Extract MediaSourceEventDispatcher from MediaSourceEventListener

Update it to allow any listener class to be registered (and thus
require the caller of dispatch() to provide the type of listener
to call).

Maintain MediaSourceEventListener.EventDispatcher as a sub-class
for now so that all existing references continue to work. This
avoids creating a huge diff in one CL. The intention is to in-line
these incrementally.

This is pre-work for issue:#6765

PiperOrigin-RevId: 298818198

library/core/src/main/java/com/google/android/exoplayer2/util/MediaSourceEventDispatcher.java

+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.android.exoplayer2.util;
+
+import android.os.Handler;
+import android.os.Looper;
+import androidx.annotation.CheckResult;
+import androidx.annotation.Nullable;
+import com.google.android.exoplayer2.C;
+import com.google.android.exoplayer2.source.MediaSource.MediaPeriodId;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+/**
+ * Event dispatcher which forwards events to a list of registered listeners.
+ *
+ * <p>Adds the correct {@code windowIndex} and {@code mediaPeriodId} values (and {@code
+ * mediaTimeOffsetMs} if needed).
+ *
+ * <p>Allows listeners of any type to be registered, calls to {@link #dispatch} then provide the
+ * type of listener to forward to, which is used to filter the registered listeners.
+ */
+// TODO: Make this final when MediaSourceEventListener.EventDispatcher is deleted.
+public class MediaSourceEventDispatcher {
+
+  /**
+   * Functional interface to send an event with {@code windowIndex} and {@code mediaPeriodId}
+   * attached.
+   */
+  public interface EventWithPeriodId<T> {
+
+    /** Sends the event to a listener. */
+    void sendTo(T listener, int windowIndex, @Nullable MediaPeriodId mediaPeriodId);
+  }
+
+  /** The timeline window index reported with the events. */
+  public final int windowIndex;
+  /** The {@link MediaPeriodId} reported with the events. */
+  @Nullable public final MediaPeriodId mediaPeriodId;
+
+  // TODO: Make these private when MediaSourceEventListener.EventDispatcher is deleted.
+  protected final CopyOnWriteArrayList<ListenerAndHandler> listenerAndHandlers;
+  // TODO: Define exactly what this means, and check it's always set correctly.
+  protected final long mediaTimeOffsetMs;
+
+  /** Creates an event dispatcher. */
+  public MediaSourceEventDispatcher() {
+    this(
+        /* listenerAndHandlers= */ new CopyOnWriteArrayList<>(),
+        /* windowIndex= */ 0,
+        /* mediaPeriodId= */ null,
+        /* mediaTimeOffsetMs= */ 0);
+  }
+
+  protected MediaSourceEventDispatcher(
+      CopyOnWriteArrayList<ListenerAndHandler> listenerAndHandlers,
+      int windowIndex,
+      @Nullable MediaPeriodId mediaPeriodId,
+      long mediaTimeOffsetMs) {
+    this.listenerAndHandlers = listenerAndHandlers;
+    this.windowIndex = windowIndex;
+    this.mediaPeriodId = mediaPeriodId;
+    this.mediaTimeOffsetMs = mediaTimeOffsetMs;
+  }
+
+  /**
+   * Creates a view of the event dispatcher with pre-configured window index, media period id, and
+   * media time offset.
+   *
+   * @param windowIndex The timeline window index to be reported with the events.
+   * @param mediaPeriodId The {@link MediaPeriodId} to be reported with the events.
+   * @param mediaTimeOffsetMs The offset to be added to all media times, in milliseconds.
+   * @return A view of the event dispatcher with the pre-configured parameters.
+   */
+  @CheckResult
+  public MediaSourceEventDispatcher withParameters(
+      int windowIndex, @Nullable MediaPeriodId mediaPeriodId, long mediaTimeOffsetMs) {
+    return new MediaSourceEventDispatcher(
+        listenerAndHandlers, windowIndex, mediaPeriodId, mediaTimeOffsetMs);
+  }
+
+  /**
+   * Adds a listener to the event dispatcher.
+   *
+   * @param handler A handler on the which listener events will be posted.
+   * @param eventListener The listener to be added.
+   */
+  public void addEventListener(Handler handler, Object eventListener) {
+    Assertions.checkNotNull(handler);
+    Assertions.checkNotNull(eventListener);
+    listenerAndHandlers.add(new ListenerAndHandler(handler, eventListener));
+  }
+
+  /**
+   * Removes a listener from the event dispatcher.
+   *
+   * @param eventListener The listener to be removed.
+   */
+  public void removeEventListener(Object eventListener) {
+    for (ListenerAndHandler listenerAndHandler : listenerAndHandlers) {
+      if (listenerAndHandler.listener == eventListener) {
+        listenerAndHandlers.remove(listenerAndHandler);
+      }
+    }
+  }
+
+  /** Dispatches {@code event} to all registered listeners of type {@code listenerClass}. */
+  @SuppressWarnings("unchecked") // The cast is gated with listenerClass.isInstance()
+  public <T> void dispatch(EventWithPeriodId<T> event, Class<T> listenerClass) {
+    for (ListenerAndHandler listenerAndHandler : listenerAndHandlers) {
+      if (listenerClass.isInstance(listenerAndHandler.listener)) {
+        postOrRun(
+            listenerAndHandler.handler,
+            () -> event.sendTo((T) listenerAndHandler.listener, windowIndex, mediaPeriodId));
+      }
+    }
+  }
+
+  private static void postOrRun(Handler handler, Runnable runnable) {
+    if (handler.getLooper() == Looper.myLooper()) {
+      runnable.run();
+    } else {
+      handler.post(runnable);
+    }
+  }
+
+  public static long adjustMediaTime(long mediaTimeUs, long mediaTimeOffsetMs) {
+    long mediaTimeMs = C.usToMs(mediaTimeUs);
+    return mediaTimeMs == C.TIME_UNSET ? C.TIME_UNSET : mediaTimeOffsetMs + mediaTimeMs;
+  }
+
+  /** Container class for a {@link Handler} and {@code listener} object. */
+  protected static final class ListenerAndHandler {
+
+    public final Handler handler;
+    public final Object listener;
+
+    public ListenerAndHandler(Handler handler, Object listener) {
+      this.handler = handler;
+      this.listener = listener;
+    }
+  }
+}