使用 Grand Central Dispatch 进行文件监控

Question

我正在使用 David Hamrick 的代码示例来使用 GCD 监视文件。

int fildes = open("/path/to/config.plist", O_RDONLY);

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_VNODE,fildes, 
                                                  DISPATCH_VNODE_DELETE | DISPATCH_VNODE_WRITE | DISPATCH_VNODE_EXTEND | DISPATCH_VNODE_ATTRIB | DISPATCH_VNODE_LINK | DISPATCH_VNODE_RENAME | DISPATCH_VNODE_REVOKE,
                                                  queue);
dispatch_source_set_event_handler(source, ^
{
    //Reload the config file
});
dispatch_source_set_cancel_handler(source, ^
{
    //Handle the cancel
});
dispatch_resume(source);

我想用它来监视plist的更改。我在第一次更改后收到通知，但不会收到以下更改。为什么？

Answer 1

当收到 DISPATCH_VNODE_DELETE 时，您确实可以重新打开文件并重新注册源（删除前一个源）。或者您可以使用专为此类场景设计的调用，即dispatch_io_create_with_path() - 它不仅会按路径监视，还会为您打开文件并让您异步读取内容。

既然您问了（不确定您要求哪种技术，但这是最简单的），这里有一个独立的代码示例：

#include <dispatch/dispatch.h>
#include <stdio.h>

int main(int ac, char *av[])
{
  int fdes = open("/tmp/pleasewatchthis", O_RDONLY);
  dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
  void (^eventHandler)(void), (^cancelHandler)(void);
  unsigned long mask = DISPATCH_VNODE_DELETE | DISPATCH_VNODE_WRITE | DISPATCH_VNODE_EXTEND | DISPATCH_VNODE_ATTRIB | DISPATCH_VNODE_LINK | DISPATCH_VNODE_RENAME | DISPATCH_VNODE_REVOKE;
  __block dispatch_source_t source;

  eventHandler = ^{
    unsigned long l = dispatch_source_get_data(source);
    if (l & DISPATCH_VNODE_DELETE) {
      printf("watched file deleted!  cancelling source\n");
      dispatch_source_cancel(source);
    }
    else {
      // handle the file has data case
      printf("watched file has data\n");
    }
  };
  cancelHandler = ^{
    int fdes = dispatch_source_get_handle(source);
    close(fdes);
    // Wait for new file to exist.
    while ((fdes = open("/tmp/pleasewatchthis", O_RDONLY)) == -1)
      sleep(1);
    printf("re-opened target file in cancel handler\n");
    source = dispatch_source_create(DISPATCH_SOURCE_TYPE_VNODE, fdes, mask, queue);
    dispatch_source_set_event_handler(source, eventHandler);
    dispatch_source_set_cancel_handler(source, cancelHandler);
    dispatch_resume(source);
  };

  source = dispatch_source_create(DISPATCH_SOURCE_TYPE_VNODE,fdes, mask, queue);
  dispatch_source_set_event_handler(source, eventHandler);
  dispatch_source_set_cancel_handler(source, cancelHandler);
  dispatch_resume(source);
  dispatch_main();
}

Answer 2

经过一番研究 我发现：

=> 我得到了旗帜

DISPATCH_VNODE_DELETE

我正在监控

~/Library/Preferences/com.apple.dock.plist

正如我发现的那样，更改扩展坞方向，删除原始文件并将其替换为新文件。

因此，监控停止。

同一作者提出了一个解决方案，在删除的情况下，GCD块调用自身来继续监控。