path: root/mobile/android/exoplayer2/src/main/java/org/mozilla/thirdparty/com/google/android/exoplayer2/DefaultRenderersFactory.java

/*
 * Copyright (C) 2017 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 org.mozilla.thirdparty.com.google.android.exoplayer2;

import android.content.Context;
import android.media.MediaCodec;
import android.os.Handler;
import android.os.Looper;
import androidx.annotation.IntDef;
import androidx.annotation.Nullable;
import org.mozilla.thirdparty.com.google.android.exoplayer2.audio.AudioCapabilities;
import org.mozilla.thirdparty.com.google.android.exoplayer2.audio.AudioProcessor;
import org.mozilla.thirdparty.com.google.android.exoplayer2.audio.AudioRendererEventListener;
import org.mozilla.thirdparty.com.google.android.exoplayer2.audio.DefaultAudioSink;
import org.mozilla.thirdparty.com.google.android.exoplayer2.audio.MediaCodecAudioRenderer;
import org.mozilla.thirdparty.com.google.android.exoplayer2.drm.DrmSessionManager;
import org.mozilla.thirdparty.com.google.android.exoplayer2.drm.FrameworkMediaCrypto;
import org.mozilla.thirdparty.com.google.android.exoplayer2.mediacodec.MediaCodecSelector;
import org.mozilla.thirdparty.com.google.android.exoplayer2.metadata.MetadataOutput;
import org.mozilla.thirdparty.com.google.android.exoplayer2.metadata.MetadataRenderer;
import org.mozilla.thirdparty.com.google.android.exoplayer2.text.TextOutput;
import org.mozilla.thirdparty.com.google.android.exoplayer2.text.TextRenderer;
import org.mozilla.thirdparty.com.google.android.exoplayer2.trackselection.TrackSelector;
import org.mozilla.thirdparty.com.google.android.exoplayer2.util.Log;
import org.mozilla.thirdparty.com.google.android.exoplayer2.video.MediaCodecVideoRenderer;
import org.mozilla.thirdparty.com.google.android.exoplayer2.video.VideoRendererEventListener;
import org.mozilla.thirdparty.com.google.android.exoplayer2.video.spherical.CameraMotionRenderer;
import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Constructor;
import java.util.ArrayList;

/**
 * Default {@link RenderersFactory} implementation.
 */
public class DefaultRenderersFactory implements RenderersFactory {

  /**
   * The default maximum duration for which a video renderer can attempt to seamlessly join an
   * ongoing playback.
   */
  public static final long DEFAULT_ALLOWED_VIDEO_JOINING_TIME_MS = 5000;

  /**
   * Modes for using extension renderers. One of {@link #EXTENSION_RENDERER_MODE_OFF}, {@link
   * #EXTENSION_RENDERER_MODE_ON} or {@link #EXTENSION_RENDERER_MODE_PREFER}.
   */
  @Documented
  @Retention(RetentionPolicy.SOURCE)
  @IntDef({EXTENSION_RENDERER_MODE_OFF, EXTENSION_RENDERER_MODE_ON, EXTENSION_RENDERER_MODE_PREFER})
  public @interface ExtensionRendererMode {}
  /**
   * Do not allow use of extension renderers.
   */
  public static final int EXTENSION_RENDERER_MODE_OFF = 0;
  /**
   * Allow use of extension renderers. Extension renderers are indexed after core renderers of the
   * same type. A {@link TrackSelector} that prefers the first suitable renderer will therefore
   * prefer to use a core renderer to an extension renderer in the case that both are able to play
   * a given track.
   */
  public static final int EXTENSION_RENDERER_MODE_ON = 1;
  /**
   * Allow use of extension renderers. Extension renderers are indexed before core renderers of the
   * same type. A {@link TrackSelector} that prefers the first suitable renderer will therefore
   * prefer to use an extension renderer to a core renderer in the case that both are able to play
   * a given track.
   */
  public static final int EXTENSION_RENDERER_MODE_PREFER = 2;

  private static final String TAG = "DefaultRenderersFactory";

  protected static final int MAX_DROPPED_VIDEO_FRAME_COUNT_TO_NOTIFY = 50;

  private final Context context;
  @Nullable private DrmSessionManager<FrameworkMediaCrypto> drmSessionManager;
  @ExtensionRendererMode private int extensionRendererMode;
  private long allowedVideoJoiningTimeMs;
  private boolean playClearSamplesWithoutKeys;
  private boolean enableDecoderFallback;
  private MediaCodecSelector mediaCodecSelector;

