在我编写的与 WeatherUnderground 一起工作的类中,我需要许多函数都几乎与此函数相同,在这种形式下可以正常工作:
func currentConditions(completion: (result: WUConditionResponse?) -> Void) -> Request {
url = urlBuilder("conditions")
let request = Alamofire.request(.GET, url)
.validate()
.responseObject { (response: Result<WUConditionResponse, NSError>) in
completion(result: response.value)
}
return request
}
我没有单独写它们,而是尝试了这个:
private typealias AFCompletion = (result: EVObject?) -> Void
private func current(command: String, onCompletion: AFCompletion) -> Request {
let function = {(completion: AFCompletion) -> Request in
self.url = self.urlBuilder(command)
let request = Alamofire.request(.GET, self.url)
.validate()
.responseObject { (response: Result<EVObject, NSError>) in
completion(result: response.value!)
}
return request
}
return function(onCompletion)
}
紧随其后
func currentConditions(completion: (result: WUConditionResponse?) -> Void) -> Request {
let myCompletion = completion as! AFCompletion
return current("conditions", onCompletion: myCompletion)
}
其中 WUConditionResponse
是 EVObject
的子类。然后,我将构造与语句一起使用,与工作中的内容保持不变:
foo.currentConditions { (conditions: WUConditionResponse?) in
print(conditions)
}
通过这种方式,我在向下转换为 AFCompletion 时遇到了 EXC_BAD_INSTRUCTION
错误。我不知所措地看到了这个问题。为什么会出现 downcast 错误?尽管有完整的答案,我该如何进一步调试?
最佳答案
按照您尝试的方式,这是不可能的。你传入的闭包
foo.currentConditions { (conditions: WUConditionResponse?) in
print(conditions)
}
接受仅 WUConditionResponse
,EVObject
的一个特定子类。如果将该闭包转换为 AFCompletion
,则生成的闭包必须接受 every EVObject
,而它不接受。
您可以做的是创建第二个调用完成
如果实际传入的对象是正确的类型而不是转换:
func currentConditions(completion: (result: WUConditionResponse?) -> Void) -> Request {
let myCompletion : AFCompletion = { res in
if res != nil || res! is WUConditionResponse {
return completion(res as! WUConditionResponse?)
}
return false // default return
}
return current("conditions", onCompletion: myCompletion)
}
关于ios - 关闭向下转换导致 EXC_BAD_INSTRUCTION 错误,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/37230150/