dispatch_async 更新变量崩溃 - 线程安全

Question

当我运行以下代码多次应用程序崩溃时：

res.append(i)

错误是致命错误：UnsafeMutablePointer.destroy 计数为负数要么正在释放的指针未分配 *** 在 malloc_error_break 中设置断点以调试

在dispatch_async里面更新一个全局变量不对吗？

class ViewController: UIViewController {

var result = Array<Int>()


func method(completion: (inner: () throws -> String)->Void){
    let group:dispatch_group_t = dispatch_group_create();
    let queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
    var res = Array<Int>()
    for i in 0..<4 {
        dispatch_group_async(group,queue){
            res.append(i)
            print(res)
            print("Block\(i)");                
            var s = 0
            for k in 0..<1000 {
                s = 2+3
            }
            print("Block\(i)End");



        }
    }

    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

    print("All background tasks are done!!");
    print(res)
}


override func viewDidLoad() {
    super.viewDidLoad()


    self.method() { (inner: () throws -> String) -> Void in
        do {
            let res = try inner()
            print(res)
        } catch let error {
            print(error)
        }
    }
}

Answer 1

是的，Array不是线程安全的，所以写入数组时，应该确保atomic。

所以可以加高性能锁：dispatch_semaphore_t.

func method(completion: (inner: () throws -> String)->Void){
    //  add lock
    let lock: dispatch_semaphore_t = dispatch_semaphore_create(1)
    let group:dispatch_group_t = dispatch_group_create();
    let queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
    var res = Array<Int>()
    for i in 0 ..< 5 {
        dispatch_group_async(group,queue){
            // lock
            dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER)
            res.append(i)
            // unlock
            dispatch_semaphore_signal(lock)
            var s = 0
            for k in 0..<1000 {
                s = 2+3
            }
        }
    }

    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

    print("All background tasks are done!!");
    print(res)
}

但是注意，如果你的异步任务不是像上面这样的耗时操作，请不要使用多线程，因为线程调度很耗时，可能会导致性能损失。

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