Swift中如何在lazy var里获取父视图的frame？

Hey there! Let's break down why your superview approach isn't working and walk through the best fixes.

Why Your Current Code Fails #

When you use a lazy var, the closure runs the first time you access the property—and at that point, your redArrowImage hasn't been added to any parent view yet. That means self.superview is nil, so self.superview.frame.size.width evaluates to 0, leaving you with an x-coordinate of -25 (which is either off-screen or nowhere near where you want it).

Solution 1: Use Auto Layout (Recommended) #

Auto Layout is the modern, reliable way to handle dynamic positioning in iOS—it automatically adapts to screen rotations, different device sizes, and parent view changes. Here's how to adjust your code:

First, set up the lazy image view without a frame (disable autoresizing masks so Auto Layout takes over):

lazy var redArrowImage: UIImageView = { 
    let imageView = UIImageView(image: UIImage(named: "red")) 
    imageView.contentMode = .scaleAspectFit 
    imageView.translatesAutoresizingMaskIntoConstraints = false // Critical for Auto Layout to work
    return imageView 
}()

Then, after adding the image view to its parent view, activate the constraints to position it correctly:

// Whenever you're adding the image view to its parent (e.g., in viewDidLoad or custom view setup)
yourParentView.addSubview(redArrowImage)

NSLayoutConstraint.activate([
    // Pin trailing edge 25 points from the parent's trailing edge
    redArrowImage.trailingAnchor.constraint(equalTo: yourParentView.trailingAnchor, constant: -25),
    // Pin top edge 12 points from the parent's top edge
    redArrowImage.topAnchor.constraint(equalTo: yourParentView.topAnchor, constant: 12),
    // Set fixed width and height
    redArrowImage.widthAnchor.constraint(equalToConstant: 15),
    redArrowImage.heightAnchor.constraint(equalToConstant: 15)
])

Solution 2: Adjust Frame After Adding to Parent View #

If you prefer to stick with frame-based layout, you need to set the frame after the image view has been added to its parent. The best place to do this is in a layout method that runs when the view hierarchy is ready:

If you're working in a custom UIView:

Override layoutSubviews (this runs whenever the view's bounds change, so it handles rotations and size updates automatically):

override func layoutSubviews() {
    super.layoutSubviews()
    // Now self (the parent view) has its final frame
    redArrowImage.frame = CGRect(x: self.frame.width - 25, y: 12, width: 15, height: 15)
}

If you're working in a ViewController:

Use viewDidLayoutSubviews (this runs after the view's subviews have been laid out):

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    // Make sure yourParentView is already initialized and has its frame
    redArrowImage.frame = CGRect(x: yourParentView.frame.width - 25, y: 12, width: 15, height: 15)
}

Key Takeaway #

Avoid relying on superview in a lazy initialization closure—since the view isn't in the hierarchy yet, that reference will be nil. Auto Layout is the most maintainable approach long-term, but frame-based adjustments in layout methods work too if you need them.

内容的提问来源于stack exchange，提问作者Ramesh