feat(Preload): Add detachAndSavePreload method

lib/player.js

@@ -1669,6 +1669,34 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
    */
   async unloadAndSavePreload(
       initializeMediaSource = true, keepAdManager = false) {
+    const preloadManager = await this.savePreload_();
+    await this.unload(initializeMediaSource, keepAdManager);
+    return preloadManager;
+  }
+
+  /**
+   * Detach the player from the current media element, if any, and returns a
+   * PreloadManager that contains the loaded manifest of that asset, if any.
+   * Allows for the asset to be re-loaded by this player faster, in the future.
+   * When in src= mode, this detach but does not make a PreloadManager.
+   * Leaves the player in a state where it cannot play media, until it has been
+   * attached to something else.
+   *
+   * @param {boolean=} keepAdManager
+   * @return {!Promise.<?shaka.media.PreloadManager>}
+   * @export
+   */
+  async detachAndSavePreload(keepAdManager = false) {
+    const preloadManager = await this.savePreload_();
+    await this.detach(keepAdManager);
+    return preloadManager;
+  }
+
+  /**
+   * @return {!Promise.<?shaka.media.PreloadManager>}
+   * @private
+   */
+  async savePreload_() {
     let preloadManager = null;
     if (this.manifest_ && this.parser_ && this.parserFactory_ &&
         this.assetUri_) {
@@ -1703,7 +1731,6 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
       this.abrManager_ = null;
       this.abrManagerFactory_ = null;
     }
-    await this.unload(initializeMediaSource, keepAdManager);
     return preloadManager;
   }
 

test/cast/cast_utils_unit.js

@@ -28,6 +28,7 @@ describe('CastUtils', () => {
       'setVideoContainer',
       'getActiveSessionsMetadata',
       'releaseAllMutexes', // Very specific to the inner workings of the player.
+      'detachAndSavePreload',
       'unloadAndSavePreload',
       'preload',
 

test/player_integration.js

@@ -1394,6 +1394,16 @@ describe('Player', () => {
     await waiter.waitUntilPlayheadReachesOrFailOnTimeout(video, 1, 10);
   });
 
+  it('detachAndSavePreload', async () => {
+    await player.load('test:sintel_compiled');
+    await video.play();
+    const preloadManager = await player.detachAndSavePreload();
+    await player.attach(video);
+    await player.load(preloadManager);
+    await video.play();
+    await waiter.waitUntilPlayheadReachesOrFailOnTimeout(video, 1, 10);
+  });
+
   it('unloadAndSavePreload', async () => {
     await player.load('test:sintel_compiled');
     await video.play();