ios – 发送消息时自我保留

@implementation TestObj

- (void)dealloc
{
    NSLog(@"%@,is main thread %u",NSStringFromSELEctor(_cmd),[NSThread ismainThread]) ;
}

- (void)doSomethingInBACkground
{
    [self performSELEctorInBACkground:@SELEctor(BACkgroundWork) withObject:nil] ;
}

- (void)BACkgroundWork
{
    sleep(5) ;
}

@end

我分配并初始化实例并向其发送doSomethingInBACkground消息并为其分配nil以便尽快释放它.

TestObj *obj = [[TestObj alloc] init] ;
[obj doSomethingInBACkground] ;
obj = nil ;

我发现dealloc将在大约5秒后运行obj = nil;,当发送方法时,系统似乎保留self [self performSELEctorInBACkground：@SELEctor(BACkgroundWork)withObject：nil];并在BACkgroundWork返回后,实例将被释放.

任何人都可以告诉我系统背后的工作.谢谢.