Add middleware method to override currentSource()

src/js/player.js

@@ -3569,7 +3569,9 @@ class Player extends Component {
    *         The current source object
    */
   currentSource() {
-    return this.cache_.source || {};
+    const mw = this.middleware_;
+
+    return middleware.currentSource(this.cache_.source, mw) || this.cache_.source || {};
   }
 
   /**

src/js/tech/middleware.js

@@ -82,6 +82,10 @@ export function setSource(player, src, next) {
   player.setTimeout(() => setSourceHelper(src, middlewares[src.type], next, player), 1);
 }
 
+export function currentSource(src, middleware) {
+  return middleware.reduceRight(middlewareIterator('currentSource'), src);
+}
+
 /**
  * When the tech is set, passes the tech to each middleware's `setTech` method.
  *

test/unit/tech/middleware.test.js

@@ -536,3 +536,17 @@ QUnit.test('a middleware without a setSource gets chosen implicitly', function(a
 
   middleware.getMiddleware('video/foo').pop();
 });
+
+QUnit.test('middleware can override currentSource', function(assert) {
+  const middlewares = [
+    {
+      currentSource: (src) => {
+        return `foo${src}`;
+      }
+    }
+  ];
+
+  const result = middleware.currentSource('bar', middlewares);
+
+  assert.equal(result, 'foobar', 'value of currentSource() is overriden');
+});