iOS 后台处理

iOS 后台处理的常见用途

1、进入后台时候删除资源：应用处于挂起状态的时候所占用的资源越少，该应用被iOS终止的风险就越低。通过从内存中清理那些易于重新创建的资源，可以增加应用驻留内存的机会，因此可以大幅加快重启速度。

2、进入后台时候保存状态:保存与用户执行的操作相关的所有信息，这样的话，用户下次回来的时候，依然可以恢复到他们离开时的进度。

3、延时执行，请求更多的后台时间：如果进入后台花费了很多时间，应用可能会从内存中移除，如果应用正在进行文件传输，没有能够完成的话，将会带来很多不便。我们可以将applicationDidEnterBackground 作为平台，告诉系统，你还有额外的工作要做，然后启动一个程序块，真正的执行该工作。

例解：

@property (strong, nonatomic) UILabel *label;
@property (strong, nonatomic) UIImage *smiley;
@property (strong, nonatomic) UIImageView *smileyView;
@property (strong, nonatomic) UISegmentedControl *segmentedControl;

@implementation BIDViewController {
    BOOL animate;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    CGRect bounds = self.view.bounds;
    CGRect labelFrame = CGRectMake(bounds.origin.x, CGRectGetMidY(bounds) - ,
                                   bounds.size.width, );

    //转动的Label
    self.label = [[UILabel alloc] initWithFrame:labelFrame];
    self.label.font = [UIFont fontWithName:@"Helvetica" size:];
    self.label.text = @"Bazinga!";
    self.label.textAlignment = NSTextAlignmentCenter;
    self.label.backgroundColor = [UIColor clearColor];

    //笑脸
    CGRect smileyFrame = CGRectMake(CGRectGetMidX(bounds) - ,
                                    CGRectGetMidY(bounds)/ - ,
                                    , );
    self.smileyView = [[UIImageView alloc] initWithFrame:smileyFrame];
    self.smileyView.contentMode = UIViewContentModeCenter;
    NSString *smileyPath = [[NSBundle mainBundle] pathForResource:@"smiley"
                                                           ofType:@"png"];

    self.smiley = [UIImage imageWithContentsOfFile:smileyPath];
    self.smileyView.image = self.smiley;

    //分段器
    self.segmentedControl = [[UISegmentedControl alloc] initWithItems:
                             [NSArray arrayWithObjects:
                              @"One", @"Two", @"Three", @"Four", nil]] ;
    self.segmentedControl.frame = CGRectMake(bounds.origin.x + ,
                                             ,
                                             bounds.size.width - , );

    [self.view addSubview:self.segmentedControl];
    [self.view addSubview:self.smileyView];
    [self.view addSubview:self.label];

    NSNumber *indexNumber = [[NSUserDefaults standardUserDefaults]
                             objectForKey:@"selectedIndex"];
    if (indexNumber) {
        NSInteger selectedIndex = [indexNumber intValue];
        self.segmentedControl.selectedSegmentIndex = selectedIndex;
    }

    //通知
    NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
    [center addObserver:self
               selector:@selector(applicationWillResignActive)
                   name:UIApplicationWillResignActiveNotification
                 object:nil];
    [center addObserver:self
               selector:@selector(applicationDidBecomeActive)
                   name:UIApplicationDidBecomeActiveNotification
                 object:nil];

    [center addObserver:self
               selector:@selector(applicationDidEnterBackground)
                   name:UIApplicationDidEnterBackgroundNotification
                 object:nil];
    [center addObserver:self
               selector:@selector(applicationWillEnterForeground)
                   name:UIApplicationWillEnterForegroundNotification
                 object:nil];
}

//Label 向下转动
- (void)rotateLabelDown
{
    [UIView animateWithDuration:0.5
                     animations:^{
                         self.label.transform = CGAffineTransformMakeRotation(M_PI);
                     }
                     completion:^(BOOL finished){
                         [self rotateLabelUp];
                     }];
}

//Label 向上转动
- (void)rotateLabelUp
{
    [UIView animateWithDuration:0.5
                     animations:^{
                         self.label.transform = CGAffineTransformMakeRotation();
                     }
                     completion:^(BOOL finished){
                         if (animate) {
                             [self rotateLabelDown];
                         }
                     }];
}

//离开活动状态
- (void)applicationWillResignActive
{
    NSLog(@"VC: %@", NSStringFromSelector(_cmd));
    animate = NO;
}

//进入活动状态
- (void)applicationDidBecomeActive
{
    NSLog(@"VC: %@", NSStringFromSelector(_cmd));
    animate = YES;
    [self rotateLabelDown];
}

//后台运行
- (void)applicationDidEnterBackground
{
    NSLog(@"VC: %@", NSStringFromSelector(_cmd));
    //先获取共享的UIApplication 实例。
    UIApplication *app = [UIApplication sharedApplication];

    //声明了taskId变量、并用__block修饰。
    __block UIBackgroundTaskIdentifier taskId;

    //告诉系统，需要更多的时间来完成某件事，并承诺在完成之后告诉它。如果系统判定我们运行了太久时间并决定停止运行，可以调用我们做为参数提供的程序块
    taskId = [app beginBackgroundTaskWithExpirationHandler:^{
        NSLog(@"Background task ran out of time and was terminated.");
        [app endBackgroundTask:taskId];
    }];

    //如果系统返回的值是UIBackgroundTaskInvalid，表明系统没有为我们提供任何多余的时间。
    if (taskId == UIBackgroundTaskInvalid) {
        NSLog(@"Failed to start background task!");
        return;
    }

    //
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, ),
    ^{
        NSLog(@"Starting background task with %f seconds remaining",
              app.backgroundTimeRemaining);

        self.smiley = nil;
        self.smileyView.image = nil;

        //存储segmentedControl的位置
        NSInteger selectedIndex = self.segmentedControl.selectedSegmentIndex;
        [[NSUserDefaults standardUserDefaults] setInteger:selectedIndex
                                                   forKey:@"selectedIndex"];

        //模拟一个25秒的过程
        [NSThread sleepForTimeInterval:];

        NSLog(@"Finishing background task with %f seconds remaining",
              app.backgroundTimeRemaining);
        //告诉系统，我们已经完成
        [app endBackgroundTask:taskId];
    });
}

//进入前台
- (void)applicationWillEnterForeground
{
    NSLog(@"VC: %@", NSStringFromSelector(_cmd));
    NSString *smileyPath = [[NSBundle mainBundle] pathForResource:@"smiley"
                                                           ofType:@"png"];
    self.smiley = [UIImage imageWithContentsOfFile:smileyPath];
    self.smileyView.image = self.smiley;
}

运行效果：