Question

I have a basic map view in my app which contains a set of user definable way points. When the view is loaded I draw a path connecting the way points. This works well.

However, when the user drags a way point around the view, I would like it to re-draw the paths and that is my problem. I dont know how to hacant get it to draw anything after the first time. Here’s my code:

- (void)drawRect:(CGRect)rect { ////// works perfectly

    context = UIGraphicsGetCurrentContext();

    CGContextSetRGBStrokeColor(context, 1, 0, 1, .7);
    CGContextSetLineWidth(context, 20.0);
    WaypointViewController *w = [arrayOfWaypoints objectAtIndex:0];
    CGPoint startPoint = w.view.center;

    CGContextMoveToPoint(context, startPoint.x, startPoint.y);

    for (int i = 1; i<[arrayOfWaypoints count]; i++) {
        WaypointViewController *w2 = [arrayOfWaypoints objectAtIndex:i];
        CGPoint nextPoint = w2.view.center;
        CGContextAddLineToPoint(context,nextPoint.x, nextPoint.y);
    }
    CGContextStrokePath(context);
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    if (moving) {
        UITouch *touch = [touches anyObject];
        currentWaypoint.view.center = [touch locationInView:self];
        [delegate setUserInteraction:NO];
        [self drawInContext:context];
        [NSThread detachNewThreadSelector:@selector(drawInContext:) toTarget:self withObject:nil];

    }

}

- (void)drawInContext:(CGContextRef)context { ///gets called, but does nothing

    CGContextSetRGBStrokeColor(context, 1, 0, 1, .7);
    CGContextSetLineWidth(context, 20.0);
    WaypointViewController *w = [arrayOfWaypoints objectAtIndex:0];
    CGPoint startPoint = w.view.center;

    CGContextMoveToPoint(context, startPoint.x, startPoint.y);

    for (int i = 1; i<[arrayOfWaypoints count]; i++) {
        WaypointViewController *w2 = [arrayOfWaypoints objectAtIndex:i];
        CGPoint nextPoint = w2.view.center;
        CGContextAddLineToPoint(context,nextPoint.x, nextPoint.y);
    }

    CGContextStrokePath(context);
}

Answer 1

You can not use the context that is active in drawRect: anywhere else.

Instead of calling [self drawInContext:context] call [self setNeedsDisplay]

The context that you are pointing to in your drawRect: might be freed or might still be active you don’t know, but either way there is no way to get the bits you put into that context onto the screen.

Also, please read the docs on drawing on iOS here