✨ amp-video-iframe integration: global jwplayer() singleton by default (

ampproject#26969)

`jwplayer()` will return the singleton instance, which makes sense as a default for a single iframed video.

Otherwise passed in explicitly like in: https://glitch.com/edit/#!/amp-video-iframe-jwplayer?path=jwplayer.html

extensions/amp-video-iframe/amp-video-iframe.md

@@ -250,15 +250,29 @@ You can additionally use [custom integration methods](#custom-integrations) if y
 
 **Default supported methods:** `pause`/`play`, `mute`/`unmute`, `hidecontrols`/`showcontrols`, `fullscreenenter`/`fullscreenexit`
 
-Pass in your [`jwplayer` instance object](https://developer.jwplayer.com/jw-player/docs/javascript-api-reference/)
-through the signature `ampIntegration.listenTo('jwplayer', myJwplayer)`. The `ampIntegration` object then knows how
-to setup the player through the instance API.
+The `amp` object knows how to setup a JwPlayer instance by using `listenTo('jwplayer')`.
+If you're embedding your player [using a video-specific script](https://support.jwplayer.com/articles/how-to-embed-a-jwplayer), you only need to register Jwplayer usage:
+
+```html
+<script src="https://cdn.jwplayer.com/players/UVQWMA4o-kGWxh33Q.js"></script>
+<script>
+  (window.AmpVideoIframe = window.AmpVideoIframe || []).push(function(
+    ampIntegration
+  ) {
+    ampIntegration.listenTo('jwplayer');
+  });
+</script>
+```
+
+Otherwise, pass in your [JwPlayer instance](https://developer.jwplayer.com/jwplayer/docs/jw8-javascript-api-reference)
+through the signature `amp.listenTo('jwplayer', instance)`:
 
 ```js
-function onAmpIntegrationReady(ampIntegration) {
-  var myJwplayer = jwplayer('my-video');
-  ampIntegration.listenTo('jwplayer', myJwplayer);
-}
+(window.AmpVideoIframe = window.AmpVideoIframe || []).push(function(
+  ampIntegration
+) {
+  ampIntegration.listenTo('jwplayer', jwplayer('my-video'));
+});
 ```
 
 ##### For Video.js

src/video-iframe-integration.js

@@ -190,30 +190,32 @@ export class AmpVideoIntegration {
 
   /**
    * @param {string} type
-   * @param {*} obj
+   * @param {*=} opt_obj
    * @param {function()=} opt_initializer For VideoJS, this optionally takes a
-   *    reference to the `videojs` function. If not provided, this reference
-   *    will be taken from the `window` object.
+   * reference to the `videojs` function. If not provided, this reference
+   * will be taken from the `window` object.
    */
-  listenTo(type, obj, opt_initializer) {
+  listenTo(type, opt_obj, opt_initializer) {
     userAssert(
       !this.usedListenToHelper_,
       '`listenTo` is meant to be used once per page.'
     );
     const types = {
       'jwplayer': () => {
-        userAssert(
-          obj.on,
-          'jwplayer integration takes a jwplayer instance as first argument.'
-        );
         userAssert(
           !opt_initializer,
-          'jwplayer integration does not take an initializer.'
+          "listenTo('jwplayer', opt_instance) does not take an initializer."
         );
-        this.listenToJwPlayer_(obj);
+        this.listenToJwPlayer_(this.getJwplayer_(opt_obj));
       },
       'videojs': () => {
-        this.listenToVideoJs_(obj, opt_initializer);
+        this.listenToVideoJs_(
+          userAssert(
+            opt_obj,
+            "listenTo('videojs', element) expects a second argument"
+          ),
+          opt_initializer
+        );
       },
     };
     userAssert(
@@ -224,6 +226,26 @@ export class AmpVideoIntegration {
     this.usedListenToHelper_ = true;
   }
 
+  /**
+   * Checks comformity for opt_player, or obtains global singleton instance.
+   * @param {?JwplayerPartialInterfaceDef=} opt_player
+   * @return {!JwplayerPartialInterfaceDef}
+   */
+  getJwplayer_(opt_player) {
+    if (opt_player) {
+      userAssert(
+        opt_player.on,
+        "listenTo('jwplayer', myjwplayer) takes a jwplayer instance as ",
+        'second argument'
+      );
+      return opt_player;
+    }
+    return userAssert(
+      this.win_.jwplayer,
+      "listenTo('jwplayer') expects a global jwplayer() in window."
+    )(); // notice the call here ;)
+  }
+
   /**
    * @param {!JwplayerPartialInterfaceDef} player
    * @private

test/unit/test-video-iframe-integration.js

@@ -223,6 +223,13 @@ describes.realWin('video-iframe-integration', {amp: false}, env => {
         });
       });
 
+      it('uses global jwplayer() to get instance when not passed in', () => {
+        const instance = {on: env.sandbox.spy()};
+        env.win.jwplayer = env.sandbox.stub().returns(instance);
+        new AmpVideoIntegration(env.win).listenTo('jwplayer');
+        expect(instance.on).to.have.been.called;
+      });
+
       function mockVideoJsPlayer() {
         return {
           ready(fn) {