How to gradually retrieve data from NSTask?

Question

I am working on a GUI (Cocoa) for a command-line tool to make it more accessible to people. Despite it being a GUI, I would like to display the output to an NSTextView. The problem is that the output is large and the analysis the tool carries out can take hours/days.

Normally, when working with NSTask and NSPipe, the output is displayed only after the task is completely finished (which can take a long time). What I want to do is split the output up and display it gradually (updating every minute for example).

So far I have placed the processing of the data in a separate thread:

[NSThread detachNewThreadSelector:@selector(processData:) toTarget:self withObject:raxmlHandle];

- (void)processData:(id)sender {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    NSString *startString = [results string];
    NSString *newString = [[NSString alloc] initWithData:[raxmlHandle readDataToEndOfFile] encoding:NSASCIIStringEncoding];
    [results setString:[startString stringByAppendingString:newString]];
    [startString release];
    [newString release];
    [pool release];
}

All this is still a bit of voodoo to me and I am not exactly sure how to deal with this challenge.

Do you have any suggestions or recommendations?

Thanks!

Answer 1

You need to use a notification provided by NSFileHandle.

First, add yourself as an observer to the NSFileHandleReadCompletionNotification

[[NSNotificationCenter defaultCenter] addObserver:self  
                                         selector:@selector(outputReceived:) 
                                             name:NSFileHandleReadCompletionNotification
                                           object:nil];

Then, prepare a task, call readInBackgrounAndNotify of the file handle of the pipe, and launch the task.

NSTask*task=[[NSTask alloc] init];
[task setLaunchPath:...];

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

// this causes the notification to be fired when the data is available
[[pipe fileHandleForReading] readInBackgroundAndNotify];  

[task launch];

Now, to actually receive the data, you need to define a method

-(void)outputReceived:(NSNotification*)notification{
    NSFileHandle*fh=[notification object];
    // it might be good to check that this file handle is the one you want to read
    ...

    NSData*d=[[aNotification userInfo] objectForKey:@"NSFileHandleNotificationDataItem"];
    ... do something with data ...
}

You might want to read Notification Programming Topics to understand what is going on.