Add a basic sky-scrollable element that scrolls

sky/framework/sky-scrollable.sky

+<!--
+// Copyright 2015 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.
+-->
+<import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement" />
+
+<sky-element
+  name="sky-scrollable"
+  on-gesturescrollstart="handleScrollStart_"
+  on-gesturescrollend="handleScrollEnd_"
+  on-gesturescrollupdate="handleScrollUpdate_"
+  on-gestureflingstart="handleFlingStart_"
+  on-gestureflingcancel="handleFlingCancel_">
+<template>
+  <style>
+  :host {
+    overflow: hidden;
+  }
+  #scrollable {
+    transform: translateY(0);
+  }
+  </style>
+  <div id="scrollable">
+    <content />
+  </div>
+</template>
+<script>
+// TODO(abarth): Move the fling curve to a separate module.
+var kFlingFriction = 0.9;
+var kFlingVelocityMin = 1;
+
+module.exports = class extends SkyElement {
+  created() {
+    this.scrollable_ = null;
+    this.scrollOffset_ = 0;
+    this.flingVelocity_ = 0;
+    this.flingAnimationId_ = null;
+  }
+
+  shadowRootReady() {
+    this.scrollable_ = this.shadowRoot.getElementById('scrollable');
+  }
+
+  scrollBy(scrollDelta) {
+    var offset = Math.max(0, Math.min(this.scrollable_.offsetHeight, this.scrollOffset_ + scrollDelta));
+    if (offset == this.scrollOffset_)
+      return false;
+    this.scrollOffset_ = offset;
+    this.applyScrollOffset_();
+    return true;
+  }
+
+  applyScrollOffset_() {
+    var transform = 'translateY(' + -this.scrollOffset_.toFixed(2) + 'px)';
+    this.scrollable_.style.transform = transform;
+  }
+
+  scheduleFlingTick_() {
+    this.flingAnimationId_ = requestAnimationFrame(this.tickFling_.bind(this));
+  }
+
+  tickFling_() {
+    this.flingAnimationId_ = null;
+    if (!this.scrollBy(this.flingVelocity_)) {
+      this.flingVelocity_ = 0;
+      return;
+    }
+    var velocity = this.flingVelocity_ * kFlingFriction;
+    if (velocity < kFlingVelocityMin)
+      velocity = 0;
+    this.flingVelocity_ = velocity;
+    if (velocity)
+      this.scheduleFlingTick_();
+  }
+
+  handleScrollStart_(event) {
+  }
+
+  handleScrollEnd_(event) {
+  }
+
+  handleScrollUpdate_(event) {
+    this.scrollBy(-event.dy);
+  }
+
+  handleFlingStart_(event) {
+    this.flingVelocity_ = -event.velocityY;
+    this.scheduleFlingTick_();
+  }
+
+  handleFlingCancel_(event) {
+    if (!this.flingAnimationId_)
+      return;
+    cancelAnimationFrame(this.flingAnimationId_);
+    this.flingVelocity_ = 0;
+    this.flingAnimationId_ = null;
+  }
+}.register();
+</script>
+</sky-element>