NSThread保留计数

Question

Why is the Retain count of my thread = 2?? It gets incremented after the start method why ?
How does Retain count work for NSThreads

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    NSThread *thread;

    @autoreleasepool
    {
        thread = [[NSThread alloc] initWithTarget:self selector:@selector(check) object:nil];
                    NSLog(@"RC == %lu",(unsigned long)[thread retainCount]);
        [thread start];
    }

    NSLog(@"RC == %lu",(unsigned long)[thread retainCount]);
}// presently stopped here on breakpoint

-(void)check{
       for (int i = 0 ; i< 100000; i++) {
           NSLog(@"NEW THREAD ==%d",i);
       }
}

@end

Answer 1

This is how it works, as you discovered: start will retain your NSThread so it does live through its execution. +[NSThread exit] will decrement that retain count once you are done with it.

On another hand, think about this: you are creating an NSThread and assigning its (retained) reference to a local variable. How are you going to decrement it? The local variable will not be visible outside of viewDidLoad , so you cannot release it.

A correct way of handling this is using an ivar for your your NSThread instance, so you can release it in dealloc , or use an autorelease d NSThread , counting on the fact that start will retain that object. So you can have:

        - (void)viewDidLoad {
            [super viewDidLoad];

            NSThread *thread;

            @autoreleasepool
            {
                thread = [[[NSThread alloc] initWithTarget:self selector:@selector(check) object:nil] autorelease];
                    NSLog(@"RC == %lu",(unsigned long)[thread retainCount]);
                [thread start];
            }

and everything will be correct.

I hope this explains why start retains the thread.