Auto Layout

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I’ve always taken for granted that whenever I make changes to constraints on a view, I’d expect the changes to get applied to the view. This chapter will look into when & how this happens. Let’s demystify auto layout.

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The Layout Cycle

The application manages the layout cycle on its main run loop as follows:

1. Handle events on the main dispatch queue
2. Check for constraints changes
3. If constraints have changed, schedule a deferred layout pass

Image from Presentation Slides, WWDC 2015, Session 219

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Registering Constraint Changes

1. Constraint changes are translated into mathematical expressions in the layout engine.
2. Based on the constraint changes, the layout engine re-computes the dimensions & coordinates of the views & controls that need to be updated.
3. These views get notified, and calls superview.setNeedsLayout(), causing the deferred layout pass to schedule.

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Deferred Layout Pass

The deferred layout pass consists of 2 passes - the update pass & the layout pass.

The Update Pass:

The intention of the update pass is to register constraint changes for current layout pass. The layout engine traverses the view hierarchy, calling:

• updateViewConstraints() on UIViewController
• updateConstraints() on each UIView

The Layout Pass:

Finally, the layout pass reposition affected views by assigning new dimensions & coordinates. In this pass, the layout engine traverses the view hierarchy, calling:

• viewWillLayoutSubviews() on UIViewController
• layoutSubviews() on each UIView

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PreEmpting the Deferred Layout Pass

What can I do if I want to effect my layout changes right away instead of waiting until the next layout pass?

• Call UIView’s setNeedsUpdateConstraints() to trigger an update + layout pass
• Call UIView’s’ setNeedsLayout() to trigger a layout pass

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Wrapping Up

Constraint Updates:

• updateConstraintsIfNeeded() triggers updateConstraints() only if constraints need updating
• setNeedsUpdateConstraints() triggers updateConstraints() unconditionally
• Thus we don’t ever call updateConstraints() directly - we can override it to group constraint updates for performance.

Layout Updates: (ie, frame changes, view added, view resizes)

• layoutIfNeeded() triggers layoutSubviews() only if layout has changed
• setNeedsLayout() triggers layoutSubviews() unconditionally
• Again, we don’t call layoutSubview() directly - we override it if autoresizing and constraint-based behaviors of the subviews do not offer the behavior you want.

Do Nots:

• Do not call setNeedsUpdateConstraints() in updateConstraints - this creates a feedback loop
• Do not call setNeedsLayout() in layoutSubview() - this creates a feedback loop
• Do not invalidate layout of views outside the view’s subtree