Make the stock app demo list view prettier

sky/examples/stocks/stock-arrow.sky

Index: sky/examples/stocks/stock-arrow.sky
diff --git a/sky/examples/stocks/stock-arrow.sky b/sky/examples/stocks/stock-arrow.sky
new file mode 100644
index 0000000000000000000000000000000000000000..0ebe9cfba3fcc5fdba3039231dd0219f524f1003
--- /dev/null
+++ b/sky/examples/stocks/stock-arrow.sky
@@ -0,0 +1,98 @@
+<!--
+// 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" />
+
+<sky-element attributes="change:number">
+<template>
+  <style>
+  :host {
+    width: 40px;
+    height: 40px;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    border-radius: 40px;
+    border: 1px solid transparent;
+  }
+  #arrow {
+    width: 0;
+    height: 0;
+    border-left: 9px solid transparent;
+    border-right: 9px solid transparent;
+  }
+  .up {
+    margin-bottom: 3px;
+    border-bottom: 9px solid white;
+  }
+  .down {
+    margin-top: 3px;
+    border-top: 9px solid white;
+  }
+  </style>
+  <div id="arrow" />
+</template>
+<script>
+import "dart:sky";
+import "dart:math";
+
+final List<String> _kRedColors = [
+  '#E57373',
+  '#EF5350',
+  '#F44336',
+  '#E53935',
+  '#D32F2F',
+  '#C62828',
+  '#B71C1C',
+];
+
+final List<String> _kGreenColors = [
+  '#81C784',
+  '#66BB6A',
+  '#4CAF50',
+  '#43A047',
+  '#388E3C',
+  '#2E7D32',
+  '#1B5E20',
+];
+
+int _colorIndexForPercentChange(double percentChange) {
+  // Currently the max is 10%.
+  double maxPercent = 10.0;
+  return max(0, ((percentChange.abs() / maxPercent) * _kGreenColors.length).floor());
+}
+
+String _colorForPercentChange(double percentChange) {
+  if (percentChange > 0)
+    return _kGreenColors[_colorIndexForPercentChange(percentChange)];
+  return _kRedColors[_colorIndexForPercentChange(percentChange)];
+}
+
+@Tagname('stock-arrow')
+class StockArrow extends SkyElement {
+  Element _arrow;
+
+  void _updateArrow(double percentChange) {
+    String border = _colorForPercentChange(percentChange).toString();
+    String type = percentChange > 0 ? 'bottom' : 'top';
+    _arrow.style['border-$type-color'] = border;
+    style['border-color'] = border;
+    _arrow.setAttribute('class', percentChange > 0 ? 'up' : 'down');
+  }
+
+  void shadowRootReady() {
+    _arrow = shadowRoot.getElementById('arrow');
+    _updateArrow(change);
+  }
+
+  void changeChanged(double oldValue, double newValue) {
+    if (_arrow != null)
+      _updateArrow(newValue);
+  }
+}
+
+_init(script) => register(script, StockArrow);
+</script>
+</sky-element>