app后台运行

1.info.plist  添加1行，选择 required background modes ， item0 写 audio (ios8会变成默认的字符串)

2. 添加avfoundation库   

3. appdelegate里面添加头文件 #import <AVFoundation/AVFoundation.h>
4  @implementation AppDelegate
{
    NSTimer * backGroudTimer;
    NSInteger backGroudNum;
}

5. - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

 NSError * categoryErr = nil;
    NSError * activationErr = nil;
    [[AVAudioSession sharedInstance]setCategory:AVAudioSessionCategoryPlayback error:&categoryErr];
    [[AVAudioSession sharedInstance]setActive:YES error:&activationErr];
    
    backGroudTimer = [NSTimer scheduledTimerWithTimeInterval:10.0 target:self selector:@selector(startBackgroud) userInfo:nil repeats:YES];
    [backGroudTimer setFireDate:[NSDate distantFuture]];

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-(void)startBackgroud{
    NSLog(@"正在后台运行");
    backGroudNum ++;
    
}

6.

-(void) applicationDidEnterBackground:(UIApplication *)application
{

 UIApplication * app = [UIApplication sharedApplication];
    __block UIBackgroundTaskIdentifier bgtask;
    bgtask = [app beginBackgroundTaskWithExpirationHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
            if (bgtask !=UIBackgroundTaskInvalid) {
                bgtask = UIBackgroundTaskInvalid;
            }
        });
    }];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_async(dispatch_get_main_queue(), ^{
            if (bgtask !=UIBackgroundTaskInvalid) {
                bgtask = UIBackgroundTaskInvalid;
            }
        });
    });
    [backGroudTimer setFireDate:[NSDate distantPast]];
    backGroudNum = 0;
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7.- (void)applicationDidBecomeActive:(UIApplication *)application
{

[backGroudTimer setFireDate:[NSDate distantFuture]];
    NSLog(@"后台计数的值是: %ld",backGroudNum);

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