Why NSTimer doesn't stop with invalidate? - objective-c

Question

I make a Cocoa application.I create a NSTimer,it can work,but can't stop with invalidate.The following is my code.Anyone else can help me?
#implementation Document
{
NSTimeInterval elapsedTime;
NSTimer *timer;
}
- (IBAction)start:(id)sender
{
elapsedTime = 0.002f;
dispatch_async(dispatch_get_main_queue(), ^{
if(timer == nil)
{
NSLog(#"Starting");
captureFrame = [[CaptureFrame alloc] init];//captureFrame is another object
timer = [NSTimer scheduledTimerWithTimeInterval:elapsedTime target:captureFrame selector:#selector(sendSingleImageImage) userInfo:nil repeats:YES];
NSThread *thread = [NSThread currentThread];
NSLog(#"Current thread is%#",thread);// I want to see the thread.
}
else
{
NSLog(#"Exist!");
[timer invalidate];
timer = nil;
}
});
[_stopButton setEnabled:YES];
[_startButton setEnabled:NO];
}
- (IBAction)stop:(id)sender
{
dispatch_async(dispatch_get_main_queue(), ^{
[timer invalidate];
timer = nil;
NSThread *sthread = [NSThread currentThread];
NSLog(#"S thread is %#",sthread);// I want to see the thread
});
[_startcastButton setEnabled:YES];
[_stopButton setEnabled:NO];
NSLog(#"Stopping");
}
I tried use the following method,but didn't work.
dispatch_async(dispatch_get_main_queue(), ^(void)block)
I also tried to find whether two actions in the same thread,and the result is yes.
So please tell me how to stop the NSTimer.
Any help is appreciated,thanks a lot in advance.