How to draw a rectangle?

Just for clarity:

If You need to draw a rectangle which has the same fill and border color, then You can replace:

CGContextSetRGBFillColor(context, 1.0, 0.0, 0.0, 1.0);
CGContextSetRGBStrokeColor(context, 1.0, 0.0, 0.0, 1.0);

with:

 [[UIColor redColor] set];

You can't do it in a viewController. You need to extend your View and add the code under "drawRect:"

this will change the drawing logic of your view.

-(void) drawRect:(CGRect)rect{    
[super drawRect:rect];  
    CGRect rectangle = CGRectMake(0, 100, 320, 100);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetRGBFillColor(context, 1.0, 0.0, 0.0, 1.0);
    CGContextSetRGBStrokeColor(context, 1.0, 0.0, 0.0, 1.0);
    CGContextFillRect(context, rectangle);
}

modern 2018 solution..

override func draw(_ rect: CGRect) {

    let r = CGRect(x: 5, y: 5, width: 10, height: 10)

    UIColor.yellow.set()
    UIRectFill(r)
}

that's it.