downloads: break downloads.js into more classes/files.

chrome/browser/resources/downloads/item.js

+// Copyright 2014 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.
+
+cr.define('downloads', function() {
+  /** @constructor */
+  function Item() {}
+
+  /**
+   * The states a download can be in. These correspond to states defined in
+   * DownloadsDOMHandler::CreateDownloadItemValue
+   * @enum {string}
+   */
+  Item.States = {
+    IN_PROGRESS: 'IN_PROGRESS',
+    CANCELLED: 'CANCELLED',
+    COMPLETE: 'COMPLETE',
+    PAUSED: 'PAUSED',
+    DANGEROUS: 'DANGEROUS',
+    INTERRUPTED: 'INTERRUPTED',
+  };
+
+  /**
+   * Explains why a download is in DANGEROUS state.
+   * @enum {string}
+   */
+  Item.DangerType = {
+    NOT_DANGEROUS: 'NOT_DANGEROUS',
+    DANGEROUS_FILE: 'DANGEROUS_FILE',
+    DANGEROUS_URL: 'DANGEROUS_URL',
+    DANGEROUS_CONTENT: 'DANGEROUS_CONTENT',
+    UNCOMMON_CONTENT: 'UNCOMMON_CONTENT',
+    DANGEROUS_HOST: 'DANGEROUS_HOST',
+    POTENTIALLY_UNWANTED: 'POTENTIALLY_UNWANTED',
+  };
+
+  Item.prototype = {
+    /**
+     * @param {!downloads.Data} data Info about the download.
+     */
+    render: function(data) {
+      /** @private {!downloads.ItemView} */
+      this.view = this.view || new downloads.ItemView;
+      this.view.update(data);
+    },
+
+    unrender: function() {
+      if (this.view) {
+        this.view.destroy();
+        delete this.view;

[asanka, 2015/03/10 04:15:51]

seems 'this.view = null' is preferred?

[Dan Beam, 2015/03/10 05:41:02]

eh, i've seen things that talk about delete not being optimized but i haven't
actually seen much evidence of this...

the difference is that `delete this.view` actually clears the enumerable
property, so

  >> this.view = null
  >> 'view' in this
  true
  >> delete this.view;
  >> 'view' in this
  false

anyways, moved to null

[asanka, 2015/03/11 19:57:49]

Cool. I'm no expert on this, and I'm okay with either. I was pointing out the
style guide preference.

+      }
+    },
+  };
+
+  return {Item: Item};
+});