25. CLKImageProvider
• backgroundImageは必須(foregroundImageは任意)
• 画像はアルファ値のみを参照する
→ユーザがカラーを設定するため
WWDC2015 session 209 creating complication with ClockKit 00:12:14∼
So you can provide a background
image and you provide it as a template image, an image that only contains
alpha information and no color of its own.
It can be -- the pixels can
be whatever color you want, but we are only going to pay
attention to the alpha channel.