Changing a view's bounds doesn't change where it draws

Question

In viewDidLoad, I create a UIView at 100,100 with a size of 200x200, then change it's bounds to have an origin of -100,-100. I had supposed that this would either move the entire view back to the top left, or only draw a smaller rectangle for the intersection of the frame and bounds. It didn't affect the drawn-into area at all.

- (void)viewDidLoad
{
    [super viewDidLoad];

    CGRect rect = CGRectMake(100, 100, 200, 200);
    greenView = [[UIView alloc] initWithFrame:rect];
    [greenView setBackgroundColor:[UIColor greenColor]];

    CGRect bounds = CGRectMake(-100, -100, 200, 200);
    [greenView setBounds:bounds];
    [greenView setClipsToBounds:YES];
    [greenView setContentMode:UIViewContentModeRedraw];

    [self.view addSubview:greenView];
}

The frame and bounds log as follows:

Frame: {{100, 100}, {200, 200}}
Bounds: {{-100, -100}, {200, 200}}

I thought that a view only draws into it's bounds rect. What am I missing about views and bounds? Why doesn't changing my view's bounds change where it draws?

Here's what it looks like:

enter image description here

@rmaddy I know that's what I _should_ do, but the purpose here isn't to move the view, it's to understand more about views and bounds. — nevan king, Nov 04 '13 at 15:58

score 2 · Accepted Answer · answered Nov 04 '13 at 16:18

As suggested by Maddy, for your use case you just change the frame. Here is what happens behind the scenes:

Changing the frame is like changing bounds and center of the view. So if you want to use bounds, also change the center. Please note that the size of the frame is not necessarily the same as the size of thebounds` because the view might be rotated.

Therefore, your statement "I thought that a view only draws into it's bounds rect." is if not incorrect at least not precise enough. If you rotate your {200,200} view by 45 degrees, its bounds would still be {200,200}, but its frame.size will be 200 * sqrt(2) for both width and height. The bounds refer to the view's own coordinate system, the frame to that of the superview.

score 0 · Answer 2 · answered Nov 04 '13 at 16:47

What Mundi said, but I would like to add, if you ever thought why bounds even existed, it makes a lot of sense when scrolling. An NSClipView's bounds of an NSScrollView is the part of the NSScrollView's document view that is visible. So scrollView.contentView.bounds changes as you scroll but scrollView.contentView.frame obviously does not.

Changing a view's bounds doesn't change where it draws

2 Answers2