什么是ios的delegate

2024-11-24

delegate design pattern

UITextView对protocol的应用

protocol CanDealWithUITextFields{
	func textFieldDidBeginEditing()
}

这样一来，每个应用这个protocol的子类就必须拥有这些textField的生命周期function，而且必须在自己类的内部定义。

事实上，这个protocol叫做UITextFieldDelegate。

protocol UITextFieldDelegate{
	func textFieldDidBeginEditing()
}

delegate内部的方法也可以是可选的Optional

// UITextFieldDelegate 的简化定义（实际上是 Objective-C 定义的）
@objc protocol UITextFieldDelegate: NSObjectProtocol {
    // 注意 @objc optional 关键字
    @objc optional func textFieldShouldBeginEditing(_ textField: UITextField) -> Bool
    @objc optional func textFieldDidBeginEditing(_ textField: UITextField)
    @objc optional func textFieldShouldEndEditing(_ textField: UITextField) -> Bool
    @objc optional func textFieldDidEndEditing(_ textField: UITextField)
    // ... 还有更多方法
}

UITextField的内部结构

这样一来，所有UITextField就可以复用这些method

var delegate: UITextFieldDelegate //property
delegate.textFieldDidBeginEditing() //method

UITextFieldDelegate的具体实现

let textField = UITextField()
textField.delegate = self //即等于类自己，所以可以重新定义protocol的相关方法
func textFieldDidBeginEditing(){
	//do something
}

关键点：

1. UITextFieldDelegate 协议定义了能做什么，即have the functions.
2. WeatherViewController 实现这些方法定义了怎么做，即define the function.
3. delegate = self 告诉 UITextField 谁来做，即call the function.

过程示例：

delegate的急救案例

protocol AdvancedLifeSupport{
    func performCPR()
}

class EmergencyCallHandler{
    var delegate: AdvancedLifeSupport?
    
    func assessSituation(){
        print("Can you tell me what happened?")
    }
    
    func medicalEmergency(){
        delegate?.performCPR()
    }
}

struct Parametic: AdvancedLifeSupport{
    init(handler: EmergencyCallHandler){
        handler.delegate = self
    }
    
    func performCPR() {
        print("The paramedic does chest compressions, 30 per second.")
    }
}

class Doctor: AdvancedLifeSupport{
    init(handler: EmergencyCallHandler){
        handler.delegate = self
    }
    
    func performCPR() {
        print("The doctor does chest compressions, 30 per second.")
    }
    
    func useStethescope(){
        print("Listening for heart sounds.")
    }
}

class Surgeon: Doctor{
    
    override func performCPR() {
        super.performCPR()
        print("Sings stying alive by the BeeGees.")
    }
    
    func useElectricDrill(){
        print("Whirr..")
    }
}

let emilio = EmergencyCallHandler()
//let pete = Parametic(handler: emilio)

let angela = Surgeon(handler: emilio)

emilio.assessSituation()
emilio.medicalEmergency()