Скачать или смотреть Understanding Why parentView of a CALayer Can Be nil During Animation in iOS

Discover the reason why the reference to a CALayer's parent view becomes `nil` during animation, and learn how to fix this issue effectively with Core Animation in Swift.
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Understanding Why parentView of a CALayer Can Be nil During Animation in iOS

When working with custom animations in iOS, developers often encounter issues with property references unexpectedly turning nil. A common problem is when a property that references a CALayer's parent view is nil during an animation. This guide will break down why this happens and provide a solution to effectively manage parent view references during animations.

The Problem: parentView Becomes nil

Imagine that you have a subclass of CALayer, and you’re trying to maintain a reference to its parent view. Surprisingly, during an animation, you find that the parentView property becomes nil. You might be wondering, "Why does this happen?"

To illustrate, suppose you have a property containerWidth that triggers an animation. While the containerWidth is animated, the parentView is shown as nil in the debug output:

[[See Video to Reveal this Text or Code Snippet]]

However, after the animation, parentView returns to its normal state with a valid reference:

[[See Video to Reveal this Text or Code Snippet]]

Why is This Happening?

The core of the issue lies in how Core Animation operates with CALayers. When an animation occurs, Core Animation essentially creates a “presentation layer” which is a copy of the original model layer.

Model State vs. Presentation State:

The model layer (the one you're interacting with) maintains the fundamental properties.

The presentation layer is what is rendered visually during the animation.

During the process of animating properties like containerWidth, your animations are executed on this copied instance, hence any property not explicitly copied, such as parentView, will not be available, resulting in a nil value.

The Solution: Initialize Properties in the Presentation Layer

To ensure your parentView is not nil during animation, you need to set it up correctly in the initializer of the presentation layer. Here’s how you can achieve that:

Override the initializer in your subclass of CALayer to capture the parentView from the model layer.

Assign properties from the model layer to the presentation layer in this initializer.

Implementation Example

Here's an implementation that demonstrates this approach:

[[See Video to Reveal this Text or Code Snippet]]

Key Steps to Implement:

Use the init(layer: Any) initializer to capture the parentView.

Call the superclass initializer and ensure that you check if the layer being initialized is of type ContainerLayer.

This way, your parentView will correctly reference its parent even during animations.

Conclusion

In summary, the parentView property of a CALayer becomes nil during animations due to Core Animation's use of a separate presentation layer that does not automatically inherit non-visible properties like UITableView. By correctly implementing the initializer for the presentation layer, you can maintain necessary references throughout the animation. This knowledge is crucial for effectively managing custom layers and animations in your iOS applications.

By understanding the underlying functionality of Core Animation and CALayers, you can develop more robust iOS applications that function smoothly, even during complex animations.