class IDSearchBar: UISearchBar {

}

var contentInset: UIEdgeInsets? {
  didSet {
    self.layoutSubviews()
  }
}

override func layoutSubviews() {
  super.layoutSubviews()

  // view是searchBar中的唯一的直接子控件
  for view in self.subviews {
    // UISearchBarBackground与UISearchBarTextField是searchBar的简介子控件
    for subview in view.subviews {

      // 找到UISearchBarTextField
      if subview.isKindOfClass(UITextField.classForCoder()) {

        if let textFieldContentInset = contentInset { // 若contentInset被赋值
          // 根据contentInset改变UISearchBarTextField的布局
          subview.frame = CGRect(x: textFieldContentInset.left, y: textFieldContentInset.top, width: self.bounds.width - textFieldContentInset.left - textFieldContentInset.right, height: self.bounds.height - textFieldContentInset.top - textFieldContentInset.bottom)
        } else { // 若contentSet未被赋值
          // 设置UISearchBar中UISearchBarTextField的默认边距
          let top: CGFloat = (self.bounds.height - 28.0) / 2.0
          let bottom: CGFloat = top
          let left: CGFloat = 8.0
          let right: CGFloat = left
          contentInset = UIEdgeInsets(top: top, left: left, bottom: bottom, right: right)
        }
      }
    }
  }
}

searchBar.frame = CGRect(x: 80, y: 100, width: 200, height: 40)

searchBar.frame = CGRect(x: 80, y: 100, width: 200, height: 40)

// 设置contentInset
searchBar.contentInset = UIEdgeInsets(top: 0, left: 8, bottom: 0, right: 8)