add lazyload

docs/js/artplayer.js

@@ -1054,6 +1054,7 @@
 
 	    this.art = art;
 	    this.option = option;
+	    this.isLoad = false;
 	    this.init();
 	  }
 
@@ -1066,14 +1067,44 @@
 	          $progress = _this$art.refs.$progress,
 	          proxy = _this$art.events.proxy;
 	      proxy($progress, 'mousemove', function (event) {
-	        _this.option.ref.style.display = 'block';
+	        _this.checkLoad(_this.art.option.thumbnails.url).then(function () {
+	          _this.option.ref.style.display = 'block';
 
-	        _this.showThumbnails(event);
+	          _this.showThumbnails(event);
+	        });
 	      });
 	      proxy($progress, 'mouseout', function () {
 	        _this.option.ref.style.display = 'none';
 	      });
 	    }
+	  }, {
+	    key: "checkLoad",
+	    value: function checkLoad(url) {
+	      var _this2 = this;
+
+	      if (this.isLoad) {
+	        return Promise.resolve(url);
+	      }
+
+	      var proxy = this.art.events.proxy;
+	      return new Promise(function (resolve, reject) {
+	        var $img = new Image();
+	        $img.src = url;
+
+	        if ($img.complete) {
+	          _this2.isLoad = true;
+	          resolve(url);
+	        }
+
+	        proxy($img, 'load', function () {
+	          _this2.isLoad = true;
+	          resolve(url);
+	        });
+	        proxy($img, 'error', function () {
+	          reject(url);
+	        });
+	      });
+	    }
 	  }, {
 	    key: "showThumbnails",
 	    value: function showThumbnails(event) {
@@ -1632,13 +1663,9 @@
 	      }
 
 	      target.addEventListener(name, callback, option);
-
-	      var destroy = function destroy() {
+	      this.destroyEvents.push(function () {
 	        target.removeEventListener(name, callback, option);
-	      };
-
-	      this.destroyEvents.push(destroy);
-	      return destroy;
+	      });
 	    }
 	  }, {
 	    key: "destroy",