两种主要的自动布局约束创建方式（代码创建）：

1. addConstraints()

   	scrollView.addSubview(statusButton)
       scrollView.addConstraints([
           statusButton.topAnchor.constraint(equalTo: scrollView.topAnchor, constant:12),
           statusButton.leftAnchor.constraint(equalTo: scrollView.leftAnchor, constant:16),
           statusButton.heightAnchor.constraint(equalToConstant: 36),
           ])
       
       scrollView.addSubview(dateRangeButton)
       scrollView.addConstraints([
           dateRangeButton.topAnchor.constraint(equalTo: scrollView.topAnchor, constant:12),
           dateRangeButton.leftAnchor.constraint(equalTo: statusButton.rightAnchor, constant:4),
           dateRangeButton.heightAnchor.constraint(equalToConstant: 36),
           ])
       
       scrollView.addSubview(transactionTypeButton)
       scrollView.addConstraints([
           transactionTypeButton.topAnchor.constraint(equalTo: scrollView.topAnchor, constant:12),
           transactionTypeButton.leftAnchor.constraint(equalTo: dateRangeButton.rightAnchor, constant:4),
           transactionTypeButton.heightAnchor.constraint(equalToConstant: 36),
           ])
   

2. activate()
   这实际上等价于 addConstraints()，NSLayoutConstraint.activate() 方法可以一次性创建所有约束，也可以调用多次，分批次创建约束，比如这样:

       stockNameLabel = UILabel()
       stockNameLabel.translatesAutoresizingMaskIntoConstraints = false
       addSubview(stockNameLabel)
       NSLayoutConstraint.activate([
           stockNameLabel.topAnchor.constraint(equalTo: topAnchor, constant: 24),
           stockNameLabel.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 16),
           ])
       stockCodeLabel = UILabel()
       stockCodeLabel.translatesAutoresizingMaskIntoConstraints = false
       addSubview(stockCodeLabel)
       NSLayoutConstraint.activate([
           stockCodeLabel.topAnchor.constraint(equalTo: stockNameLabel.bottomAnchor, constant: 8),
           stockCodeLabel.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 16),
           ])
       
   

3. 修改约束

   首先创建一个约束并保持该约束的引用：

   // 创建约束，同时保持一个引用
   separatorViewTop = separatorView.topAnchor.constraint(equalTo:view.topAnchor, constant: investmentAccountSection.intrinsicHeight)
   // 添加约束
   NSLayoutConstraint.activate([
          ...
           // Separator view
           separatorView.heightAnchor.constraint(equalToConstant: 1),
   		separatorViewTop!,
           separatorView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
           separatorView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
           ...
   ])
   

   修改约束并立即调用 updateConstraintsIfNeeded()：

    self.separatorViewTop?.constant = self.investmentAccountSection.intrinsicHeight
    self.view.updateConstraintsIfNeeded()