Drawing with finger iOS - ios

Question

I'm writing application which user can draw line with his finger.
This is code:
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
NSLog(#"BEGAN");//TEST OK
UITouch* tap=[touches anyObject];
start_point=[tap locationInView:self];
}
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
NSLog(#"MOVED");//TEST OK
UITouch* tap=[touches anyObject];
current_point=[tap locationInView:self];
[self DrawLine:start_point end:current_point];
start_point=current_point;
}
-(void)DrawLine: (CGPoint)start end:(CGPoint)end
{
context= UIGraphicsGetCurrentContext();
CGColorSpaceRef space_color= CGColorSpaceCreateDeviceRGB();
CGFloat component[]={1.0,0.0,0.0,1};
CGColorRef color = CGColorCreate(space_color, component);
//draw line
CGContextSetLineWidth(context, 1);
CGContextSetStrokeColorWithColor(context, color);
CGContextMoveToPoint(context, start.x, start.y);
CGContextAddLineToPoint(context,end.x, end.y);
CGContextStrokePath(context);
}
My problem is when I draw line on screen but the line is not visible.
P.S I draw on main View of application

Answer 1

context= UIGraphicsGetCurrentContext();
You're calling UIGraphicsGetCurrentContext() from outside the drawRect: method. So it will return nil. Therefore the following functions try to draw on a context that is actually nil which obviously cannot work

Answer 2

As #Jorg mentioned, there is no current context outside of drawRect: method, so UIGraphicsGetCurrentContext() will return nil most probably.
You can use a CGLayerRef for drawing offscreen, while in your drawRect: method, you can draw the contents of the layer on your view.
First, you'll need to declare the layer as a member of your class, so in your #interface declare CGLayerRef _offscreenLayer;. You may also create a property for it, however, I'll use it directly in this example.
Somewhere in your init method:
CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(NULL, self.frame.size.width, self.frame.size.height, 8, 4 * self.frame.size.width, colorspace, (uint32_t)kCGImageAlphaPremultipliedFirst);
CGColorSpaceRelease(colorspace);
_offscreenLayer = CGLayerCreateWithContext(context, self.frame.size, NULL);
Now, let's handle the drawing:
-(void)DrawLine: (CGPoint)start end:(CGPoint)end
{
CGContextRef context = CGLayerGetContext(_offscreenLayer);
// ** draw your line using context defined above
[self setNeedsDisplay]; // or even better, use setNeedsDisplayInRect:, and compute the dirty rect using start and end points
}
-(void)drawRect:(CGRect)rect {
CGContextRef currentContext = UIGraphicsGetCurrentContext(); // this will work now, since we're in drawRect:
CGRect drawRect = CGRectMake(0, 0, self.bounds.size.width, self.bounds.size.height);
CGContextDrawLayerInRect(currentContext, drawRect, _offscreenLayer);
}
Note that you may need to make small changes in order for the code to work, but should give you some idea on how you can implement it.