Defer

.

A defer statement is used for executing code just before transferring program control outside of the scope that the defer statement appears in.

defer {
    statuments
}

“func f() {
    defer { print("First") }
    defer { print("Second") }
    defer { print("Third") }
    print("Four")
}
f()
// Prints "Four"
// Prints "Third"
// Prints "Second"
// Prints "First”

Alamofire中用defer来确保get和set中的操作能线程安全

subscript(task: URLSessionTask) -> Request? {
    get {
        lock.lock(); defer { lock.unlock() }
        return requests[task.taskIdentifier]
    }
    set {
        lock.lock(); defer { lock.unlock() }
        requests[task.taskIdentifier] = newValue
    }
}

defer 的 block 执行顺序和书写的顺序是相反的，这种相反的顺序是必要的，是为了确保每样东西的 defer block 在被创建的时候，该元素依然在当前上下文中存在。