creating a line between two UIlabels using the long press gesture recognizer

Question

i am developing a ER diagram editor, i have a bunch of draggable UILabels but all of them have the same name. i want to be able to create a line between two UIlabels when both are pressed together using the long press gesture recognizer. any help will be most appreciated

Answer 1

You can create your long press gesture on the superview shared by these two labels, e.g.:

UILongPressGestureRecognizer *twoTouchLongPress = [[UILongPressGestureRecognizer alloc] initWithTarget:self
                                                                                                action:@selector(handleLongPress:)];
twoTouchLongPress.numberOfTouchesRequired = 2;
[self.view addGestureRecognizer:twoTouchLongPress];

You can then write a gesture handler:

- (void)handleLongPress:(UILongPressGestureRecognizer *)gesture
{
    if (gesture.state == UIGestureRecognizerStateBegan)
    {
        CGPoint location0 = [gesture locationOfTouch:0 inView:gesture.view];
        CGPoint location1 = [gesture locationOfTouch:1 inView:gesture.view];

        if ((CGRectContainsPoint(self.label0.frame, location0) && CGRectContainsPoint(self.label1.frame, location1)) ||
            (CGRectContainsPoint(self.label1.frame, location0) && CGRectContainsPoint(self.label0.frame, location1)))
        {
            NSLog(@"success; draw your line");
        }
        else
        {
            NSLog(@"failure; don't draw your line");
        }
    }
}

---

In the updated comments, you suggest that you're creating a local UILabel variable, and then adding the resulting label to the view. That's fine, but you really want to maintain a backing model, that captures what you're doing in the view. For simplicity's sake, let me assume that you'll have array of these labels, e.g.:

@property (nonatomic, strong) NSMutableArray *labels;

Which you then initialize at some point (e.g. viewDidLoad):

self.labels = [[NSMutableArray alloc] init];

Then as you add labels to your view, add a reference to them in your array:

UILabel *label = [[UILabel alloc]initWithFrame:CGRectMake(xVal, yVal, 200.0f, 60.0f)]; 
label.text = sentence; 
label.layer.borderColor = [UIColor blueColor].CGColor;
label.layer.borderWidth = 0.0; 
label.backgroundColor = [UIColor clearColor]; 
label.font = [UIFont systemFontOfSize:19.0f];
[self.view addSubview:label];

[self.labels addObject:label];

Then, your gesture can do something like:

- (UILabel *)labelForLocation:(CGPoint)location
{
    for (UILabel *label in self.labels)
    {
        if (CGRectContainsPoint(label.frame, location))
            return label;                                // if found one, return that `UILabel`
    }
    return nil;                                          // if not, return nil
}

- (void)handleLongPress:(UILongPressGestureRecognizer *)gesture
{
    if (gesture.state == UIGestureRecognizerStateBegan)
    {
        CGPoint location0 = [gesture locationOfTouch:0 inView:gesture.view];
        CGPoint location1 = [gesture locationOfTouch:1 inView:gesture.view];

        UILabel *label0 = [self labelForLocation:location0];
        UILabel *label1 = [self labelForLocation:location1];

        if (label0 != nil && label1 != nil && label0 != label1)
        {
            NSLog(@"success; draw your line");
        }
        else
        {
            NSLog(@"failure; don't draw your line");
        }
    }
}

Frankly, I'd rather see this backed by a proper model, but that's a more complicated conversation beyond the scope of a simple Stack Overflow answer. But hopefully the above gives you an idea of what it might look like. (BTW, I just typed in the above without assistance of Xcode, so I'll apologize in advance for typos.)