  /** @param context A {@link Context}. */
  public DefaultRenderersFactory(Context context) {
    this.context = context;
    extensionRendererMode = EXTENSION_RENDERER_MODE_OFF;
    allowedVideoJoiningTimeMs = DEFAULT_ALLOWED_VIDEO_JOINING_TIME_MS;
    mediaCodecSelector = MediaCodecSelector.DEFAULT;
  }

  /**
   * @deprecated Use {@link #DefaultRenderersFactory(Context)} and pass {@link DrmSessionManager}
   *     directly to {@link SimpleExoPlayer.Builder}.
   */
  @Deprecated
  @SuppressWarnings("deprecation")
  public DefaultRenderersFactory(
      Context context, @Nullable DrmSessionManager<FrameworkMediaCrypto> drmSessionManager) {
    this(context, drmSessionManager, EXTENSION_RENDERER_MODE_OFF);
  }

  /**
   * @deprecated Use {@link #DefaultRenderersFactory(Context)} and {@link
   *     #setExtensionRendererMode(int)}.
   */
  @Deprecated
  @SuppressWarnings("deprecation")
  public DefaultRenderersFactory(
      Context context, @ExtensionRendererMode int extensionRendererMode) {
    this(context, extensionRendererMode, DEFAULT_ALLOWED_VIDEO_JOINING_TIME_MS);
  }

  /**
   * @deprecated Use {@link #DefaultRenderersFactory(Context)} and {@link
   *     #setExtensionRendererMode(int)}, and pass {@link DrmSessionManager} directly to {@link
   *     SimpleExoPlayer.Builder}.
   */
  @Deprecated
  @SuppressWarnings("deprecation")
  public DefaultRenderersFactory(
      Context context,
      @Nullable DrmSessionManager<FrameworkMediaCrypto> drmSessionManager,
      @ExtensionRendererMode int extensionRendererMode) {
    this(context, drmSessionManager, extensionRendererMode, DEFAULT_ALLOWED_VIDEO_JOINING_TIME_MS);
  }

  /**
   * @deprecated Use {@link #DefaultRenderersFactory(Context)}, {@link
   *     #setExtensionRendererMode(int)} and {@link #setAllowedVideoJoiningTimeMs(long)}.
   */
  @Deprecated
  @SuppressWarnings("deprecation")
  public DefaultRenderersFactory(
      Context context,
      @ExtensionRendererMode int extensionRendererMode,
      long allowedVideoJoiningTimeMs) {
    this(context, null, extensionRendererMode, allowedVideoJoiningTimeMs);
  }

  /**
   * @deprecated Use {@link #DefaultRenderersFactory(Context)}, {@link
   *     #setExtensionRendererMode(int)} and {@link #setAllowedVideoJoiningTimeMs(long)}, and pass
   *     {@link DrmSessionManager} directly to {@link SimpleExoPlayer.Builder}.
   */
  @Deprecated
  public DefaultRenderersFactory(
      Context context,
      @Nullable DrmSessionManager<FrameworkMediaCrypto> drmSessionManager,
      @ExtensionRendererMode int extensionRendererMode,
      long allowedVideoJoiningTimeMs) {
    this.context = context;
    this.extensionRendererMode = extensionRendererMode;
    this.allowedVideoJoiningTimeMs = allowedVideoJoiningTimeMs;
    this.drmSessionManager = drmSessionManager;
    mediaCodecSelector = MediaCodecSelector.DEFAULT;
  }

  /**
   * Sets the extension renderer mode, which determines if and how available extension renderers are
   * used. Note that extensions must be included in the application build for them to be considered
   * available.
   *
   * <p>The default value is {@link #EXTENSION_RENDERER_MODE_OFF}.
   *
   * @param extensionRendererMode The extension renderer mode.
   * @return This factory, for convenience.
   */
  public DefaultRenderersFactory setExtensionRendererMode(
      @ExtensionRendererMode int extensionRendererMode) {
    this.extensionRendererMode = extensionRendererMode;
    return this;
  }

