iOS7 Programming Cookbook 第十六章学习笔记

####Completing a Long-Running Task in the Background

#import "AppDelegate.h"

@interface AppDelegate ()
//UIBackgroundTaskIdentifier : 一个独特的标志,这个标志标识请求在后台运行
@property (nonatomic, unsafe_unretained) UIBackgroundTaskIdentifier backgroundTaskIdentifier;
@property (nonatomic, strong) NSTimer *myTimer;

@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{
    
    self.window = [[UIWindow alloc] initWithFrame:
                   [[UIScreen mainScreen] bounds]];
    
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];
    return YES;
}

//结束后台任务
- (void)endBackgroundTask{
    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    __weak AppDelegate *weakSelf = self;
    dispatch_async(mainQueue, ^(void) {
        AppDelegate *strongSelf = weakSelf;
        if (strongSelf != nil){
            //时间停止计时
            [strongSelf.myTimer invalidate];
            [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTaskIdentifier];
            //这个常数应该用于初始化变量或检查错误。
            strongSelf.backgroundTaskIdentifier = UIBackgroundTaskInvalid;
        }
        
    });
    
}

//一个布尔值表示是否支持多任务当前的设备上。(只读)
- (BOOL)isMultitaskingSupported{
    BOOL result = NO;
    if ([[UIDevice currentDevice] respondsToSelector:@selector(isMultitaskingSupported)]){
        result = [[UIDevice currentDevice] isMultitaskingSupported];
    }
    return result;
    
}

- (void)timerMethod:(NSTimer *)paramSender{
    
    NSTimeInterval backgroundTimeRemaining =
    [[UIApplication sharedApplication] backgroundTimeRemaining];
    
    if (backgroundTimeRemaining == DBL_MAX){
        NSLog(@"背景剩余时间=待定");
    } else {
        NSLog(@"Background Time Remaining = %.02f Seconds",
              backgroundTimeRemaining);
    }
    
}

//应用程序已经在后台
- (void)applicationDidEnterBackground:(UIApplication *)application{
    
    if ([self isMultitaskingSupported] == NO){
        return;
    }
    //添加方法
    self.myTimer = [NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:@selector(timerMethod:) userInfo:nil repeats:YES];
    //标志着一个新的开始长时间运行的后台任务。
    self.backgroundTaskIdentifier = [application beginBackgroundTaskWithExpirationHandler:^(void) {
        [self endBackgroundTask];
    }];
    
}

//应用程序即将进入前景
- (void)applicationWillEnterForeground:(UIApplication *)application{
    if (self.backgroundTaskIdentifier != UIBackgroundTaskInvalid){
        [self endBackgroundTask];
    }
    
}

@end

Reference

• O’Reilly Shop iOS 7 Programming Cookbook