Matching subview's width to it's superview using autolayout

Question

Goal:

Have a UIWebView be the same width as it\'s superview, which is a UIScrollView, using autolayout constraints.

Answer 1

Improving on Rob's answer, as requested.

As Rob already mentioned, UIScrollViews have peculiar behavior under Auto Layout.

What is of interest in this case is the fact that the scrollView total width is determined by using its subviews total width. So while the scrollView already asks the webView for its width, you're telling the webView to also ask the scrollView for its width. That's why it doesn't work. One is asking another, and no one knows the answer. You need another reference view to use as a constraint for the webView, and then the scrollView will also be able to successfully ask about its expected width.

An easy way this could be done: create another view, containerView, and add the scrollView as a subview to that. Then set the proper constraints for containerView. Let's say you wanted the scrollView centered on a viewController, with some padding on the edges. So do it for the containerView:

NSDictionary *dict = NSDictionaryOfVariableBindings(containerView);
[self.view addConstraints:[NSLayoutConstraints constraintsWithVisualFormat:@"H|-(100)-[containerView]-(100)-|" options:0 metrics:0 views:dict];
[self.view addConstraints:[NSLayoutConstraints constraintsWithVisualFormat:@"V|-(100)-[containerView]-(100)-|" options:0 metrics:0 views:dict];

Then you can proceed adding the webView as a subview to the scrollView and setting its width:

NSLayoutConstraint *makeWidthTheSameAsScrollView =[NSLayoutConstraint
                                                       constraintWithItem:self.questionWebView
                                                       attribute:NSLayoutAttributeWidth
                                                       relatedBy:0
                                                       toItem:containerView
                                                       attribute:NSLayoutAttributeWidth
                                                       multiplier:1.0
                                                       constant:0];

[self.view addConstraint:makeWidthTheSameAsScrollView];

This would make the scrollview as large and tall as the webView, and they both would be placed as intended (with the constraints set on containerView).

Answer 2

Scrollviews are a bit strange in how they interact with auto layout. See TN2154 (UIScrollView and Autolayout).

See also UIScrollView doesn't use autolayout constraints.

In general, you need to get the width of the contained view some other way than "the current width of the scrollview" since in auto layout the scrollview's width (i.e. content width) is defined in terms of its content. Thus your current request is circular.