iOS10 today extension 填坑

1. Height
   The height of the widget in iOS 10 is exactly 110 in compact mode. It can be set to whatever height you want in expanded mode, but in compact mode it will always be 110 and that can't be overwritten.

2. iOS10 today extension 打开时，先闪现空界面，然后绘制UI
   按照Apple文档的介绍，today extension应该显示历史截图，而不应该先为空，这样就不会有闪现的效果。原因是没有正确理解lifecycle，不应在viewWillAppear中刷新数据，应该在widgetPerformUpdateWithCompletionHandler中更新数据。调用顺序：viewDidLoad->widgetPerformUpdateWithCompletionHandler(调用completionHandler(NCUpdateResultNewData)表明数据已更新)->viewWillAppear。

Today Extension view flashes when redrawing