iOS Autolayout with UIScrollview: Why does content view of scroll view not fill the scroll view?

Question

The following code (called in viewDidLoad) results in a fully red screen. I would expect it to be a fully green screen. Why is it red? And how can I make it all green?

UIScrollView* scrollView = [UIScrollView new];
scrollView.translatesAutoresizingMaskIntoConstraints = NO;
scrollView.backgroundColor = [UIColor redColor];
[self.view addSubview:scrollView];

UIView* contentView = [UIView new];
contentView.translatesAutoresizingMaskIntoConstraints = NO;
contentView.backgroundColor = [UIColor greenColor];
[scrollView addSubview:contentView];

NSDictionary* viewDict = NSDictionaryOfVariableBindings(scrollView,contentView);

[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[scrollView]|" options:0 metrics:0 views:viewDict]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[scrollView]|" options:0 metrics:0 views:viewDict]];

[scrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[contentView]|" options:0 metrics:0 views:viewDict]];
[scrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[contentView]|" options:0 metrics:0 views:viewDict]];

Answer 1

Constraints with scroll views work slightly differently than it does with other views. The constraints between of contentView and its superview (the scrollView) are to the scrollView's contentSize, not to its frame. This might seem confusing, but it is actually quite useful, meaning that you never have to adjust the contentSize, but rather the contentSize will automatically adjust to fit your content. This behavior is described in Technical Note TN2154.

If you want to define the contentView size to the screen or something like that, you'd have to add a constraint between the contentView and the main view, for example. That's, admittedly, antithetical to putting content into the scrollview, so I probably wouldn't advise that, but it can be done.

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To illustrate this concept, that the size of contentView will be driven by its content, not by the bounds of the scrollView, add a label to your contentView:

UIScrollView* scrollView = [UIScrollView new];
scrollView.translatesAutoresizingMaskIntoConstraints = NO;
scrollView.backgroundColor = [UIColor redColor];
[self.view addSubview:scrollView];

UIView* contentView = [UIView new];
contentView.translatesAutoresizingMaskIntoConstraints = NO;
contentView.backgroundColor = [UIColor greenColor];
[scrollView addSubview:contentView];

UILabel *randomLabel = [[UILabel alloc] init];
randomLabel.text = @"this is a test";
randomLabel.translatesAutoresizingMaskIntoConstraints = NO;
randomLabel.backgroundColor = [UIColor clearColor];
[contentView addSubview:randomLabel];

NSDictionary* viewDict = NSDictionaryOfVariableBindings(scrollView, contentView, randomLabel);

[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[scrollView]|" options:0 metrics:0 views:viewDict]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[scrollView]|" options:0 metrics:0 views:viewDict]];

[scrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[contentView]|" options:0 metrics:0 views:viewDict]];
[scrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[contentView]|" options:0 metrics:0 views:viewDict]];

[contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-[randomLabel]-|" options:0 metrics:0 views:viewDict]];
[contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[randomLabel]-|" options:0 metrics:0 views:viewDict]];

Now you'll see that the contentView (and, therefore, the contentSize of the scrollView) are adjusted to fit the label with standard margins. And because I didn't specify the width/height of the label, that will adjust based upon the text you put into that label.

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If you want the contentView to also adjust to the width of the main view, you could do redefine your viewDict like so, and then add these additional constraints (in addition to all the others, above):

UIView *mainView = self.view;

NSDictionary* viewDict = NSDictionaryOfVariableBindings(scrollView, contentView, randomLabel, mainView);

[mainView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:[contentView(==mainView)]" options:0 metrics:0 views:viewDict]];

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There is a known issue (bug?) with multiline labels in scrollviews, that if you want it to resize according to the amount of text, you have to do some sleight of hand, such as:

dispatch_async(dispatch_get_main_queue(), ^{
    randomLabel.preferredMaxLayoutWidth = self.view.bounds.size.width;
});