Autoresizing Mask Strange Behavior

Question

I'm in an UIViewController, self.view points to a valid view with a frame of 300x480.

UIView *redView;
UIView *blueView;

//[[self view] setAutoresizingMask:YES]; //WRONG!
[[self view] setAutoresizesSubviews:YES];
[[self view] setClipsToBounds:YES];

redView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 200, 200)];

[redView setBackgroundColor:[UIColor redColor]];

[redView setAutoresizingMask:UIViewAutoresizingFlexibleLeftMargin];

[[self view] addSubview:redView];

blueView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];

[blueView setBackgroundColor:[UIColor blueColor]];

[blueView setAutoresizingMask:UIViewAutoresizingFlexibleRightMargin];

[[self view] addSubview:blueView];

With this snippet of code i was trying

1. To align the 200x200 red view to the right margin,
2. To align the 100x100 blue view to the left margin;

but the result as you can see is far from being what i was expecting...

http://imageshack.us/photo/my-images/27/iossimulatorscreenshoto.png/

I've read every single bit of apple documentation and every google result about the autoresizing mask usage, but i still can't figure out why this is happening.

Can someone please explain to me what is happening?

Thanks in advance.

Answer 1

Keep in mind it's an autoREsizing mask, not an autosizing mask. It doesn't do anything until something changes size.

What you want to do is define your views' frames exactly where you want them to be according to the current size of your parent view, i.e. 300 x 480. So for example

redView = [[UIView alloc] initWithFrame:CGRectMake(100, 0, 200, 200)];

blueView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];

You are setting their autoresizing masks correctly as far as the horizontal alignment. If self.view changes size, then redView and blueView will remain locked to the margins.

You should set at least one of flexible top margin, flexible bottom margin, or flexible height. Otherwise the behavior is undefined if the parent view changes height. It's impossible for all three of those parameters to remain fixed within two different heights.

This line of code doesn't make sense:

[[self view] setAutoresizingMask:YES];

It isn't a compiler error because BOOL happens to be castable to UIViewAutoresizing, which is an enum. The effect is that YES = 1 = UIViewAutoresizingFlexibleLeftMargin. Probably not what you wanted. More likely you wanted:

[[self view] setAutoresizingMask:UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight];

If you can, you should do all this in Interface Builder. Since it's a WYSIWYG editor, you can SWYG ("see what you get"). You can quickly play around with the options and learn how they work. Much faster than edit - compile - run to see the effects. Even if you can't / won't use Interface Builder for your project, try a sample project with it to learn how autoresizing works. Once you know you can do it in code with less trial and error. But watch out for the gotcha that turning a margin bar on in Interface Builder is equivalent to turning the corresponding FlexibleXYZMargin option off in code.