MD Settings: Save and restore scroll position in iron-list

ui/webui/resources/cr_elements/cr_scrollable_behavior.js

Index: ui/webui/resources/cr_elements/cr_scrollable_behavior.js
diff --git a/ui/webui/resources/cr_elements/cr_scrollable_behavior.js b/ui/webui/resources/cr_elements/cr_scrollable_behavior.js
index 1858ccffd07cde6c343ae84479f8a9c344b99457..ba34a8fd78dcc5cc8240b161820667e4c08f8577 100644
--- a/ui/webui/resources/cr_elements/cr_scrollable_behavior.js
+++ b/ui/webui/resources/cr_elements/cr_scrollable_behavior.js
@@ -36,10 +36,17 @@
 
 /** @polymerBehavior */
 var CrScrollableBehavior = {
-  properties: {
-    /** @private {number|null} */
-    intervalId_: {type: Number, value: null}
-  },
+
+  /** @private {number|null} */
+  intervalId_: null,
+
+  /**
+   * A dictionary of integer arrays, one entry per scrollable contianer id.
+   * Each array keeps a queue of scroll positions. This ensures that multiple
+   * calls to save/restore scroll do not interfere with each other.
+   * @private {!Object<!Array<number>>}
+   */
+  saveScrollTop_: {},
 
   ready: function() {
     var scrollableElements = this.root.querySelectorAll('[scrollable]');
@@ -93,6 +100,25 @@ var CrScrollableBehavior = {
     }.bind(this), 10);
   },
 
+  /** @param {!IronListElement} list */
+  saveScroll: function(list) {
+    let target = list.scrollTarget;

[Dan Beam, 2017/02/01 06:13:17]

no let until we vulcanize

[stevenjb, 2017/02/02 01:17:47]

Oops, missed that. Will vulcanize be part of the build step so that these will
eventually be caught?

[Dan Beam, 2017/02/02 04:18:00]

yes

+    this.saveScrollTop_[target.id] = this.saveScrollTop_[target.id] || [];

[Dan Beam, 2017/02/01 06:13:17]

just save on the list itself, i.e.

list.prevScrollTops_ = list.prevScrollTops_ || [];
list.prevScrollTops_.push(list.scrollTarget.scrollTop);

list.scroll(0, list.prevScrollTops_.pop());

also, do we really need an array?  why?

[stevenjb, 2017/02/02 01:17:47]

The C++ programmer in me cringes at decorating another class with it's own
cruft, but yeah, that works too, and I guess is more Javascripty? It does
certainly simplify the code.

We need a fifo here because we may get multiple updates in a single frame, so we
want to apply the last non zero update. It is also important that we don't apply
a stale update, so we can't just always use the latest non-zero value since we
wouldn't know when to clear it. I'll add a comment.

+    this.saveScrollTop_[target.id].push(target.scrollTop);
+  },
+
+  /** @param {!IronListElement} list */
+  restoreScroll: function(list) {
+    let target = list.scrollTarget;
+    this.async(function() {
+      let scrollTop = this.saveScrollTop_[target.id].shift();
+      // Ignore scrollTop of 0 in case it was intermittent (we do not need to
+      // explicity scroll to 0).
+      if (scrollTop != 0)
+        list.scroll(0, scrollTop);
+    });
+  },
+
   /**
    * Event wrapper for updateScroll_.
    * @param {!Event} event