dispatch_suspend不起作用

Question

我是iOS编程新手。我使用以下代码 ViewController.h

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController

@property dispatch_queue_t background;
- (IBAction)stop_resume:(UIBarButtonItem *)sender;
@end

ViewController.m

#import "ViewController.h"
@interface ViewController ()
@end
@implementation ViewController
@synthesize background;
- (void)viewDidLoad
{
    [super viewDidLoad];
    background = dispatch_queue_create("background", NULL);
    dispatch_async(background, ^{
        for(int i=0; ; i++)
        {
            NSLog(@"Number is %d\n", i);
        }
    });
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
}

- (IBAction)stop_resume:(UIBarButtonItem *)sender
{
    dispatch_suspend(background);
}
@end

当我按下按钮后，后台线程不会被挂起，循环继续运行。