Variable Scope in Objective-C

Question

I'm getting started with Objective-C, and there is something about variables scope that I still didn't get. I searched about it, but I still couldn't catch what I'm doing wrong.

I'm trying to create a code that will give me the x,y difference between two point. The first NSLog inside the first IF shows the right value for pointWhereDragBegan.x and .y, but when I try to get the value of the pointWhereDragBegan in the second IF statement, the value I get for pointWhereDragBegan.x is -1.998683 and .y is 0.0.

I'm sure it is something really simple, I just can't catch my mistake.

- (void)drag:(UILongPressGestureRecognizer *)drag{

CGPoint pointWhereDragBegan;
if(drag.state == UIGestureRecognizerStateBegan){
    pointWhereDragBegan = [drag locationInView:self];        
    NSLog(@"Drag started at %f,%f",pointWhereDragBegan.x,pointWhereDragBegan.y);        
}

if(drag.state == UIGestureRecognizerStateEnded){
    CGPoint pointWhereDragEnded = [drag locationInView:self];  

    float xDragged = pointWhereDragEnded.x - pointWhereDragBegan.x;
    float yDragged = pointWhereDragEnded.y - pointWhereDragBegan.y;



    NSLog(@"Drag ended at %f,%f",pointWhereDragEnded.x,pointWhereDragEnded.y);
    NSLog(@"The user moved %f, %f",xDragged,yDragged);
}
}

You aren't logging pointWhereDragBegan in the second if statement. You're only logging pointWhereDragEnded and the results of the subtractions. — Peter Hosey
– Peter Hosey, Commented Apr 22, 2012 at 21:08
I mean't that in the case I logged pointWhereDragBegan in the second if, the values would be -1.998683 and 0. — rantunes
– rantunes, Commented Apr 22, 2012 at 21:13

Jeremy Roman · Accepted Answer · 2012-04-22 20:49:13Z

4

drag.state will never be simultaneously UIGestureRecognizerStateBegan and UIGestureRecognizerStateEnded. This method should be invoked twice: once in each state.

As a result, in order to fix your issue, you'll need to persist pointWhereDragBegan outside the method scope. For example, you might use an instance variable.

answered Apr 22, 2012 at 20:49

Jeremy Roman

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2 Comments

rantunes Over a year ago

Thank you for the answer, I could make it work creating an instance variable. I was expecting to have an way of doing it without an instance variable. I thought that as I declared the variable outside the IF block, it would reference the same variable anywhere inside the method scope.

Jeremy Roman Over a year ago

But only across one invocation. If the method is called a second time, values assigned in the first invocation are not preserved.

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