to only emit TabLayout.OnTabSelectedListener in this case.

(Related: why is the Java Callable Wrapper generator emitting both interface types in the first place, when the C# code is class MainActivity : AppCompatActivity, TabLayout.IOnTabSelectedListener2?)

[jonpryor]

A possible workaround is to bypass this bug in the Java Callable Wrapper generator by binding a new Java class which has the appropriate inheritance hierarchy:

package jcw_kludge;

public class MainAppCompatActivity extends androidx.appcompat.app.AppCompatActivity
    implements com.google.android.material.tabs.TabLayout.OnTabSelectedListener
{
    public void onTabSelected(com.google.android.material.tabs.TabLayout.Tab tab) {}
    public void onTabUnselected(com.google.android.material.tabs.TabLayout.Tab tab) {}
    public void onTabReselected(com.google.android.material.tabs.TabLayout.Tab tab) {}
}

Compile jcw_kludge.MainAppCompatActivity into a new .jar file, bind the .jar file, and then the C# class can do:

public class MainActivity : JcwKludge.MainAppCompatActivity {
    // ...
}

This should in turn result in a Java Callable Wrapper for MainActivity which compiles.

[jonpryor]

Possible Workaround number 2 -- only sketched out here -- doesn't require an intermediate .jar file, but does require the same jcw_kludge.MainAppCompatActivity Java class.

Step 1: Add a MainAppCompatActivity.java to the .csproj, with a Build action of @(AndroidJavaSource).

Step 2: Contents of MainAppCompatActivity.java:

package jcw_kludge;

public class MainAppCompatActivity extends androidx.appcompat.app.AppCompatActivity
    implements com.google.android.material.tabs.TabLayout.OnTabSelectedListener
{
    public void onTabSelected(com.google.android.material.tabs.TabLayout.Tab tab) {}
    public void onTabUnselected(com.google.android.material.tabs.TabLayout.Tab tab) {}
    public void onTabReselected(com.google.android.material.tabs.TabLayout.Tab tab) {}
}

Then in C#-land, "hand-bind" this type

    [Register ("jcw_kludge. MainAppCompatActivity", DoNotGenerateAcw=true)]
    public class AppCompatActivityKludge : AppCompatActivity, TabLayout.IOnTabSelectedListener2 {

        [Register ("onTabReselected", "()V", "")]
        public virtual void OnTabReselected(TabLayout.Tab tab)
        {
            throw new NotImplementedException();
        }

        public virtual void OnTabSelected(TabLayout.Tab tab)
        {
            throw new NotImplementedException();
        }

        public virtual void OnTabUnselected(TabLayout.Tab tab)
        {
            throw new NotImplementedException();
        }
    }

Then the C# MainActivity class can inherit from AppCompatActivityKludge.

Note: This solution is incomplete. In order to fully work, AppCompatActivityKludge must be fully "hand-written", including proper [Register] values so that the methods can be overridden.

Binding the .jar is likely to be easier.

[jonpryor]

The bug is here:

https://github.com/xamarin/java.interop/blob/56955d9ad3952070de3bb1718375b368437f7427/src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers/JavaCallableWrapperGenerator.cs#L547-L550

The problem is that we iterate over all C# interfaces implemented by the type (e.g. MainActivity), and this iteration returns both TabLayout.IOnTabSelectedListener2 and TabLayout.IOnTabSelectedListener, even though one implements the other.

The fix is to filter out interfaces "already implemented" by another interface found in the iteration.