[cr-action-menu] Allow configurable anchors.

ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.js

 // Copyright 2016 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.
 
+/**
+ * @typedef {{
+ *   top: number,
+ *   left: number,
+ *   width: (number| undefined),
+ *   height: (number| undefined),
+ *   anchorConfigX: (number| undefined),
+ *   anchorConfigY: (number| undefined),
+ *   minX: (number| undefined),
+ *   minY: (number| undefined),
+ *   maxX: (number| undefined),
+ *   maxY: (number| undefined),
+ * }}
+ */
+var ShowConfig;
+
 Polymer({
   is: 'cr-action-menu',
   extends: 'dialog',
 
   /**
    * List of all options in this action menu.
    * @private {?NodeList<!Element>}
    */
   options_: null,
 
   /**
    * The element which the action menu will be anchored to. Also the element
-   * where focus will be returned after the menu is closed.
+   * where focus will be returned after the menu is closed. Only populated if
+   * menu is opened with showAt().
    * @private {?Element}
    */
   anchorElement_: null,
 
   /**
    * Bound reference to an event listener function such that it can be removed
    * on detach.
    * @private {?Function}
    */
   boundClose_: null,
@@ skipping 116 matching lines @@
     } while (!nextOption && counter < numOptions);
 
     return nextOption;
   },
 
   /** @override */
   close: function() {
     // Removing 'resize' and 'popstate' listeners when dialog is closed.
     this.removeListeners_();
     HTMLDialogElement.prototype.close.call(this);
-    this.anchorElement_.focus();
-    this.anchorElement_ = null;
+    if (this.anchorElement_) {
+      this.anchorElement_.focus();
+      this.anchorElement_ = null;
+    }
   },
 
   /**
    * Shows the menu anchored to the given element.
    * @param {!Element} anchorElement
    */
   showAt: function(anchorElement) {
     this.anchorElement_ = anchorElement;
+    this.anchorElement_.scrollIntoViewIfNeeded();
+    var rect = this.anchorElement_.getBoundingClientRect();
+    this.showAtPosition({
+      top: rect.top,
+      left: rect.left,
+      height: rect.height,
+      width: rect.width,
+      // Default to anchoring towards the left.
+      anchorConfigX: -1,
+    });
+  },
+
+  /**
+   * Shows the menu anchored to the given box. The anchor configuration is
+   * specified as an X and Y alignment which represents a point in the anchor
+   * and the menu that will be aligned where (0, 0) and (1, 1) are the center
+   * and bottom-right of each box respectively.
+   *
+   * For example, having the menu centered around the anchor box would use an
+   * anchor config of (0, 0), whereas aligning the menu to the top-right edge of
+   * the anchor would use a config of (1, -1).

[dpapad, 2017/04/25 17:42:43]

This explanation is much better than before. Can we include an ASCII graph
though to make it even better? For example:

x indicates the center point of the menu to be displayed.
(anchorConfigX, anchorConfigY) indicates the relative coordinates to the anchor
box.


 (-1, -1)   (0, -1)   (1, -1)
         x_____x_____x
         |           |
         |           |
         |   (0, 0)  |
 (-1, 0) x     x     x (1, 0)
         |           |
         |anchor box |
         |_____ _____|
        x      x      x
 (-1, 1)     (0, 1)    (1, 1)

[calamity, 2017/04/26 03:15:50]

Done.

+   *
+   * To align the menu outside the given box, use either a point or a line
+   * instead of the full box. e.g aligning a dropdown to center beneath a div
+   * at ((0, 0), (100, 100)) would use ((0, 100), (100, 100)) as the box and
+   * (0, 1) as the anchor, effectively using the bottom edge of the div as the
+   * anchor and centering horizontally beneath that.
+   *
+   * @param {!ShowConfig} config
+   */
+  showAtPosition: function(config) {
+    var c = Object.assign(this.getDefaultShowConfig_(), config);
+
+    var top = c.top;
+    var left = c.left;
+    var bottom = top + c.height;
+    var right = left + c.width;
+
     this.boundClose_ = this.boundClose_ || function() {
       if (this.open)
         this.close();
     }.bind(this);
     window.addEventListener('resize', this.boundClose_);
     window.addEventListener('popstate', this.boundClose_);
 
     // Reset position to prevent previous values from affecting layout.
     this.style.left = '';
     this.style.right = '';
     this.style.top = '';
 
-    this.anchorElement_.scrollIntoViewIfNeeded();
     this.showModal();
 
-    var rect = this.anchorElement_.getBoundingClientRect();
-    if (getComputedStyle(this.anchorElement_).direction == 'rtl') {
-      var right = window.innerWidth - rect.left - this.offsetWidth;
-      this.style.right = right + 'px';
+    // Flip the X anchor in RTL.
+    var rtl = getComputedStyle(this).direction == 'rtl';
+    if (rtl)
+      c.anchorConfigX *= -1;
+
+    var menuLeft = this.getStartPointWithAnchor_(
+        left, right, this.offsetWidth, c.anchorConfigX, c.minX, c.maxX);
+
+    if (rtl) {
+      var menuRight = window.innerWidth - menuLeft - this.offsetWidth;
+      this.style.right = menuRight + 'px';
     } else {
-      var left = rect.right - this.offsetWidth;
-      this.style.left = left + 'px';
+      this.style.left = menuLeft + 'px';
     }
 
-    // Attempt to show the menu starting from the top of the rectangle and
-    // extending downwards. If that does not fit within the window, fallback to
-    // starting from the bottom and extending upwards.
-    var top = rect.top + this.offsetHeight <= window.innerHeight ? rect.top :
-                                                                   rect.bottom -
-            this.offsetHeight - Math.max(rect.bottom - window.innerHeight, 0);
+    var menuTop = this.getStartPointWithAnchor_(
+        top, bottom, this.offsetHeight, c.anchorConfigY, c.minY, c.maxY);
+    this.style.top = menuTop + 'px';
+  },
 
-    this.style.top = top + 'px';
+  /**
+   * Returns the point to start along the X or Y axis given a start and end
+   * point to anchor to, the length of the target and the direction to anchor
+   * in. If honoring the anchor would force the menu outside of min/max, this
+   * will ignore the anchor position and try to keep the menu within min/max.
+   * @private
+   * @param {number} start
+   * @param {number} end
+   * @param {number} length
+   * @param {number} anchorConfig
+   * @param {number} min
+   * @param {number} max
+   * @return {number}
+   */
+  getStartPointWithAnchor_: function(
+      start, end, length, anchorConfig, min, max) {
+    var startPoint =
+        ((start + end - length) + (start - end + length) * anchorConfig) / 2;
+    if (startPoint + length > max)
+      startPoint = end - length;
+    if (startPoint < min)
+      startPoint = start;
+    return startPoint;
+  },
+
+
+  /**
+   * @private
+   * @return {!ShowConfig}
+   */
+  getDefaultShowConfig_: function() {
+    return {
+      top: 0,
+      left: 0,
+      height: 0,
+      width: 0,
+      anchorConfigX: 1,
+      anchorConfigY: 1,
+      minX: 0,
+      minY: 0,
+      maxX: window.innerWidth,
+      maxY: window.innerHeight,
+    };
   },
 });