Question

I’m trying to add an undo/redo function to an iOS app. I want to be able to draw several lines, then undo each one.. I can erase the whole thing, but that’s not good enough… I would really appreciate help as this is my first foray into using CG..

My .h declarations include:

CGPoint lastPoint;
NSMutableArray *pathArray;
UIBezierPath *myPath;

In .m, I have:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    NSLog(@"%s", __FUNCTION__);
    UITouch *touch = [touches anyObject];
    myPath=[[UIBezierPath alloc]init];
    lastPoint = [touch locationInView:self.view];
    [myPath moveToPoint:lastPoint];
    lastPoint.y -= 20;
    [pathArray addObject:myPath];
    NSLog(@"pathArray count is %i", [pathArray count]);

}


- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    NSLog(@"%s", __FUNCTION__);
    UITouch *touch = [touches anyObject];   
    CGPoint currentPoint = [touch locationInView:self.view];
    currentPoint.y -= 20;   
    UIGraphicsBeginImageContext(self.view.frame.size);
    [drawImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];

    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), brush);
    CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), red, green, blue, 1.0);
    CGContextBeginPath(UIGraphicsGetCurrentContext());
    CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
    CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
    CGContextStrokePath(UIGraphicsGetCurrentContext());

    drawImage.image = UIGraphicsGetImageFromCurrentImageContext();
    savedImage.image = drawImage.image;

    UIGraphicsEndImageContext();

    lastPoint = currentPoint;

}

At the end of touchesBegin, the pathArray count is always zero.. 🙁

To implement the undo, I’m using this code:

- (void)undoButtonTapped {
    NSLog(@"%s", __FUNCTION__);
    NSLog(@"pathArray count is %i", [pathArray count]);
    if([pathArray count]>0){
        UIBezierPath *_path=[pathArray lastObject];
        [bufferArray addObject:_path];
        [pathArray removeLastObject];
        [self.view setNeedsDisplay];
    }

}

count here is also zero..

All of this is being handled in a UIViewController. I would welcome any advice/improvements/suggestions.

Thanks

Answer 1

I’d say that the reason that [pathArray count] is zero in touchesBegan is that nowhere in your code do I see

pathArray = [[NSMutableArray alloc] init];

So you are sending messages to a null pointer (which is allowed but does nothing).

So are you allocating pathArray? Or is it null??