在View/ImageVIew中显示Drawable
API
View.setBackground(Drawable)
ImageView.setImagDrawable(Drawable)
源码分析
View.mBackground在View.draw(Canvas)中绘制，调用Drawable.draw(Canvas)
ImageView.mDrawable在ImageView.onDraw(Canvas)中绘制，调用Drawable.draw(Canvas)
Drawable对View的更新操作的实现
API
Drawable.invalidateSelf
更新
Drawable.scheduleSelf(Runnable, long)
延时更新
Drawable.unscheduleSelf
取消延时更新
源码分析：委托模式
Drawable.Callback
接口，定义更新相关方法invalidateDrawable、scheduleDrawable、unscheduleDrawable
