深入探究updateConstraints()实现：调用snp.updateConstraints()后需调用布局方法吗？

Great question—this is a common point of confusion when working with SnapKit, so let’s break it down clearly:

Short Answer #

You don’t need to manually call setNeedsLayout() after using view.snp.updateConstraints(). SnapKit’s implementation handles marking the view’s constraints as needing updates, which triggers the system’s layout cycle automatically.

As for layoutIfNeeded(), you only need to call it if you want to force an immediate layout update (like within an animation block), rather than waiting for the next runloop cycle.

Longer Explanation #

Let’s dive into what’s happening under the hood:

1. When you call view.snp.updateConstraints { make in ... }, SnapKit updates the constraints defined in the closure and automatically calls setNeedsUpdateConstraints() on the view. This tells the system that the view’s constraints have changed and need to be recalculated.
2. The system then schedules an update to the layout in the next runloop cycle—you don’t need to trigger this manually with setNeedsLayout().
3. layoutIfNeeded() is only necessary if you need the layout changes to take effect right away. For example, if you’re animating constraint changes, you’ll wrap the layoutIfNeeded() call inside the animation block to ensure the changes animate smoothly:

UIView.animate(withDuration: 0.3) {
    myView.snp.updateConstraints { make in
        make.height.equalTo(300)
    }
    myView.superview?.layoutIfNeeded() // Forces immediate layout update for animation
}

For regular, non-animated constraint updates, the system will handle the layout update automatically without needing this call.

Key Takeaways #

• Skip setNeedsLayout() entirely—SnapKit takes care of triggering the constraint update flow.
• Use layoutIfNeeded() only when you require immediate layout synchronization (e.g., animations, accessing updated frame values right after changing constraints).

内容的提问来源于stack exchange，提问作者Dave