  /**
   * Sets whether renderers are permitted to play clear regions of encrypted media prior to having
   * obtained the keys necessary to decrypt encrypted regions of the media. For encrypted media that
   * starts with a short clear region, this allows playback to begin in parallel with key
   * acquisition, which can reduce startup latency.
   *
   * <p>The default value is {@code false}.
   *
   * @param playClearSamplesWithoutKeys Whether renderers are permitted to play clear regions of
   *     encrypted media prior to having obtained the keys necessary to decrypt encrypted regions of
   *     the media.
   * @return This factory, for convenience.
   */
  public DefaultRenderersFactory setPlayClearSamplesWithoutKeys(
      boolean playClearSamplesWithoutKeys) {
    this.playClearSamplesWithoutKeys = playClearSamplesWithoutKeys;
    return this;
  }

  /**
   * Sets whether to enable fallback to lower-priority decoders if decoder initialization fails.
   * This may result in using a decoder that is less efficient or slower than the primary decoder.
   *
   * @param enableDecoderFallback Whether to enable fallback to lower-priority decoders if decoder
   *     initialization fails.
   * @return This factory, for convenience.
   */
  public DefaultRenderersFactory setEnableDecoderFallback(boolean enableDecoderFallback) {
    this.enableDecoderFallback = enableDecoderFallback;
    return this;
  }

  /**
   * Sets a {@link MediaCodecSelector} for use by {@link MediaCodec} based renderers.
   *
   * <p>The default value is {@link MediaCodecSelector#DEFAULT}.
   *
   * @param mediaCodecSelector The {@link MediaCodecSelector}.
   * @return This factory, for convenience.
   */
  public DefaultRenderersFactory setMediaCodecSelector(MediaCodecSelector mediaCodecSelector) {
    this.mediaCodecSelector = mediaCodecSelector;
    return this;
  }

  /**
   * Sets the maximum duration for which video renderers can attempt to seamlessly join an ongoing
   * playback.
   *
   * <p>The default value is {@link #DEFAULT_ALLOWED_VIDEO_JOINING_TIME_MS}.
   *
   * @param allowedVideoJoiningTimeMs The maximum duration for which video renderers can attempt to
   *     seamlessly join an ongoing playback, in milliseconds.
   * @return This factory, for convenience.
   */
  public DefaultRenderersFactory setAllowedVideoJoiningTimeMs(long allowedVideoJoiningTimeMs) {
    this.allowedVideoJoiningTimeMs = allowedVideoJoiningTimeMs;
    return this;
  }

  @Override
  public Renderer[] createRenderers(
      Handler eventHandler,
      VideoRendererEventListener videoRendererEventListener,
      AudioRendererEventListener audioRendererEventListener,
      TextOutput textRendererOutput,
      MetadataOutput metadataRendererOutput,
      @Nullable DrmSessionManager<FrameworkMediaCrypto> drmSessionManager) {
    if (drmSessionManager == null) {
      drmSessionManager = this.drmSessionManager;
    }
    ArrayList<Renderer> renderersList = new ArrayList<>();
    buildVideoRenderers(
        context,
        extensionRendererMode,
        mediaCodecSelector,
        drmSessionManager,
        playClearSamplesWithoutKeys,
        enableDecoderFallback,
        eventHandler,
        videoRendererEventListener,
        allowedVideoJoiningTimeMs,
        renderersList);
    buildAudioRenderers(
        context,
        extensionRendererMode,
        mediaCodecSelector,
        drmSessionManager,
        playClearSamplesWithoutKeys,
        enableDecoderFallback,
        buildAudioProcessors(),
        eventHandler,
        audioRendererEventListener,
        renderersList);
    buildTextRenderers(context, textRendererOutput, eventHandler.getLooper(),
        extensionRendererMode, renderersList);
    buildMetadataRenderers(context, metadataRendererOutput, eventHandler.getLooper(),
        extensionRendererMode, renderersList);
    buildCameraMotionRenderers(context, extensionRendererMode, renderersList);
    buildMiscellaneousRenderers(context, eventHandler, extensionRendererMode, renderersList);
    return renderersList.toArray(new Renderer[0]);
  }

