为什么将 DispatchQueue 延迟函数放在另一个 DispatchQueue 中在 Swift 中不起作用

问题描述

预期行为:

对于 i = 0，打印语句在 0 秒后调用.

For i = 0, print statement called after 0 second .

对于 i = 1，print 语句在 1.1 秒后调用

For i = 1, print statement called after 1.1 seconds

对于 i = 2，打印语句在 2.2 秒后调用

For i = 2, print statement called after 2.2 seconds

实际行为:

分别在 0、1、2、3 秒后调用打印语句，即忽略内部延迟函数.

Print statement called after 0, 1, 2, 3 seconds respectively i.e. inner delay function is ignored.

为什么会出现这种差异?

So why the discrepancy?

    for i in 0...3 {

        DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(i), execute: {

            DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(i/10), execute: {
                print("function was called")
            })
        })
    }

推荐答案

有多种方法可以做到这一点.这里有 3 个:

There are a number of ways you could do this. Here are 3:

使用计时器每秒重复一个块

var i = 0
Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { timer in
    i += 1
    print("hello (i)")
    if i == 5 {
        timer.invalidate()
    }
}

分派多个异步任务

for i in 1...5 {
    DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(i)) {
        print("hello (i)")
    }
}

注意:这会将所有任务排在前面，如果有大量任务，队列可能会溢出.

Note: This queues all of the tasks up front and could overflow a queue if there were a large number of them.

在后台运行循环并睡眠并切换到前台打印

DispatchQueue.global().async {
    for i in 1...5 {
        sleep(1)
        DispatchQueue.main.async {
            print("hello (i)")
        }
    }
}

注意:如果循环中的工作需要大量时间，这会稍微漂移(不是相隔 1 秒).

Note: This will drift a little (not be 1 second apart) if the work in the loop takes a significant amount of time.

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