fix: Respect existing app usage of Cast SDK (#4523)

Closes #4521

AUTHORS

@@ -82,3 +82,4 @@ ViacomCBS <*@viacomcbs.com>
 Vincent Valot <valot.vince@gmail.com>
 Wayne Morgan <wayne.morgan.dev@gmail.com>
 Raymond Cheng <raycheng100@gmail.com>
+Blue Billywig <*@bluebillywig.com>

CONTRIBUTORS

@@ -120,3 +120,4 @@ Vincent Valot <valot.vince@gmail.com>
 Wayne Morgan <wayne.morgan.dev@gmail.com>
 Yohann Connell <robinconnell@google.com>
 Raymond Cheng <raycheng100@gmail.com>
+Janroel Koppen <j.koppen@bluebillywig.com>

lib/cast/cast_sender.js

@@ -186,21 +186,14 @@ shaka.cast.CastSender = class {
       // If the API becomes available before this instance dies, init will be
       // called again.
 
-      // A note about this value: In our testing environment, we load both
-      // uncompiled and compiled code.  This global callback in uncompiled mode
-      // can be overwritten by the same in compiled mode.  The two versions will
-      // each have their own instances_ map.  Therefore the callback must have a
-      // name, as opposed to being anonymous.  This way, the CastSender tests
-      // can invoke the named static method instead of using a global that could
-      // be overwritten.
-      if (!window.__onGCastApiAvailable) {
-        window.__onGCastApiAvailable = shaka.cast.CastSender.onSdkLoaded_;
-      }
-      if (window.__onGCastApiAvailable != shaka.cast.CastSender.onSdkLoaded_) {
-        shaka.log.alwaysWarn('A global Cast SDK hook is already installed! ' +
-          'Shaka Player will be unable to receive a notification when the ' +
-          'Cast SDK is ready.');
+      // Check if our callback is already installed.
+      if (window.__onGCastApiAvailable !== CastSender.onGCastApiAvailable_) {
+        // Save pre-existing __onGCastApiAvailable in order to restore later.
+        CastSender.__onGCastApiAvailable_ =
+          window.__onGCastApiAvailable || null;
+        window.__onGCastApiAvailable = CastSender.onGCastApiAvailable_;
       }
+
       return;
     }
 
@@ -747,6 +740,9 @@ shaka.cast.CastSender.hasReceivers_ = false;
 /** @private {chrome.cast.Session} */
 shaka.cast.CastSender.session_ = null;
 
+/** @private {?function(boolean)} */
+shaka.cast.CastSender.__onGCastApiAvailable_ = null;
+
 /**
  * A set of all living CastSender instances.  The constructor and destroy
  * methods will add and remove instances from this set.
@@ -774,3 +770,32 @@ shaka.cast.CastSender.onSdkLoaded_ = (loaded) => {
     }
   }
 };
+
+/**
+ * @param {boolean} available
+ * @private
+ */
+shaka.cast.CastSender.onGCastApiAvailable_ = (available) => {
+  // Restore callback from saved.
+  if (shaka.cast.CastSender.__onGCastApiAvailable_) {
+    window.__onGCastApiAvailable =
+      shaka.cast.CastSender.__onGCastApiAvailable_;
+  } else {
+    delete window.__onGCastApiAvailable;
+  }
+  shaka.cast.CastSender.__onGCastApiAvailable_ = null;
+
+  // A note about this value: In our testing environment, we load both
+  // uncompiled and compiled code.  This global callback in uncompiled mode
+  // can be overwritten by the same in compiled mode.  The two versions will
+  // each have their own instances_ map.  Therefore the callback must have a
+  // name, as opposed to being anonymous.  This way, the CastSender tests
+  // can invoke the named static method instead of using a global that could
+  // be overwritten.
+  shaka.cast.CastSender.onSdkLoaded_(available);
+
+  // call restored callback (if any)
+  if (typeof window.__onGCastApiAvailable === 'function') {
+    window.__onGCastApiAvailable(available);
+  }
+};