  /**
   * Builds video renderers for use by the player.
   *
   * @param context The {@link Context} associated with the player.
   * @param extensionRendererMode The extension renderer mode.
   * @param mediaCodecSelector A decoder selector.
   * @param drmSessionManager An optional {@link DrmSessionManager}. May be null if the player will
   *     not be used for DRM protected playbacks.
   * @param playClearSamplesWithoutKeys Whether renderers are permitted to play clear regions of
   *     encrypted media prior to having obtained the keys necessary to decrypt encrypted regions of
   *     the media.
   * @param enableDecoderFallback Whether to enable fallback to lower-priority decoders if decoder
   *     initialization fails. This may result in using a decoder that is slower/less efficient than
   *     the primary decoder.
   * @param eventHandler A handler associated with the main thread's looper.
   * @param eventListener An event listener.
   * @param allowedVideoJoiningTimeMs The maximum duration for which video renderers can attempt to
   *     seamlessly join an ongoing playback, in milliseconds.
   * @param out An array to which the built renderers should be appended.
   */
  protected void buildVideoRenderers(
      Context context,
      @ExtensionRendererMode int extensionRendererMode,
      MediaCodecSelector mediaCodecSelector,
      @Nullable DrmSessionManager<FrameworkMediaCrypto> drmSessionManager,
      boolean playClearSamplesWithoutKeys,
      boolean enableDecoderFallback,
      Handler eventHandler,
      VideoRendererEventListener eventListener,
      long allowedVideoJoiningTimeMs,
      ArrayList<Renderer> out) {
    out.add(
        new MediaCodecVideoRenderer(
            context,
            mediaCodecSelector,
            allowedVideoJoiningTimeMs,
            drmSessionManager,
            playClearSamplesWithoutKeys,
            enableDecoderFallback,
            eventHandler,
            eventListener,
            MAX_DROPPED_VIDEO_FRAME_COUNT_TO_NOTIFY));

    if (extensionRendererMode == EXTENSION_RENDERER_MODE_OFF) {
      return;
    }
    int extensionRendererIndex = out.size();
    if (extensionRendererMode == EXTENSION_RENDERER_MODE_PREFER) {
      extensionRendererIndex--;
    }

    try {
      // Full class names used for constructor args so the LINT rule triggers if any of them move.
      // LINT.IfChange
      Class<?> clazz = Class.forName("org.mozilla.thirdparty.com.google.android.exoplayer2.ext.vp9.LibvpxVideoRenderer");
      Constructor<?> constructor =
          clazz.getConstructor(
              long.class,
              android.os.Handler.class,
              org.mozilla.thirdparty.com.google.android.exoplayer2.video.VideoRendererEventListener.class,
              int.class);
      // LINT.ThenChange(../../../../../../../proguard-rules.txt)
      Renderer renderer =
          (Renderer)
              constructor.newInstance(
                  allowedVideoJoiningTimeMs,
                  eventHandler,
                  eventListener,
                  MAX_DROPPED_VIDEO_FRAME_COUNT_TO_NOTIFY);
      out.add(extensionRendererIndex++, renderer);
      Log.i(TAG, "Loaded LibvpxVideoRenderer.");
    } catch (ClassNotFoundException e) {
      // Expected if the app was built without the extension.
    } catch (Exception e) {
      // The extension is present, but instantiation failed.
      throw new RuntimeException("Error instantiating VP9 extension", e);
    }

    try {
      // Full class names used for constructor args so the LINT rule triggers if any of them move.
      // LINT.IfChange
      Class<?> clazz = Class.forName("org.mozilla.thirdparty.com.google.android.exoplayer2.ext.av1.Libgav1VideoRenderer");
      Constructor<?> constructor =
          clazz.getConstructor(
              long.class,
              android.os.Handler.class,
              org.mozilla.thirdparty.com.google.android.exoplayer2.video.VideoRendererEventListener.class,
              int.class);
      // LINT.ThenChange(../../../../../../../proguard-rules.txt)
      Renderer renderer =
          (Renderer)
              constructor.newInstance(
                  allowedVideoJoiningTimeMs,
                  eventHandler,
                  eventListener,
                  MAX_DROPPED_VIDEO_FRAME_COUNT_TO_NOTIFY);
      out.add(extensionRendererIndex++, renderer);
      Log.i(TAG, "Loaded Libgav1VideoRenderer.");
    } catch (ClassNotFoundException e) {
      // Expected if the app was built without the extension.
    } catch (Exception e) {
      // The extension is present, but instantiation failed.
      throw new RuntimeException("Error instantiating AV1 extension", e);
    }
  }

