UIView.convert(_ point:to:)方法异常行为的排查求助

我尝试转换若干视图的中心点CGPoint以便绘制连线，但原本垂直对齐的点，convert返回的值不符合预期。复现代码如下：

class ViewController: UIViewController {
    
    private lazy var backgroundView: UIView = {
        let view = UIView()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.backgroundColor = .systemBackground
        return view
    }()
    
    private lazy var firstView: UIView = {
        let view = UIView()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.backgroundColor = .systemRed
        return view
    }()
    
    private lazy var secondView: UIView = {
        let view = UIView()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.backgroundColor = .systemBlue
        return view
    }()
    
    private lazy var firstSubview: UIImageView = {
        let view = UIImageView()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.backgroundColor = .systemMint
        return view
    }()
    
    private lazy var secondSubview: UIImageView = {
        let view = UIImageView()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.backgroundColor = .systemGreen
        return view
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.addSubview(backgroundView)
        
        backgroundView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor).isActive = true
        backgroundView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor, constant: 20).isActive = true
        backgroundView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor, constant: -20).isActive = true
        backgroundView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor).isActive = true
        
        backgroundView.addSubview(firstView)
        backgroundView.addSubview(secondView)
        
        firstView.topAnchor.constraint(equalTo: backgroundView.topAnchor).isActive = true
        firstView.leadingAnchor.constraint(equalTo: backgroundView.leadingAnchor).isActive = true
        firstView.trailingAnchor.constraint(equalTo: backgroundView.trailingAnchor).isActive = true
        
        secondView.topAnchor.constraint(equalTo: firstView.bottomAnchor, constant: 10).isActive = true
        secondView.leadingAnchor.constraint(equalTo: backgroundView.leadingAnchor).isActive = true
        secondView.trailingAnchor.constraint(equalTo: backgroundView.trailingAnchor).isActive = true
        
        firstView.addSubview(firstSubview)
        
        firstSubview.topAnchor.constraint(equalTo: firstView.topAnchor).isActive = true
        firstSubview.bottomAnchor.constraint(equalTo: firstView.bottomAnchor).isActive = true
        firstSubview.leadingAnchor.constraint(equalTo: firstView.leadingAnchor).isActive = true
        firstSubview.heightAnchor.constraint(equalToConstant: 50).isActive = true
        firstSubview.widthAnchor.constraint(equalToConstant: 50).isActive = true
        
        secondView.addSubview(secondSubview)
        
        secondSubview.topAnchor.constraint(equalTo: secondView.topAnchor).isActive = true
        secondSubview.bottomAnchor.constraint(equalTo: secondView.bottomAnchor).isActive = true
        secondSubview.leadingAnchor.constraint(equalTo: secondView.leadingAnchor, constant: 10).isActive = true
        secondSubview.heightAnchor.constraint(equalToConstant: 30).isActive = true
        secondSubview.widthAnchor.constraint(equalToConstant: 30).isActive = true
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
                
        let firstCenter1 = firstSubview.convert(firstSubview.center, to: view)
        let secondCenter1 = secondSubview.convert(secondSubview.center, to: view)
        
        print(firstCenter1, secondCenter1)
    }
}

打印结果为(45.0, 84.0) (55.0, 134.0)，预期应输出(45.0, 84.0) (45.0, 134.0)，因为secondSubview的相对center.x理应与firstSubview一致为45.0。

错误原因 #

你混淆了UIView.center的坐标系和convert(_:to:)方法的参数要求：

• UIView.center属性表示的是该视图在父视图坐标系中的中心点，而非自身坐标系的点。
• convert(_ point: CGPoint, to view: UIView?)方法要求传入的point是调用该方法的视图自身坐标系中的点。

当你执行secondSubview.convert(secondSubview.center, to: view)时，相当于把「secondSubview在父视图中的中心点坐标」当成「secondSubview自身坐标系中的点」来转换，这自然会得到错误的结果。

另外你的代码中存在约束冲突：secondSubview同时设置了top/bottom与父视图对齐和固定height，AutoLayout会自动调整约束优先级，这也可能间接影响坐标计算。

解决方案 #

有两种正确的方式获取目标坐标系下的中心点：

方式1：通过父视图转换子视图的center #

利用子视图的center（父坐标系中的点），让父视图执行转换：

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
            
    let firstCenter1 = firstView.convert(firstSubview.center, to: view)
    let secondCenter1 = secondView.convert(secondSubview.center, to: view)
    
    print(firstCenter1, secondCenter1) // 输出预期的(45.0, 84.0) (45.0, 134.0)
}

方式2：转换子视图自身bounds的中心点 #

子视图自身坐标系的中心点是bounds.center，直接用子视图转换这个点：

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
            
    let firstCenter1 = firstSubview.convert(firstSubview.bounds.center, to: view)
    let secondCenter1 = secondSubview.convert(secondSubview.bounds.center, to: view)
    
    print(firstCenter1, secondCenter1) // 输出预期结果
}

额外修复：解决约束冲突 #

移除secondSubview的top/bottom约束，保留height和垂直居中约束，避免AutoLayout自动调整布局：

// 替换原secondSubview的约束代码
secondSubview.centerYAnchor.constraint(equalTo: secondView.centerYAnchor).isActive = true
secondSubview.leadingAnchor.constraint(equalTo: secondView.leadingAnchor, constant: 10).isActive = true
secondSubview.heightAnchor.constraint(equalToConstant: 30).isActive = true
secondSubview.widthAnchor.constraint(equalToConstant: 30).isActive = true

内容的提问来源于stack exchange，提问作者Faruk