When is layoutSubviews called?

Question

I had a similar question, but wasn't satisfied with the answer (or any I could find on the net), so I tried it in practice and here is what I got:

• init does not cause layoutSubviews to be called (duh)
• addSubview: causes layoutSubviews to be called on the view being added, the view it’s being added to (target view), and all the subviews of the target
• view setFrame intelligently calls layoutSubviews on the view having its frame set only if the size parameter of the frame is different
• scrolling a UIScrollView causes layoutSubviews to be called on the scrollView, and its superview
• rotating a device only calls layoutSubview on the parent view (the responding viewControllers primary view)
• Resizing a view will call layoutSubviews on its superview (Important: views with an intrinsic content size will re-size if the content that determines their size changes; for example, updating the text on a UILabel will cause the intrinsic content size to be updated and thus call layoutSubviews on its superview)

My results - http://blog.logichigh.com/2011/03/16/when-does-layoutsubviews-get-called/

Building on the previous answer by @BadPirate, I experimented a bit further and came up with some clarifications/corrections. I found that layoutSubviews: will be called on a view if and only if:

• Its own bounds (not frame) changed.
• The bounds of one of its direct subviews changed.
• A subview is added to the view or removed from the view.

Some relevant details:

• The bounds are considered changed only if the new value is different, including a different origin. Note specifically that is why layoutSubviews: is called whenever a UIScrollView scrolls, as it performs the scrolling by changing its bounds' origin.
• Changing the frame will only change the bounds if the size has changed, as this is the only thing propagated to the bounds property.
• A change in bounds of a view that is not yet in a view hierarchy will result in a call to layoutSubviews: when the view is eventually added to a view hierarchy.
• And just for completeness: these triggers do not directly call layoutSubviews, but rather call setNeedsLayout, which sets/raises a flag. Each iteration of the run loop, for all views in the view hierarchy, this flag is checked. For each view where the flag is found raised, layoutSubviews: is called on it and the flag is reset. Views higher up the hierarchy will be checked/called first.

https://developer.apple.com/library/prerelease/tvos/documentation/WindowsViews/Conceptual/ViewPG_iPhoneOS/CreatingViews/CreatingViews.html#//apple_ref/doc/uid/TP40009503-CH5-SW1

Layout changes can occur whenever any of the following events happens in a view:

a. The size of a view’s bounds rectangle changes.
b. An interface orientation change occurs, which usually triggers a change in the root view’s bounds rectangle.
c. The set of Core Animation sublayers associated with the view’s layer changes and requires layout.
d. Your application forces layout to occur by calling the setNeedsLayout or layoutIfNeeded method of a view.
e. Your application forces layout by calling the setNeedsLayout method of the view’s underlying layer object.

Some of the points in BadPirate's answer are only partially true:

1. For addSubView point

   addSubview causes layoutSubviews to be called on the view being added, the view it’s being added to (target view), and all the subviews of the target.

   It depends on the view's (target view) autoresize mask. If it has autoresize mask ON, layoutSubview will be called on each addSubview. If it has no autoresize mask then layoutSubview will be called only when the view's (target View) frame size changes.

   Example: if you created UIView programmatically (it has no autoresize mask by default), LayoutSubview will be called only when UIView frame changes not on every addSubview.

   It is through this technique that the performance of the application also increases.

2. For the device rotation point

   Rotating a device only calls layoutSubview on the parent view (the responding viewController's primary view)

   This can be true only when your VC is in the VC hierarchy (root at window.rootViewController), well this is most common case. In iOS 5, if you create a VC, but it is not added into any another VC, then this VC would not get any noticed when device rotate. Therefore its view would not get noticed by calling layoutSubviews.

I tracked the solution down to Interface Builder's insistence that springs cannot be changed on a view that has the simulated screen elements turned on (status bar, etc.). Since the springs were off for the main view, that view could not change size and hence was scrolled down in its entirety when the in-call bar appeared.

Turning the simulated features off, then resizing the view and setting the springs correctly caused the animation to occur and my method to be called.

An extra problem in debugging this is that the simulator quits the app when the in-call status is toggled via the menu. Quit app = no debugger.

calling [self.view setNeedsLayout]; in viewController makes it to call viewDidLayoutSubviews

have you looked at layoutIfNeeded?

The documentation snippet is below. Does the animation work if you call this method explicitly during the animation?

layoutIfNeeded Lays out the subviews if needed.

- (void)layoutIfNeeded

Discussion Use this method to force the layout of subviews before drawing.

Availability Available in iPhone OS 2.0 and later.