  /**
   * Builds audio renderers for use by the player.
   *
   * @param context The {@link Context} associated with the player.
   * @param extensionRendererMode The extension renderer mode.
   * @param mediaCodecSelector A decoder selector.
   * @param drmSessionManager An optional {@link DrmSessionManager}. May be null if the player will
   *     not be used for DRM protected playbacks.
   * @param playClearSamplesWithoutKeys Whether renderers are permitted to play clear regions of
   *     encrypted media prior to having obtained the keys necessary to decrypt encrypted regions of
   *     the media.
   * @param enableDecoderFallback Whether to enable fallback to lower-priority decoders if decoder
   *     initialization fails. This may result in using a decoder that is slower/less efficient than
   *     the primary decoder.
   * @param audioProcessors An array of {@link AudioProcessor}s that will process PCM audio buffers
   *     before output. May be empty.
   * @param eventHandler A handler to use when invoking event listeners and outputs.
   * @param eventListener An event listener.
   * @param out An array to which the built renderers should be appended.
   */
  protected void buildAudioRenderers(
      Context context,
      @ExtensionRendererMode int extensionRendererMode,
      MediaCodecSelector mediaCodecSelector,
      @Nullable DrmSessionManager<FrameworkMediaCrypto> drmSessionManager,
      boolean playClearSamplesWithoutKeys,
      boolean enableDecoderFallback,
      AudioProcessor[] audioProcessors,
      Handler eventHandler,
      AudioRendererEventListener eventListener,
      ArrayList<Renderer> out) {
    out.add(
        new MediaCodecAudioRenderer(
            context,
            mediaCodecSelector,
            drmSessionManager,
            playClearSamplesWithoutKeys,
            enableDecoderFallback,
            eventHandler,
            eventListener,
            new DefaultAudioSink(AudioCapabilities.getCapabilities(context), audioProcessors)));

    if (extensionRendererMode == EXTENSION_RENDERER_MODE_OFF) {
      return;
    }
    int extensionRendererIndex = out.size();
    if (extensionRendererMode == EXTENSION_RENDERER_MODE_PREFER) {
      extensionRendererIndex--;
    }

    try {
      // Full class names used for constructor args so the LINT rule triggers if any of them move.
      // LINT.IfChange
      Class<?> clazz = Class.forName("org.mozilla.thirdparty.com.google.android.exoplayer2.ext.opus.LibopusAudioRenderer");
      Constructor<?> constructor =
          clazz.getConstructor(
              android.os.Handler.class,
              org.mozilla.thirdparty.com.google.android.exoplayer2.audio.AudioRendererEventListener.class,
              org.mozilla.thirdparty.com.google.android.exoplayer2.audio.AudioProcessor[].class);
      // LINT.ThenChange(../../../../../../../proguard-rules.txt)
      Renderer renderer =
          (Renderer) constructor.newInstance(eventHandler, eventListener, audioProcessors);
      out.add(extensionRendererIndex++, renderer);
      Log.i(TAG, "Loaded LibopusAudioRenderer.");
    } catch (ClassNotFoundException e) {
      // Expected if the app was built without the extension.
    } catch (Exception e) {
      // The extension is present, but instantiation failed.
      throw new RuntimeException("Error instantiating Opus extension", e);
    }

