Question

I’m trying to build some iOS apps while learning and am having some trouble understanding the proper way to do this.

What I have currently is a view that is a subclass of UIView. It is clear and I want to use it as a drawing surface. It will sit on top of something else, like tracing paper.

The user should be able to click on one spot then another spot and a line should draw between the 2 points. I’m getting the touch data, I have the points, and I am able to draw stuff from inside of drawRect: initially.

The problem is I’m not sure how to update stuff later. When everything loads up and drawRect: is calle, it will draw a line just fine. But how do I make it draw new stuff or alter stuff that is already drawn based on what the user is doing. I know I need to call setNeedsDisplay, but not how to get the data to the view to draw stuff.

I’ve read a bunch of tutorials/examples and they all stop at “Override drawRect: and draw some stuff… done!”. How do I pass data down in to the view to tell it “hey, redraw this stuff and add this new line”.

Or am I going about this all the wrong way?

EDIT:
I’ll try to explain better the setup I have.

I have a VC. Inside this VC’s view I have a toolbar at the bottom. The rest of the area is taken up by 2 views. One is an image view that holds a reference image. One is the custom view that is clear (tracing paper) that sits at the top. They click a button on the toolbar which turns on a gesturerecognizer. They click on the screen, and I collect the tap data, turn off the gesturerecognizer and HOPEFULLY draw a line. I’ve got it all working except the drawing part.

Answer 1

You are on the right track. Sounds like you need to keep track of the points.

Here’s some example code.

LineDrawView.h

#import <UIKit/UIKit.h>
@interface LineDrawView : UIView 
{
    NSMutableArray *lines;
    CGPoint pointA, pointB;
    BOOL activeLine;
}
@end

LineDrawView.m

#import "LineDrawView.h"
@implementation LineDrawView
- (id)initWithFrame:(CGRect)frame 
{
    self = [super initWithFrame:frame];
    if (self) 
    {
        self.backgroundColor = [UIColor blackColor];
        lines = [[NSMutableArray alloc] init];
    }
    return self;
}
- (void)dealloc 
{
    [lines release];
    [super dealloc];
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesBegan:touches withEvent:event];
    CGPoint point = [[touches anyObject] locationInView:self];
    if ([lines count] == 0) pointA = point;
    else pointB = point;
    activeLine = YES;
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesMoved:touches withEvent:event];
    pointB = [[touches anyObject] locationInView:self];
    [self setNeedsDisplay];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesEnded:touches withEvent:event];
    pointB = [[touches anyObject] locationInView:self];
    [lines addObject:[NSArray arrayWithObjects:[NSValue valueWithCGPoint:pointA], [NSValue valueWithCGPoint:pointB], nil]];
    pointA = pointB;
    pointB = CGPointZero;
    activeLine = NO;
    [self setNeedsDisplay];
}

- (void)drawRect:(CGRect)rect
{
    CGContextRef c = UIGraphicsGetCurrentContext();
    CGContextSetLineWidth(c, 2);
    CGContextSetLineCap(c, kCGLineCapRound);
    CGContextSetLineJoin(c, kCGLineJoinRound);
    CGContextSetStrokeColorWithColor(c, [UIColor grayColor].CGColor);
    for (NSArray *line in lines)
    {
        CGPoint points[2] = { [[line objectAtIndex:0] CGPointValue], [[line objectAtIndex:1] CGPointValue] };
        CGContextAddLines(c, points, 2);
    }
    CGContextStrokePath(c);
    if (activeLine)
    {
        CGContextSetStrokeColorWithColor(c, [UIColor whiteColor].CGColor);
        CGPoint points2[2] = { pointA, pointB };
        CGContextAddLines(c, points2, 2);
        CGContextStrokePath(c);
    }
}
@end