UITextField似乎未启用"Touch Up Inside"，但"Touch Down"有效.

It seems "Touch Up Inside" is not enabled for UITextField, but "Touch Down" works.

因此解决方案如下:

Swift 4.x

myTextField.addTarget(self, action: #selector(myTargetFunction), for: .touchDown)
@objc func myTargetFunction(textField: UITextField) {
    print("myTargetFunction")
}

Swift 3.x

myTextField.addTarget(self, action: #selector(myTargetFunction), for: UIControlEvents.touchDown)

@objc func myTargetFunction(textField: UITextField) {
    print("myTargetFunction")
}