当App进入到后台时候，会自动调用AppDelegate代理方法
- (void)applicationDidEnterBackground:(UIApplication *)application；接下来就是在applicationDidEnterBackground的代理方法中设置
- (void)applicationDidEnterBackground:(UIApplication *)application { [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:nil]; [self backTask];}- (void)backTask{ if ([[UIApplication sharedApplication] backgroundTimeRemaining] < 61.0) { [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:nil]; } NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:2 target:self selector:@selector(test) userInfo:nil repeats:YES]; self.timer = timer; self.i = 1;}- (void)test{ NSLog(@"这是第几次输出====%d",self.i); self.i++ ;}这样做就可以是App进入到后台仍可以运行，看到控制输出台上一直打印。
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