Swift 在使用 NSTimer 时无法输出 [英] Swift cannot output when using NSTimer

问题描述

我尝试使用 NSTimer 每秒输出到 STDOUT.

I tried output to STDOUT every second by using NSTimer.

我编写了以下代码并将其保存为 sample.swift.

I wrote the following code and saved it as sample.swift.

#!/usr/bin/swift

import AppKit

class Foo : NSObject {
  override init() {
    super.init()
    NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: "bar", userInfo: nil, repeats: true)
  }

  func bar() {
    print("buz")
  }
}

let h = Foo()

// Output successful
// while true {
//   print("Foo!")
// }

NSRunLoop.mainRunLoop().run()

然后，我执行了下面的命令，可以看到buz.

Then, I executed the following command and can see buz.

okada@iyokan$ ./sample.swift
buz
buz
buz
buz

所以，我执行了以下命令，但我看不到任何buz.

So, I executed the following command but I cannot see any buz.

okada@iyokan$ ./sample.swift > sample.log # wait 5 seconds
okada@iyokan$ cat sample.log
okada@iyokan$ 

顺便说一下，我尝试了while循环版本(激活注释掉上面的代码)然后我可以通过两个程序获得buz.

By the way, I tried while loop version (activated comment out the above code) then I could get buz by two procedures.

为什么我找不到任何buz?请教我.

Why I could not get any buz? Please teach me.

推荐答案

这是 stdio 库的正常"行为.文件描述符可以缓冲"，这意味着所有 printf() 和其他输出操作首先进入内部缓冲区.当那个缓冲区达到一定大小(例如 8KB)，将其内容写入实际文件.

That is a "normal" behaviour of the stdio library. A file descriptor can be "buffered", which means that the result of all printf() and other output operations goes to an internal buffer first. When that buffer reaches a certain size (e.g. 8KB), its contents is written to the actual file.

共有三种模式:unbuffered"(所有输出都写入文件立即)，行缓冲"(收集输出直到换行字符被打印)，和完全缓冲".

There are three modes: "unbuffered" (all output is written to the file immediately), "line buffered" (output is collected until a newline characters is printed), and "fully buffered".

根据 stdout 行是缓冲的、未缓冲的还是不确定的默认?:

Unix 惯例是 stdin 和 stdout 在与终端关联时是行缓冲的，否则为全缓冲(又名块缓冲).

Unix convention is that stdin and stdout are line-buffered when associated with a terminal, and fully-buffered (aka block-buffered) otherwise.

这解释了为什么你看到每个打印操作的输出当输出到达终端时立即，但不是当输出被重定向到一个文件.

This explains why you see the output of each print operation immediately when the output goes to the terminal, but not when output is redirected to a file.

与

print("buz")
fflush(stdout)

您可以强制将输出立即写入文件.或者，您可以将标准输出设置为行缓冲模式通过调用

you can force the output to be written to the file immediately. Alternatively, you can set standard output into line buffering mode by calling

setlinebuf(stdout)

在打印任何东西之前.

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