在swift中使用带有多个参数的闭包

Question

这个问题很大程度上基于这个问题:

链接

主要区别在于我也想将参数传递给闭包.说我有这样的事情:

func someFunctionThatTakesAClosure(completionClosure: (venues: Dictionary<String, AnyObject>, error: NSError) -> ()) {
        // function body goes here
        var error: NSError?
        let responseDictionary: Dictionary<String, AnyObject> = ["test" : "test2"]
        completionClosure(venues: responseDictionary, error: error!)
    }

这里没有错误.但是当我在主视图控制器中调用此函数时,我尝试了几种方法,但所有结果都会导致不同的错误:

venueService.someFunctionThatTakesAClosure(completionClosure(venues: Dictionary<String, AnyObject>, error: NSError){

        })

或者像这样:

venueService.someFunctionThatTakesAClosure((venues: Dictionary<String, AnyObject>, error: NSError){

        })

或者甚至喜欢这样:

venueService.someFunctionThatTakesAClosure(completionClosure: (venues: Dictionary<String, AnyObject>, error: NSError) -> (){

            });

我可能只是累了,但任何帮助都会非常感激!

Answer 1

venueService.someFunctionThatTakesAClosure({
  venues, error in
  // do stuff
})

您可以选择指定类型(但由于编译器知道闭包应该提供什么类型,因此您不必严格要求:

venueService.someFunctionThatTakesAClosure({
  (venues: Dictionary<String, AnyObject>, error: NSError) -> () in
  // do stuff
})

但是我在你的调用代码中看到另一个问题:

completionClosure(venues: responseDictionary, error: error!)
//            No Bueno. What if there's no error?    ^^^^^^

你不应该强行打开错误,因为它完全有可能它仍然是零.强制展开nil会导致错误.所以你想要这个:

completionClosure(venues: responseDictionary, error: error)

并且您想要更改闭包以获取可选错误.总共:

func someFunctionThatTakesAClosure(completionClosure: (venues: Dictionary<String, AnyObject>, error: NSError?) -> ()) {
    …
    completionClosure(venues: responseDictionary, error: error)
}

// when calling:
venueService.someFunctionThatTakesAClosure({
  (venues: Dictionary<String, AnyObject>, error: NSError?) -> () in
  // do stuff
})

如果你想传递额外的参数,请记住swift是针对作为最后一个参数传递的闭包进行优化的(并且它是objective-c API中广泛遵循的约定):

// in venueService:
func someArgAndClosureFunction(arg1: String, arg2: Int, 
                  completionClosure: (venues: Dictionary<String, AnyObject>, error: NSError?) -> ()) {
    // do stuff
}

// When calling:
venueService.someArgAndClosureFunction("string arg 1", arg2: 10) {
  (venues: Dictionary<String, AnyObject>, error: NSError?) -> () in
  // do stuff
}

在这个例子中,我使用了尾随闭包语法,它允许你在函数调用parens 之外传递闭包(但它仍然作为最后一个参数传递).