Programming iOS 6, 3rd Edition

Content Mode

A view that draws something within itself, as opposed to merely having a background color and subviews (as in the previous chapter), has content. This means that its contentMode property becomes important whenever the view is resized. As I mentioned earlier, the drawing system will avoid asking a view to redraw itself from scratch if possible; instead, it will use the cached result of the previous drawing operation (the bitmap backing store). So, if the view is resized, the system may simply stretch or shrink or reposition the cached drawing, if your contentMode setting instructs it to do so.

It’s a little tricky to illustrate this point, because I have to arrange for the view to be resized without also causing it to be redrawn (that is, without triggering a call to drawRect:). Here’s how I’ll do that. As the app starts up, I’ll create an instance of a UIView subclass that knows how to draw our arrow. Then I’ll use delayed performance to resize the instance after the window has shown and the interface has been initially displayed:

void (^resize) (void) = ^{
    CGRect f = mv.bounds; // mv is the MyView instance
    f.size.height *= 2;
    mv.bounds = f;
};
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, NSEC_PER_SEC);
dispatch_after(popTime, dispatch_get_main_queue(), resize);

We double the height of the view without causing drawRect: to be called. The result is that the view’s drawing appears at double its correct height. For example, if our view’s drawRect: code is the ...