当类型是关联类型时，无法将类型的值转换为预期的参数类型

Posted 2023-03-14

【中文标题】当类型是关联类型时，无法将类型的值转换为预期的参数类型

【英文标题】：Cannot convert value of type to expected argument type when the type is an associated type

【发布时间】：2018-02-07 08:20:28

【问题描述】：

这里是 swift 4 代码：

protocol BarProtocol 
    associatedtype Bar
    var bar:Bar? get set


class FooClass : BarProtocol 
    //typealias Bar = (String) -> ()
    typealias Bar = (String)
    var bar: Bar?


func configFooClass<T:BarProtocol>(fooClass:inout T, bar:T.Bar) 
    fooClass.bar = bar

func setupFooClass(fooClass:inout FooClass, bar:FooClass.Bar) 
    configFooClass(fooClass: &fooClass, bar: bar)


var fooClass = FooClass()
var barString = "barString"
//let barAction:(String) -> () =  text in
//    print(text)
//

setupFooClass(fooClass:&fooClass, bar:barString)
print(fooClass.bar) // print: Optional("barString")

到目前为止一切顺利，它将在控制台中打印“Optional("barString")”。 FooClass类中的关联类型现在是String类型，所以它可以存储字符串值“barString”。

但是如果我把这段代码改成：

protocol BarProtocol 
    associatedtype Bar
    var bar:Bar? get set


class FooClass : BarProtocol 
    typealias Bar = (String) -> ()
//    typealias Bar = (String)
    var bar: Bar?


func configFooClass<T:BarProtocol>(fooClass:inout T, bar:T.Bar) 
    fooClass.bar = bar

func setupFooClass(fooClass:inout FooClass, bar:FooClass.Bar) 
    configFooClass(fooClass: &fooClass, bar: bar) 
// Error here: Cannot convert value of type '(String) -> ()' to expected argument type '_.Bar'


var fooClass = FooClass()
var barString = "barString"
let barAction:(String) -> () =  text in
    print(text)


//setupFooClass(fooClass:&fooClass, bar:barString)
//print(fooClass.bar)
setupFooClass(fooClass:&fooClass, bar:barAction)
fooClass.bar!("lalalalala")

其实我只是把FooClass的关联类型从String改成(String) -> ()，会报错：Cannot convert value of type '(String) -> ()' to expected argument type '_.Bar '

我不明白，似乎闭包可以作为函数的参数传递。但是为什么'String'类型很好用，而'(String) -> ()'却不行呢？

【参考方案1】：

看来加个@escaping 就可以解决这个问题了……

func setupFooClass(fooClass:inout FooClass, bar:@escaping FooClass.Bar) 
    configFooClass(fooClass: &fooClass, bar: bar)