OS X Cocoa Auto Layout hidden elements

Question

I am trying to use the new Auto Layout in Lion because it seems quite nice. But I can not find good information about how to do things. For example:

I have two labels:

Answer 1

This category makes collapsing Auto Layout constrained views really simple:

https://github.com/depth42/AutolayoutExtensions

I just added it to a project and it works great.

Answer 2

Here's an example of how I handled this programmatically rather than using Interface Builder. In summary; I only add the view if it's enabled and then iterate over subviews, adding vertical constraints as I go.

Note that the views in question are initialized prior to this.

/*
  Begin Auto Layout
*/
NSMutableArray *constraints = [NSMutableArray array];
NSMutableDictionary *views = [[NSMutableDictionary alloc] init];


/*
  Label One
*/
if (enableLabelOne) {
    [contentView addSubview:self.labelOne];

    self.labelOne.translatesAutoresizingMaskIntoConstraints = NO;

    [views setObject:self.labelOne
              forKey:@"_labelOne"];

    [constraints addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[_labelOne(44)]"
                                                                             options:0
                                                                             metrics:nil
                                                                               views:views]];

    [constraints addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-[_labelOne]-|"
                                                                             options:0
                                                                             metrics:nil
                                                                               views:views]];
}

/*
    Label Two
*/
if (enableLabelTwo) {
    [contentView addSubview:self.labelTwo];

    self.labelTwo.translatesAutoresizingMaskIntoConstraints = NO;

    [views setObject:self.labelTwo
              forKey:@"_labelTwo"];

    [constraints addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[_labelTwo(44)]"
                                                                             options:0
                                                                             metrics:nil
                                                                               views:views]];
}

[constraints addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-[_labelTwo]-|"
                                                                         options:0
                                                                         metrics:nil
                                                                           views:views]];

/*
  Dynamically add vertical spacing constraints to subviews
*/
NSArray *subviews = [contentView subviews];

if ([subviews count] > 0) {
    UIView *firstView = [subviews objectAtIndex:0];
    UIView *secondView = nil;
    UIView *lastView = [subviews lastObject];

    [constraints addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[firstView]"
                                                                             options:0
                                                                             metrics:nil
                                                                               views:NSDictionaryOfVariableBindings(firstView)]];

    for (int i = 1; i < [subviews count]; i++) {
        secondView = [subviews objectAtIndex:i];
        [constraints addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[firstView]-10-[secondView]"
                                                                                 options:0
                                                                                 metrics:nil
                                                                                   views:NSDictionaryOfVariableBindings(firstView, secondView)]];
        firstView = secondView;
    }

    [constraints addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[lastView]-|"
                                                                             options:0
                                                                             metrics:nil
                                                                               views:NSDictionaryOfVariableBindings(lastView)]];
}


[self addConstraints:constraints];

I'm only setting the lastView constraint because this code was adapted from something inside of a UIScrollView.

I originally implemented this based on this Stack Overflow answer and changed things to suit my own needs.