Core Layout Runtime

In addition to the “display” pass Cocoa has always used, Cocoa Auto Layout adds two more passes that occur when rendering a window: updateConstraintsIfNeeded and layoutIfNeeded. These occur in order: update constraints, layout, display. If you invoke display manually, it invokes layout and layout invokes updateConstraints.

You can think of the update constraints pass as a measurement pass that communicates changes in geometry to the layout system. For example, if you change the title of a button at some point the text has to be measured, and a constraint has to be set up that communicates that information into the layout system. The update constraints pass is bottom-up.

It is in the layout pass that the frames of views are updated. The layout pass is top-down.

These passes have parallel sets of methods:

If you’re implementing a custom view, the most common methods to interact with are the view-level overridable methods and the invalidation methods.

• You can override updateConstraints to perform deferred constraint setup. You invoke setNeedsUpdateConstraints: when you have set up you want to defer. This is particularly useful when a nib file is loaded and properties are changing a lot—you won’t receive updateConstraints until things have settled down.

• You should only override layout only if you want to do custom layout. If you do, then you also need to invoke setNeedsLayout: when something that feeds into your custom layout changes.

To reiterate: you never need to call setNeedsLayout: unless you override layout. Based on changes to constraints, the layout engine determines when layout is required.

A controller object is typically more concerned with the NSWindow and NSView methods that perform the pass. These uses are more rare, though still valid and needed on occasion.

• Perform the update constraints pass manually to make sure all constraints are up to date.

  You might do this before introspecting a constraint to extract a metric.

• Perform the layout pass manually if you want to extract frames from views and make decisions based on them. If you don’t make sure layout is up to date first, the frames won’t be meaningful.

  An NSAlert object uses this as part of its layout. The constraint-based system doesn’t deal with flowed text and line breaking any better (or worse) than springs and struts does. NSAlert sets up all its constraints except for anything giving the height of the text fields. Then it calls layoutSubtreeIfNeeded. At this point the text fields have the correct width but not the correct height. It then uses NSTextField methods to find the appropriate height for the fields given their width, and adds the height constraints.