# Swift Escaping Closures

* 當closure作為一個function的argument傳入，但會在function return後被繼續使用的話，就需要在parameter’s type 前面加上 @escaping

* 例如

    var completionHandlers: [() -> Void] = []
    func someFunctionWithEscapingClosure(completionHandler: @escaping () -> Void) {
        completionHandlers.append(completionHandler)
    }

* completionHandlers 是一個含 "() -> Void" 的 array
* someFunctionWithEscapingClosure 將一個closure做為argument (completionHandler) 傳入，append到completionHandlers中，做為之後使用。
* 因為closure在someFunctionWithEscapingClosure return後還需要被使用，如果沒有加上@escaping, 會發生compile-time error

* 當 escaping closure 使用到self是指 class的instance時，需要明確的將self標注出來以表示意圖跟提醒自己避免造成strong reference cycle. 因為該reference會在function return後繼續存活，沒注意的話可能就會有strong reference cycle發生。
* 例如

    // 有escaping
    var completionHandlers: [() -> Void] = []
    func someFunctionWithEscapingClosure(completionHandler: @escaping () -> Void) {
        completionHandlers.append(completionHandler)
    }
    
    // 沒有escaping
    func someFunctionWithNonescapingClosure(closure: () -> Void) {
        closure()
    }

    class SomeClass {
        var x = 10
        func doSomething() {
            someFunctionWithEscapingClosure { self.x = 100 } //有escaping
            someFunctionWithNonescapingClosure { x = 200 } // 沒有escaping
        }
    }
    
        let instance = SomeClass()
    
    // dosomething 在沒有escaping把x設為200,
    // 在escaping加入 {()->Void in self.x = 100 } 到completionHandlers中
    
    instance.doSomething() 
    print(instance.x) // Prints "200"

    completionHandlers.first?()
    print(instance.x) // Prints "100"
    
    
    // 也可以用capturelist來指稱self: [self] default 是strong
    class SomeOtherClass {
        var x = 10
        func doSomething() {
            someFunctionWithEscapingClosure { [self] in x = 100 }
            someFunctionWithNonescapingClosure { x = 200 }
        }
    }
    
    // weak 可以避免 strong reference cycle
    class AnotherClass {
        var x = 10
        func doSomething() {
            someFunctionWithEscapingClosure { [weak self] in
                guard let self = self else { return }
                self.x = 100
            }
            someFunctionWithNonescapingClosure { x = 200 }
        }
    }

* 如果self is an instance of a structure or an enumeration 就不用特別標示self(coz struct is not a reference),但因為Structures and enumerations don’t allow shared mutability, 在self is an instance of a structure or an enumeration的情況下，escaping closure根本也沒辦法capture a mutable reference to self

    struct SomeStruct {
    var x = 10
    mutating func doSomething() {
            someFunctionWithEscapingClosure { x = 100 }     // Error
            someFunctionWithNonescapingClosure { x = 200 }  // Ok
        }
    }

###### Referece: [swift.org](https://docs.swift.org/swift-book/documentation/the-swift-programming-language/closures#Closure-Expression-Syntax)
###### tags: `Swift`