Setting A CGContext Transparent Background

I am still struggling with drawing a line with CGContext. I have actually go to line to draw, but now I need the background of the Rect to be transparent so the existing background shows thru. Here's my test code:

(void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context, [UIColor clearColor].CGColor);
    CGContextSetAlpha(context,0.0);
    CGContextFillRect(context, rect);
    
    CGContextSetStrokeColorWithColor(context, [UIColor whiteColor].CGColor);
    CGContextSetLineWidth(context, 5.0);
    CGContextMoveToPoint(context, 100.0,0.0);
    CGContextAddLineToPoint(context,100.0, 100.0);
    CGContextStrokePath(context);
}

Any ideas?

After UIGraphicsGetCurrentContext() call CGContextClearRect(context,rect)

Edit: Alright, got it.

Your custom view with the line should have the following:

- (id)initWithFrame:(CGRect)frame {
    if (self = [super initWithFrame:frame]) {
        [self setBackgroundColor:[UIColor clearColor]];
    }
    return self;
}

- (void)drawRect:(CGRect)rect {
    CGContextRef context = UIGraphicsGetCurrentContext();
	CGContextClearRect(context, rect);
	CGContextSetStrokeColorWithColor(context, [UIColor whiteColor].CGColor);
	CGContextSetLineWidth(context, 5.0);
	CGContextMoveToPoint(context, 100.0,0.0);
	CGContextAddLineToPoint(context,100.0, 100.0);
	CGContextStrokePath(context);
}

My test used this as a very basic UIViewController:

- (void)viewDidLoad {
    [super viewDidLoad];
	UIImageView *v = [[UIImageView alloc] initWithFrame:self.view.bounds];
	[v setBackgroundColor:[UIColor redColor]];
	[self.view addSubview:v];
	TopView *t = [[TopView alloc] initWithFrame:self.view.bounds];
	[self.view addSubview:t];
	[v release];
	[t release];
}

Easy way:

- (id)initWithFrame:(CGRect)frame
{
    if ((self = [super initWithFrame:frame]))
    {       
        self.opaque = NO;
    }
    return self;
}

- (void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextClearRect(context, rect);
    //your code
}

Init a context with opaque == false, Swift 3

UIGraphicsBeginImageContextWithOptions(size, false, UIScreen.main.scale) 

opaque > A Boolean flag indicating whether the bitmap is opaque. If you know the bitmap is fully opaque, specify true to ignore the alpha channel and optimize the bitmap’s storage. Specifying false means that the bitmap must include an alpha channel to handle any partially transparent pixels.

This is what worked for me with a UIImage which had been manually added using InterfaceBuilder.

- (id)initWithCoder:(NSCoder *)aDecoder {

    if(self = [super initWithCoder:aDecoder]) {
        self.backgroundColor = [UIColor clearColor];
    }
    
    return self;
}


-(void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();    
    CGContextSetStrokeColorWithColor(context, [UIColor whiteColor].CGColor);
    CGContextSetLineWidth(context, 5.0);
    CGContextMoveToPoint(context, 100.0,0.0);
    CGContextAddLineToPoint(context,100.0, 100.0);
    CGContextStrokePath(context);
}

David Kanarek's answer only works when you're manually creating your own UIImageView. If you've created a UIView and manually added it via Interface Builder then you will need a different approach like this calling the initWithCoder method instead.

I have the same problem, then I find it is. I overwrite the init Method is -(id)initWithFrame:(CGRect)rect. In this method self.background = [UIColor clearColor]; but i use this view in xib file !!! That will call the init method is

-(id)initWithCoder:(NSCoder*)aDecoder;

So overwrite all the init Method and setup BackgroundColor will work OK.

CGContextClearRect(context,rect) 

If the provided context is a window or bitmap context, Quartz effectively clears the rectangle. For other context types, Quartz fills the rectangle in a device-dependent manner. However, you should not use this function in contexts other than window or bitmap contexts.

you can create a image context with this code:

cacheContext = CGBitmapContextCreate (cacheBitmap, size.width, size.height, 8, bitmapBytesPerRow, CGColorSpaceCreateDeviceRGB(), kCGImageAlphaPremultipliedLast);
CGContextSetRGBFillColor(cacheContext, 0, 0, 0, 0);
CGContextFillRect(cacheContext, (CGRect){CGPointZero, size});

the key is kCGImageAlphaPremultipliedLast.

Having trouble understanding the question here, but if you're unable to have a "background" UIView show through a "top" view into which you're drawing, one solution is topView.backgroundColor = [UIColor clearColor]; I was having (I think) this same problem and this solved it for me.