在CGContext上绘制图像而不使用initWithFrame

Question

I currently have an UIImage object which gets initialized with initWithFrame and then I have some 2D drawings in -(void)drawRect:(CGRect)rect{} with `CGContext. Very basic. All I want to know is, how can I draw through the push of eg a button or any event (not only on initialization)?

I tried copying the code inside drawRect to a new method called redraw but this results in an obvious error:

CGContextMoveToPoint: invalid context 0x0

I only want to know how to be able to draw through CGContext while the UIView has been created. Not eg how to listen for an UIButton event

Thank you.

Answer 1

You can only draw inside a UIView's drawRect: method or by creating your own image context. You cannot draw to the screen by any means other than drawRect: or implementing drawing for a CALayer. You need to set variables on your view accordingly, then call setNeedsDisplay to have it redrawn. For example, add a property @property(retain) UIImage *image; to your view, set the new image on it and then call setNeedsDisplay (or even better, implement the setImage: method and call [self setNeedsDisplay]; here).