为什么UIButton使用目标动作设计模式而不是UITextField的委派模式和反之

Question

Can any one help in understanding why Apple decides to uses Target Action design pattern for event handling of UIButton not Delegate Pattern?

Or, I can say why Apple choose delegation Design pattern for UITextField even same thing can be achieved by Target Action also.

Answer 1

There are various trade-offs to the different approaches. I think the clincher for button actions, though, is that you can add multiple target/actions to a button. (A one-to-many relationship.) Delegation is a one-to-one relationship, so having a button trigger multiple actions, possibly to different targets, would not be possible with the delegation design pattern.

I think if Apple were designing button handling now they'd use blocks/closures instead of IBActions. The control could hold an array of blocks and the events that trigger each.

Answer 2

All @IBAction functions have one of three signatures;

• functionName() -> Void
• functionName(sender: Any) -> Void
• functionName(sender: Any, forEvent event: UIEvent) -> Void

An IBAction cannot accept any other arguments and does not return a value.

A delegate allows the use of functions that take different parameters and return values while actions are a standard approach across UIView subclasses.

In some classes, such as UITextField , certain action can be handled through both delegate and action methods. I guess this just gives you some choice; if you are already implementing delegate functions then you don't need to implement action handlers as well.

Answer 3

这是由于Apple希望在代码中显示不同的模式。