Objective c 为什么WaitForDataInBackground和Notify不'；无法使用具有多个参数的实时监视器？

我从其中一个API中找到了以下代码，并尝试使用它对vm_stat 1进行实时监控：

BOOL terminated = NO;

-(void)realTimeMonitor
{

NSTask *task = [[NSTask alloc] init];
[task setLaunchPath:@"/bin/bash"];
[task setArguments:[NSArray arrayWithObjects:@"-c", @"/usr/bin/vm_stat 1", nil]]; //works fine

// [task setArguments:[NSArray arrayWithObjects:@"-c", @"/usr/bin/vm_stat 1 | /usr/bin/awk '{print $1}'", nil]]; not working

NSPipe *pipe = [NSPipe pipe];
NSPipe *errorPipe = [NSPipe pipe];
[task setStandardOutput:pipe];
[task setStandardError:errorPipe];

NSFileHandle *outFile = [pipe fileHandleForReading];
NSFileHandle *errFile = [errorPipe fileHandleForReading];


[task launch];
[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(terminated:)
                                             name:NSTaskDidTerminateNotification
                                           object:task];

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(outData:)
                                             name:NSFileHandleDataAvailableNotification
                                           object:outFile];

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(errData:)
                                             name:NSFileHandleDataAvailableNotification
                                           object:errFile];


[outFile waitForDataInBackgroundAndNotify];
[errFile waitForDataInBackgroundAndNotify];
while(!terminated)
{
    if (![[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]])
    {
        break;
    }
}
}

-(void) outData: (NSNotification *) notification
{
NSFileHandle *fileHandle = (NSFileHandle*) [notification object];
NSData *data = [fileHandle availableData];

if ([data length]) {

    NSString *currentString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
    // do something 
}

[fileHandle waitForDataInBackgroundAndNotify]; //Checks to see if data is available in a background thread.
}


-(void) errData: (NSNotification *) notification
{
NSLog(@"errData");
NSFileHandle *fileHandle = (NSFileHandle*) [notification object];
[fileHandle waitForDataInBackgroundAndNotify];
}

- (void) terminated: (NSNotification *)notification
{
NSLog(@"Task terminated");
[[NSNotificationCenter defaultCenter] removeObserver:self];
terminated =YES;
}

========

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请看setArguments行。第一行工作得非常好“/usr/bin/vm_stat 1”，但是第二行不工作“/usr/bin/vm_stat 1 |/usr/bin/awk'{print$1}'”（根本没有输出），但是如果我在终端上运行它，它工作得非常好。这是为什么？

也许

awk

正在缓冲其标准输出流，因为它认为它不会将标准输出流写入tty设备

这可能最终导致完全死锁，因为如果

awk

的stdin被填满，并且

awk

的stdout上没有使用者，

vm_stat

可能无法写入其stdout

因此，请尝试使用其内置的

fflush（）

函数刷新

awk

的标准输出

[task setArguments:[NSArray arrayWithObjects:@"-c", @"/usr/bin/vm_stat 1 | /usr/bin/awk '{print $1; fflush();}'", nil]]; 

我可以使用（稍加修改）运行您的代码