fix: wait for source before preloading the entry (#45)

Following kaltura/playkit-js#197 playing an ad can be before sourceselected event, so we have to consider this use case when ima preloads the source

src/ima-middleware.js

@@ -44,9 +44,7 @@ export default class ImaMiddleware extends BaseMiddleware {
    */
   play(next: Function): void {
     if (!this._isPlayerLoaded) {
-      this._context.player.load();
-      this._isPlayerLoaded = true;
-      this._context.logger.debug("Player loaded");
+      this._loadPlayer();
     }
     this._context.loadPromise.then(() => {
       let sm = this._context.getStateMachine();
@@ -107,4 +105,22 @@ export default class ImaMiddleware extends BaseMiddleware {
       }
     }
   }
+
+  /**
+   * Load the player.
+   * @returns {void}
+   * @private
+   */
+  _loadPlayer(): void {
+    const loadPlayer = () => {
+      this._context.logger.debug("Load player by ima middleware");
+      this._context.player.load();
+      this._isPlayerLoaded = true;
+    };
+    if (this._context.player.engineType) { // player has source to play
+      loadPlayer();
+    } else {
+      this._context.player.addEventListener(this._context.player.Event.SOURCE_SELECTED, () => loadPlayer());
+    }
+  }
 }