[video_player] Move most Android event logic to Dart (#10128)

Rather than interpreting ExoPlayer event callbacks in Java and then sending event channel events that match the Dart platform interface, send the events directly and intepret them in Dart. This continues the process of reducing Java-side logic in the plugin.

In order to make this change less error-prone, it also adopts Pigeon event channels, so that the new events are type-safe; in order to do that, this switches to the Kotlin Pigeon generator, since Pigeon event channels haven't been implemented for Java.

Now that the buffer progress event synthesis is in Dart, this also fixes an existing TODO by reworking that logic to be based on an independent timer, rather than driven by playback position polling from the app-facing package. This both makes the logic clearer, and makes it work regardless of whether or not the video is paused.

Part of flutter/flutter#172763

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: stuartmorgan-g

packages/video_player/video_player_android/CHANGELOG.md

@@ -1,3 +1,10 @@
+## 2.8.17
+
+* Moves video event processing logic to Dart, and fixes an issue where buffer
+  range would not be updated for a paused video.
+* Switches to Kotlin for Pigeon-generated code.
+* Adopts type-safe event channels for internal communication.
+
 ## 2.8.16
 
 * Updates Java compatibility version to 17 and minimum supported SDK version to Flutter 3.35/Dart 3.9.

packages/video_player/video_player_android/android/build.gradle

@@ -2,13 +2,15 @@ group = 'io.flutter.plugins.videoplayer'
 version = '1.0-SNAPSHOT'
 
 buildscript {
+    ext.kotlin_version = '2.2.10'
     repositories {
         google()
         mavenCentral()
     }
 
     dependencies {
         classpath 'com.android.tools.build:gradle:8.12.1'
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
     }
 }
 
@@ -20,6 +22,7 @@ rootProject.allprojects {
 }
 
 apply plugin: 'com.android.library'
+apply plugin: 'kotlin-android'
 
 android {
     namespace = "io.flutter.plugins.videoplayer"
@@ -38,6 +41,13 @@ android {
         sourceCompatibility = JavaVersion.VERSION_17
         targetCompatibility = JavaVersion.VERSION_17
     }
+    kotlinOptions {
+        jvmTarget = JavaVersion.VERSION_17.toString()
+    }
+
+    sourceSets {
+        main.java.srcDirs += 'src/main/kotlin'
+    }
 
     dependencies {
         def exoplayer_version = "1.5.1"

packages/video_player/video_player_android/android/src/main/java/io/flutter/plugins/videoplayer/ExoPlayerEventListener.java

@@ -10,7 +10,6 @@
 import androidx.media3.exoplayer.ExoPlayer;
 
 public abstract class ExoPlayerEventListener implements Player.Listener {
-  private boolean isBuffering = false;
   private boolean isInitialized = false;
   protected final ExoPlayer exoPlayer;
   protected final VideoPlayerCallbacks events;
@@ -47,47 +46,34 @@ public ExoPlayerEventListener(
     this.events = events;
   }
 
-  private void setBuffering(boolean buffering) {
-    if (isBuffering == buffering) {
-      return;
-    }
-    isBuffering = buffering;
-    if (buffering) {
-      events.onBufferingStart();
-    } else {
-      events.onBufferingEnd();
-    }
-  }
-
   protected abstract void sendInitialized();
 
   @Override
   public void onPlaybackStateChanged(final int playbackState) {
+    PlatformPlaybackState platformState = PlatformPlaybackState.UNKNOWN;
     switch (playbackState) {
       case Player.STATE_BUFFERING:
-        setBuffering(true);
-        events.onBufferingUpdate(exoPlayer.getBufferedPosition());
+        platformState = PlatformPlaybackState.BUFFERING;
         break;
       case Player.STATE_READY:
+        platformState = PlatformPlaybackState.READY;
         if (!isInitialized) {
           isInitialized = true;
           sendInitialized();
         }
         break;
       case Player.STATE_ENDED:
-        events.onCompleted();
+        platformState = PlatformPlaybackState.ENDED;
         break;
       case Player.STATE_IDLE:
+        platformState = PlatformPlaybackState.IDLE;
         break;
     }
-    if (playbackState != Player.STATE_BUFFERING) {
-      setBuffering(false);
-    }
+    events.onPlaybackStateChanged(platformState);
   }
 
   @Override
   public void onPlayerError(@NonNull final PlaybackException error) {
-    setBuffering(false);
     if (error.errorCode == PlaybackException.ERROR_CODE_BEHIND_LIVE_WINDOW) {
       // See
       // https://exoplayer.dev/live-streaming.html#behindlivewindowexception-and-error_code_behind_live_window