Made demo properly re-connect to cast sessions.

Note that this does not fully fix the problem; issue shaka-project#1948 means
that if the ui is configured more than once, we cannot cast at all.
Also, the cast receiver does not currently work; once a v2.5.1 version
is released that will no longer be an issue, but the v2.5.0 version
currently uploaded is missing goog.require calls.
I have manually verified that once those issues are fixed, though,
this will allow for the player to re-connect.

Issue shaka-project#1948

Change-Id: I26201cc696a61906fac3c320f9ac5e4b61fe1920

demo/main.js

@@ -163,7 +163,9 @@ class ShakaDemoMain {
     this.fullyLoaded_ = true;
     this.remakeHash();
 
-    if (asset) {
+    if (asset && !this.selectedAsset) {
+      // If an asset has begun loading in the meantime (for example, due to
+      // re-joining an existing cast session), don't play this.
       this.loadAsset(asset);
     }
   }
@@ -858,20 +860,31 @@ class ShakaDemoMain {
     this.remakeHash();
   }
 
+  /**
+   * Performs all visual operations that should be performed when a new asset
+   * begins playing. The video bar is un-hidden, the screen is scrolled, and so
+   * on.
+   *
+   * @private
+   */
+  showPlayer_() {
+    const videoBar = document.getElementById('video-bar');
+    this.showNode_(videoBar);
+    this.closeError_();
+    this.video_.poster = ShakaDemoMain.mainPoster_;
+
+    // Scroll to the top of the page, so that if the page is scrolled down,
+    // the user won't need to manually scroll up to see the video.
+    videoBar.scrollIntoView({behavior: 'smooth', block: 'start'});
+  }
+
   /**
    * @param {ShakaDemoAssetInfo} asset
    */
   async loadAsset(asset) {
     try {
       this.selectedAsset = asset;
-      const videoBar = document.getElementById('video-bar');
-      this.showNode_(videoBar);
-      this.closeError_();
-      this.video_.poster = ShakaDemoMain.mainPoster_;
-
-      // Scroll to the top of the page, so that if the page is scrolled down,
-      // the user won't need to manually scroll up to see the video.
-      videoBar.scrollIntoView({behavior: 'smooth', block: 'start'});
+      this.showPlayer_();
 
       // The currently-selected asset changed, so update asset cards.
       this.dispatchEventWithName_('shaka-main-selected-asset-changed');
@@ -1235,7 +1248,13 @@ class ShakaDemoMain {
    * @private
    */
   onCastStatusChange_(connected) {
-    // TODO: Handle.
+    if (connected && !this.selectedAsset) {
+      // You joined an existing session.
+      // The exact asset playing is unknown. Just have a selectedAsset, to show
+      // that this is playing something.
+      this.selectedAsset = ShakaDemoAssetInfo.makeBlankAsset();
+      this.showPlayer_();
+    }
   }
 }