UIGraphicsGetCurrentContext vs UIGraphicsBeginImageContext/UIGraphicsEndImageContext

Question

I am new to these parts of iOS API and here are some questions that are causing an infinite loop in my mind

1. Why does ..BeginImageContext have a size but ..GetCurrentContext does not have a size? If ..GetCurrentContext does not have a size, where does it draw? What are the bounds?

2. Why did they have to have two contexts, one for image and one for general graphics? Isn't an image context already a graphic context? What was the reason for the separation (I am trying to know what I don't know)

Answer 1

UIGraphicsGetCurrentContext() returns a reference to the current graphics context. It doesn't create one. This is important to remember because if you view it in that light, you see that it doesn't need a size parameter because the current context is just the size the graphics context was created with.

UIGraphicsBeginImageContext(aSize) is for creating graphics contexts at the UIKit level outside of UIView's drawRect: method.

Here is where you would use them.

If you had a subclass of UIView you could override its drawRect: method like so:

- (void)drawRect:(CGRect)rect
{
    //the graphics context was created for you by UIView
    //you can now perform your custom drawing below

    //this gets you the current graphic context
    CGContextRef ctx = UIGraphicsGetCurrentContext();

    //set the fill color to blue
    CGContextSetFillColorWithColor(ctx, [UIColor blueColor].CGColor);

    //fill your custom view with a blue rect
    CGContextFillRect(ctx, rect);
}

In this case, you didn't need to create the graphics context. It was created for you automatically and allows you to perform your custom drawing in the drawRect: method.

Now, in another situation, you might want to perform some custom drawing outside of the drawRect: method. Here you would use UIGraphicsBeginImageContext(aSize)

You could do something like this:

UIBezierPath *circle = [UIBezierPath
                        bezierPathWithOvalInRect:CGRectMake(0, 0, 200, 200)];  

UIGraphicsBeginImageContext(CGSizeMake(200, 200));

//this gets the graphic context
CGContextRef context = UIGraphicsGetCurrentContext();

//you can stroke and/or fill
CGContextSetStrokeColorWithColor(context, [UIColor blueColor].CGColor);
CGContextSetFillColorWithColor(context, [UIColor lightGrayColor].CGColor);
[circle fill];
[circle stroke];

//now get the image from the context
UIImage *bezierImage = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

UIImageView *bezierImageView = [[UIImageView alloc]initWithImage:bezierImage];

I hope this helps to clear things up for you. Also, you should be using UIGraphicsBeginImageContextWithOptions(size, opaque, scale). For further explanation of custom drawing with graphics contexts, see my answer here