how to draw a picture in iPhone with Quartz2d

Question

I'm studying Quartz now and want to do a demo like this: when your finger moves on the iPhone screen, it shows the track in red color。 The code is like:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
UITouch *touch = [touches anyObject];
_firstPoint = [touch locationInView:self];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{
UITouch *touch = [touches anyObject];
_endPoint = [touch locationInView:self];
[self setNeedsDisplay];
_firstPoint = _endPoint;
 }

then

- (void)drawRect:(CGRect)rect {

// Drawing code.
CGContextRef _context = UIGraphicsGetCurrentContext();

CGContextSetRGBStrokeColor(_context, 1, 0, 0, 1);
CGContextMoveToPoint(_context, _firstPoint.x, _firstPoint.y);
CGContextAddLineToPoint(_context, _endPoint.x, _endPoint.y);

CGContextStrokePath(_context);
}

Here,_firstPoint and _endPoint are CGPoint to record positions. However, it doesn't show the track. I don't know what is the problem. Please give any tips.

Finally, I'd like to consultant whether it is right to fulfill such a kind of App.

thanks!

Answer 1

To your point about where the collection of points making up the lines is stored, it is not stored in this example.

EDITED

Yeah, to store them, I'd just add to a NSMutableArray.

something like

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    if (!_points) _points = [[NSMutableArray array] retain];
     UITouch *touch = [touches anyObject];
    [_points addObject:[NSValue valueWithCGPoint:[touch locationInView:self]];
}
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    [_points addObject:[NSValue valueWithCGPoint:[touch locationInView:self]];
    [self setNeedsDisplay];
 }

The setNeedsDisplay is going to invoke the drawRect that's where you use the points and your draw methods.