Programming iOS 4

Drawing a UIView

The most flexible way to draw a UIView is to draw it yourself. Actually, you don’t draw a UIView; you subclass UIView and endow the subclass with the ability to draw itself. When a UIView needs drawing, its drawRect: method is called. Overriding that method is your chance to draw. At the time that drawRect: is called, the current graphics context has already been set to the view. You can use Core Graphics functions or UIKit convenience methods to draw into that context.

You should never call drawRect: yourself. If a view needs updating and you want its drawRect: called, send the view the setNeedsDisplay message. This will cause drawRect: to be called at the next proper moment.

Warning

If you subclass a built-in UIView subclass, don’t override drawRect: unless you are assured that this is legal. For example, it is not legal to override drawRect: in a subclass of UIImageView; you cannot combine your drawing with that of the UIImageView.

So let’s begin again. We’ll have a UIView subclass called MyView, in which we’ll do all our drawing. How this class gets instantiated, and how the instance gets into our view hierarchy, isn’t important. Here, I’ll do it in code as the app launches:

MyView* mv = [[MyView alloc] initWithFrame:
               CGRectMake(0, 0, self.window.bounds.size.width - 50, 150)];
mv.center = self.window.center;
[self.window addSubview: mv];
mv.opaque = NO;
[mv release];

The only really new thing here is that we set our UIView instance’s opaque property to NO. If we don’t ...