MD Settings: Lazy load the contents of the "advanced" settings.

chrome/browser/resources/settings/controls/settings_idle_load.js

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/**
+ * @fileoverview
+ * settings-idle-load is a simple variant of dom-if designed for lazy
+ * loading and rendering of elements that are accessed imperatively. A URL is
+ * given that holds the elements to be loaded lazily.
+ */
+Polymer({
+  is: 'settings-idle-load',
+  extends: 'template',
+
+  behaviors: [Polymer.Templatizer],
+
+  properties: {
+    /**
+     * If specified, it will be loaded via an HTML import before stamping the
+     * template.
+     */
+    url: String,
+  },
+
+  /** @private {TemplatizerNode} */
+  child_: null,
+
+  /** @private {number} */
+  idleCallback_: 0,
+
+  /** @override */
+  attached: function() {
+    this.idleCallback_ = requestIdleCallback(this.get.bind(this));
+  },
+
+  /** @override */
+  detached: function() {
+    // No-op if callback already fired.
+    cancelIdleCallback(this.idleCallback_);
+  },
+
+  /**
+   * @return {!Promise<Element>} Child element which has been stamped into the
+   *     DOM tree.
+   */
+  get: function() {
+    if (this.loading_)
+      return this.loading_;
+
+    this.loading_ = new Promise(function(resolve, reject) {
+      this.importHref(this.url, function() {
+        assert(!this.ctor);
+        this.templatize(this);
+        assert(this.ctor);
+
+        var instance = this.stamp({});
+
+        assert(!this.child_);
+        this.child_ = instance.root.firstElementChild;
+
+        this.parentNode.insertBefore(instance.root, this);
+        resolve(this.child_);
+
+        this.fire('lazy-loaded');
+      }.bind(this), reject, true);
+    }.bind(this));
+
+    return this.loading_;
+  },
+
+  /**
+   * @param {string} prop
+   * @param {Object} value
+   */
+  _forwardParentProp: function(prop, value) {
+    if (this.child_)
+      this.child_._templateInstance[prop] = value;
+  },
+
+  /**
+   * @param {string} path
+   * @param {Object} value
+   */
+  _forwardParentPath: function(path, value) {
+    if (this.child_)
+      this.child_._templateInstance.notifyPath(path, value, true);
+  }
+});