[MD settings] idle load basic and advanced pages

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

Index: chrome/browser/resources/settings/controls/settings_idle_render.js
diff --git a/chrome/browser/resources/settings/controls/settings_idle_render.js b/chrome/browser/resources/settings/controls/settings_idle_render.js
new file mode 100644
index 0000000000000000000000000000000000000000..f19c11d31b576dabfca3b803217b747396550d88
--- /dev/null
+++ b/chrome/browser/resources/settings/controls/settings_idle_render.js
@@ -0,0 +1,76 @@
+// 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-render is a simple variant of dom-if designed for lazy
+ * rendering of elements that are accessed imperatively.
+ * If a use for idle time expansion is found outside of settings, this code
+ * should be replaced by cr-lazy-render after that feature is merged into
+ * ui/webui/resources/cr_elements/cr_lazy_render/cr_lazy_render.js
+ */
+
+Polymer({
+  is: 'settings-idle-render',
+  extends: 'template',
+
+  behaviors: [Polymer.Templatizer],
+
+  /** @private {TemplatizerNode} */
+  child_: null,
+
+  /** @private {number} */
+  idleCallback_: 0,
+
+  /** @override */
+  attached: function() {
+    this.idleCallback_ = requestIdleCallback(this.render_.bind(this));

[dpapad, 2017/02/01 00:30:04]

In both tests we are calling get() explicitly. Is the case where the
requestIdleCallback takes effect tested somehow (directly or indirectly)?

[dschuyler, 2017/02/01 02:28:15]

Added browser tests.

Done.

+  },
+
+  /** @override */
+  detached: function() {
+    if (this.idleCallback_ != 0)
+      cancelIdleCallback(this.idleCallback_);

[dpapad, 2017/02/01 00:30:04]

By the time this runs, the callback might have already executed, so canceling
could be a no-op. Should we do the following instead?

if (this.idleCallback != 0 && !this.child_)
  cancelIdleCallback(this.idleCallback_);

I would also further argue that the framework guarantees that attached() is
called before detached() so the first part of the check could be omitted.

[dschuyler, 2017/02/01 02:28:15]

After chatting offline, we reasoned that no conditional at all was a good
choice.
I included the comment as requested: // No-op if callback already fired.

+  },
+
+  /**
+   * Stamp the template into the DOM tree synchronously
+   * @return {Element} Child element which has been stamped into the DOM tree.
+   */
+  get: function() {
+    if (!this.child_)
+      this.render_();
+    return this.child_;
+  },
+
+  /** @private */
+  render_: function() {
+    if (!this.ctor)
+      this.templatize(this);
+    var parentNode = this.parentNode;
+    if (parentNode && !this.child_) {
+      var instance = this.stamp({});
+      this.child_ = instance.root.firstElementChild;
+      parentNode.insertBefore(instance.root, this);
+    }
+  },
+
+  /**
+   * @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);
+  }
+});