    try {
      // Full class names used for constructor args so the LINT rule triggers if any of them move.
      // LINT.IfChange
      Class<?> clazz = Class.forName("org.mozilla.thirdparty.com.google.android.exoplayer2.ext.flac.LibflacAudioRenderer");
      Constructor<?> constructor =
          clazz.getConstructor(
              android.os.Handler.class,
              org.mozilla.thirdparty.com.google.android.exoplayer2.audio.AudioRendererEventListener.class,
              org.mozilla.thirdparty.com.google.android.exoplayer2.audio.AudioProcessor[].class);
      // LINT.ThenChange(../../../../../../../proguard-rules.txt)
      Renderer renderer =
          (Renderer) constructor.newInstance(eventHandler, eventListener, audioProcessors);
      out.add(extensionRendererIndex++, renderer);
      Log.i(TAG, "Loaded LibflacAudioRenderer.");
    } catch (ClassNotFoundException e) {
      // Expected if the app was built without the extension.
    } catch (Exception e) {
      // The extension is present, but instantiation failed.
      throw new RuntimeException("Error instantiating FLAC extension", e);
    }

    try {
      // Full class names used for constructor args so the LINT rule triggers if any of them move.
      // LINT.IfChange
      Class<?> clazz =
          Class.forName("org.mozilla.thirdparty.com.google.android.exoplayer2.ext.ffmpeg.FfmpegAudioRenderer");
      Constructor<?> constructor =
          clazz.getConstructor(
              android.os.Handler.class,
              org.mozilla.thirdparty.com.google.android.exoplayer2.audio.AudioRendererEventListener.class,
              org.mozilla.thirdparty.com.google.android.exoplayer2.audio.AudioProcessor[].class);
      // LINT.ThenChange(../../../../../../../proguard-rules.txt)
      Renderer renderer =
          (Renderer) constructor.newInstance(eventHandler, eventListener, audioProcessors);
      out.add(extensionRendererIndex++, renderer);
      Log.i(TAG, "Loaded FfmpegAudioRenderer.");
    } catch (ClassNotFoundException e) {
      // Expected if the app was built without the extension.
    } catch (Exception e) {
      // The extension is present, but instantiation failed.
      throw new RuntimeException("Error instantiating FFmpeg extension", e);
    }
  }

  /**
   * Builds text renderers for use by the player.
   *
   * @param context The {@link Context} associated with the player.
   * @param output An output for the renderers.
   * @param outputLooper The looper associated with the thread on which the output should be called.
   * @param extensionRendererMode The extension renderer mode.
   * @param out An array to which the built renderers should be appended.
   */
  protected void buildTextRenderers(
      Context context,
      TextOutput output,
      Looper outputLooper,
      @ExtensionRendererMode int extensionRendererMode,
      ArrayList<Renderer> out) {
    out.add(new TextRenderer(output, outputLooper));
  }

  /**
   * Builds metadata renderers for use by the player.
   *
   * @param context The {@link Context} associated with the player.
   * @param output An output for the renderers.
   * @param outputLooper The looper associated with the thread on which the output should be called.
   * @param extensionRendererMode The extension renderer mode.
   * @param out An array to which the built renderers should be appended.
   */
  protected void buildMetadataRenderers(
      Context context,
      MetadataOutput output,
      Looper outputLooper,
      @ExtensionRendererMode int extensionRendererMode,
      ArrayList<Renderer> out) {
    out.add(new MetadataRenderer(output, outputLooper));
  }

  /**
   * Builds camera motion renderers for use by the player.
   *
   * @param context The {@link Context} associated with the player.
   * @param extensionRendererMode The extension renderer mode.
   * @param out An array to which the built renderers should be appended.
   */
  protected void buildCameraMotionRenderers(
      Context context, @ExtensionRendererMode int extensionRendererMode, ArrayList<Renderer> out) {
    out.add(new CameraMotionRenderer());
  }

  /**
   * Builds any miscellaneous renderers used by the player.
   *
   * @param context The {@link Context} associated with the player.
   * @param eventHandler A handler to use when invoking event listeners and outputs.
   * @param extensionRendererMode The extension renderer mode.
   * @param out An array to which the built renderers should be appended.
   */
  protected void buildMiscellaneousRenderers(Context context, Handler eventHandler,
      @ExtensionRendererMode int extensionRendererMode, ArrayList<Renderer> out) {
    // Do nothing.
  }

  /**
   * Builds an array of {@link AudioProcessor}s that will process PCM audio before output.
   */
  protected AudioProcessor[] buildAudioProcessors() {
    return new AudioProcessor[0];
  }

}