Question

I am doing this multithreading application just to see how the @synchronized directive works.I’ve read that if all the threads have the same object as argument of @synchronized, then they all wait on the same lock.So I thought to use self as argument, since it’s the same for all threads.
In this application there is a text field being edited multiple times by all the thread.I don’t care about performance, it’s just a test so I don’t put the @synchronized directive before the for, but inside it.

The properties that I use:

@property (weak) IBOutlet NSTextField *textField;
@property (nonatomic, copy) NSNumber* value;
@property (nonatomic,copy) NSMutableArray* threads;

The code:

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    value= @0;
    textField.objectValue= value;
    for(int i=0; i<10; i++)
    {
        NSThread* thread=[[NSThread alloc]initWithTarget: self selector: @selector(routine:) object: nil];
        [threads addObject: thread];
        [thread start];
    }
}

- (void) routine : (id) argument
{
    for(NSUInteger i=0; i<100; i++)
    {
        @synchronized(self)
        {
            value= @(value.intValue+1);
            textField.objectValue= value;
        }
    }
}

Sometimes the application has success and I see 1000 as text field value.But sometimes not, I fear that this is starvation and I don’t see anything on the text field, it’s empty.I tried the debug but it’s hard to see what is wrong, since the criteria of failure seems casual to me, sometimes it just works fine.

SOLUTION

@synthesize threads,value, textField;

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    value= @0;
    threads=[[NSMutableArray alloc]initWithCapacity: 100];
    for(NSUInteger i=0; i<100; i++)
    {
        NSThread* thread=[[NSThread alloc]initWithTarget: self selector: @selector(routine:) object: nil ];
        [threads addObject: thread];
        [thread start];
    }
}

- (void) routine : (id) arg
{
    for(NSUInteger i=0; i<1000; i++)
    {
        @synchronized(self)
        {
            value= @(value.intValue+1);
            [textField performSelectorOnMainThread: @selector(setObjectValue:) withObject: value waitUntilDone: NO];
        }
    }
}

Answer 1

You are accessing an NSTextField, which is a subclass of NSView, from a non-main thread. That is not a safe thing to do. The results are undefined; sometimes it appears to work, sometimes it doesn’t.