swift - 以编程方式添加负约束

Question

为什么我必须为 titleLabel 的 rigthAnchor 约束赋予负值，以便从 View 的右侧留出 28 个边距。如果我只给它一个正值，它就会超出 View 28 个像素。

private func setupLayout() {
    view.addSubview(imageView)
    view.addSubview(titleLabel)

    // *** imageView ***
    if #available(iOS 11.0, *) {
        imageView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 116).isActive = true
    } else {
        imageView.topAnchor.constraint(equalTo: view.topAnchor, constant: 116).isActive = true
    }
    imageView.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
    imageView.leftAnchor.constraint(equalTo: view.leftAnchor).isActive = true
    imageView.heightAnchor.constraint(equalToConstant: 224).isActive = true

    // *** titleLabel ***
    titleLabel.topAnchor.constraint(equalTo: imageView.bottomAnchor, constant: 25).isActive = true
    titleLabel.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -28).isActive = true
    titleLabel.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 28).isActive = true
}

Answer 1

因为根据这个

titlelbl.right = view.right - 28

等于说

view.right = titlelbl.right + 28

firstItem.anchor = seconfItem.anchor * multiplier +/- constant

转化为

titleLabel.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -28).isActive = true

或

view.rightAnchor.constraint(equalTo: titleLabel.rightAnchor, constant: 28).isActive = true

如果你不喜欢负值

编辑:

约束为-28时的含义

你告诉自动布局将 titlbl 的最大 x 值设置为等于 View 的最大 x 值减去 28

| - lbl + 28 - |

但是 +28

| - lbl - | + 28 <<< out of view max x