subviewA.translatesAutoresizingMaskIntoConstraints = false  // 這是純程式碼寫 Auto Layout 的大雷：必須設定這一行！
view.addSubview(subviewA)
var constraints = [NSLayoutConstraint]()
constraints.append(NSLayoutConstraint(item: subviewA, attribute: .centerX, relatedBy: .equal,
                                      toItem: view, attribute: .centerX, multiplier: 1.0, constant: 0))
// subviewA.centerX = 1.0 * view.centerX + 0 即 subviewA.centerX = view.centerX
constraints.append(NSLayoutConstraint(item: subviewA, attribute: .centerY, relatedBy: .equal,
                                      toItem: view, attribute: .centerY, multiplier: 1.0, constant: 0))
// subviewA.centerY = 1.0 * view.centerY + 0 即 subviewA.centerY = view.centerY
constraints.append(NSLayoutConstraint(item: subviewA, attribute: .width, relatedBy: .equal,
                                      toItem: nil, attribute: .notAnAttribute, multiplier: 0, constant: 200))
// subviewA.width = 0 * nil + 200.0 即 subviewA.width = 200
constraints.append(NSLayoutConstraint(item: subviewA, attribute: .height, relatedBy: .equal,
                                      toItem: nil, attribute: .notAnAttribute, multiplier: 0, constant: 100))
// subviewA.centerX = 0 * nil + 100.0 即 subviewA.height = 100
NSLayoutConstraint.activate(constraints)	// 這些 constraint 必須 activate 才會啟用