WIP: integrate onBackPressed flow as callback

This sets up a little JavaFlow wrapper so we can register callbacks on
the back press flow in Java-land.

Inspired by one of the answers in
https://stackoverflow.com/questions/60605176/kotlin-flows-java-interop-callback

Kotlin generates default interface instances, but only if
`-Xjvm-default=all` is set in the compiler flags. The Java IDE
would propose using a lambda, which would fail because the kotlin
compiler would not generate the right ABI without that flag.

app/build.gradle

@@ -88,6 +88,11 @@ android {
 
     kotlinOptions {
         jvmTarget = JavaVersion.VERSION_17
+        freeCompilerArgs += [
+                // Generate default method implementations for interfaces
+                // https://kotlinlang.org/docs/java-to-kotlin-interop.html#default-methods-in-interfaces
+                '-Xjvm-default=all'
+        ]
     }
 
     sourceSets {

app/src/main/java/org/schabi/newpipe/fragments/detail/VideoDetailFragment.java

@@ -103,6 +103,7 @@ import org.schabi.newpipe.util.Constants;
 import org.schabi.newpipe.util.DeviceUtils;
 import org.schabi.newpipe.util.ExtractorHelper;
 import org.schabi.newpipe.util.InfoCache;
+import org.schabi.newpipe.util.JavaFlow;
 import org.schabi.newpipe.util.ListHelper;
 import org.schabi.newpipe.util.Localization;
 import org.schabi.newpipe.util.NavigationHelper;
@@ -130,6 +131,7 @@ import io.reactivex.rxjava3.android.schedulers.AndroidSchedulers;
 import io.reactivex.rxjava3.disposables.CompositeDisposable;
 import io.reactivex.rxjava3.disposables.Disposable;
 import io.reactivex.rxjava3.schedulers.Schedulers;
+import kotlin.Unit;
 
 @UnstableApi
 @AndroidEntryPoint
@@ -311,6 +313,11 @@ public final class VideoDetailFragment
         newPlayerViewModel = new ViewModelProvider(this).get(NewPlayerViewModelImpl.class);
         newPlayerViewModel.setNewPlayer(this.newPlayer);
         newPlayerViewModel.setContentFitMode(ContentScale.FIT_INSIDE);
+        new JavaFlow<Unit>().collect(newPlayerViewModel.getOnBackPressed(), result -> {
+            Log.d(TAG, "NewPlayer forwarded backpress");
+            newPlayerViewModel.setNewPlayer(null);
+            newPlayer.pause();
+        });
 
         final SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(activity);
         showComments = prefs.getBoolean(getString(R.string.show_comments_key), true);

app/src/main/java/org/schabi/newpipe/util/JavaFlow.kt

@@ -0,0 +1,36 @@
+package org.schabi.newpipe.util
+
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.cancel
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.onCompletion
+import kotlinx.coroutines.flow.onStart
+import kotlinx.coroutines.launch
+
+class JavaFlow<T>(
+    private val coroutineScope: CoroutineScope = CoroutineScope(Dispatchers.Default)
+) {
+
+    interface OperatorCallback <T> {
+        fun onStart() = Unit
+        fun onCompletion(thr: Throwable?) = Unit
+        fun onResult(result: T)
+    }
+
+    fun collect(
+        flow: Flow<T>,
+        operatorCallback: OperatorCallback<T>,
+    ) {
+        coroutineScope.launch {
+            flow
+                .onStart { operatorCallback.onStart() }
+                .onCompletion { operatorCallback.onCompletion(it) }
+                .collect { operatorCallback.onResult(it) }
+        }
+    }
+
+    fun close() {
+        coroutineScope.cancel()
+    }
+}