UIView Drawing Cycle and ContentMode

UIView 的绘制模式是按需要(on-demand),当这个View第一次出现在屏幕上时,系统请求View绘制他的content,然后系统缓存该snapshot 

When a view first appears on
the screen, the system asks it to draw its content. The system captures a snapshot of this content and uses
that snapshot as the view’s visual representation. If you never change the view’s content, the view’s drawing
code may never be called again. The snapshot image is reused for most operations involving the view. If you
do change the content, you notify the system that the view has changed. The view then repeats the process
of drawing the view and capturing a snapshot of the new results.
When the contents of your view change, you do not redraw those changes directly. Instead, you invalidate the
view using either the setNeedsDisplay or setNeedsDisplayInRect: method. These methods tell the
system that the contents of the view changed and need to be redrawn at the next opportunity. The system
waits until the end of the current run loop before initiating any drawing operations. This delay gives you a
chance to invalidate multiple views, add or remove views from your hierarchy, hide views, resize views, and
reposition views all at once. All of the changes you make are then reflected at the same time