[摘要]本文是對iOS 後台處理的講解，對學習IOS蘋果軟件開發有所幫助,與大家分享。

iOS 後台處理的常見用途

1、進入後台時候刪除資源：應用處於掛起狀態的時候所占用的資源越少，該應用被iOS終止的風險就越低。通過從內存中清理那些易於重新創建的資源，可以增加應用駐留內存的機會，因此可以大幅加快重啟速度。

2、進入後台時候保存狀態:保存與用戶執行的操作相關的所有信息，這樣的話，用戶下次回來的時候，依然可以恢復到他們離開時的進度。

3、延時執行，請求更多的後台時間：如果進入後台花費了很多時間，應用可能會從內存中移除，如果應用正在進行文件傳輸，沒有能夠完成的話，將會帶來很多不便。我們可以將applicationDidEnterBackground 作為平台，告訴系統，你還有額外的工作要做，然後啟動一個程序塊，真正的執行該工作。

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) - 50,
                                   bounds.size.width, 100);
    
    //轉動的Label
    self.label = [[UILabel alloc] initWithFrame:labelFrame];
    self.label.font = [UIFont fontWithName:@"Helvetica" size:70];
    self.label.text = @"Bazinga!";
    self.label.textAlignment = NSTextAlignmentCenter;
    self.label.backgroundColor = [UIColor clearColor];
    
    
    //笑臉
    CGRect smileyFrame = CGRectMake(CGRectGetMidX(bounds) - 42,
                                    CGRectGetMidY(bounds)/2 - 35,
                                    84, 84);
    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 + 20,
                                             50,
                                             bounds.size.width - 40, 30);
    
    
    [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 向下轉動
- (void)rotateLabelDown
{
    [UIView animateWithDuration:0.5
                     animations:^{
                         self.label.transform = CGAffineTransformMakeRotation(M_PI);
                     }
                     completion:^(BOOL finished){
                         [self rotateLabelUp];
                     }];
}

//Label 向上轉動
- (void)rotateLabelUp
{
    [UIView animateWithDuration:0.5
                     animations:^{
                         self.label.transform = CGAffineTransformMakeRotation(0);
                     }
                     completion:^(BOOL finished){
                         if (animate) {
                             [self rotateLabelDown];
                         }
                     }];
}

//離開活動狀態
- (void)applicationWillResignActive
{
    NSLog(@"VC: %@", NSStringFromSelector(_cmd));
    animate = NO;
}

//進入活動狀態
- (void)applicationDidBecomeActive
{
    NSLog(@"VC: %@", NSStringFromSelector(_cmd));
    animate = YES;
    [self rotateLabelDown];
}

//後台運行
- (void)applicationDidEnterBackground
{
    NSLog(@"VC: %@", NSStringFromSelector(_cmd));
    //先獲取共享的UIApplication 實例。
    UIApplication *app = [UIApplication sharedApplication];
    
    //聲明了taskId變量、並用__block修飾。
    __block UIBackgroundTaskIdentifier taskId;
    
    //告訴系統，需要更多的時間來完成某件事，並承諾在完成之後告訴它。如果系統判定我們運行了太久時間並決定停止運行，可以調用我們做為參數提供的程序塊
    taskId = [app beginBackgroundTaskWithExpirationHandler:^{
        NSLog(@"Background task ran out of time and was terminated.");
        [app endBackgroundTask:taskId];
    }];
    
    //如果系統返回的值是UIBackgroundTaskInvalid，表明系統沒有為我們提供任何多余的時間。
    if (taskId == UIBackgroundTaskInvalid) {
        NSLog(@"Failed to start background task!");
        return;
    }
    
    //
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
    ^{
        NSLog(@"Starting background task with %f seconds remaining",
              app.backgroundTimeRemaining);
        
        self.smiley = nil;
        self.smileyView.image = nil;
        
        //存儲segmentedControl的位置
        NSInteger selectedIndex = self.segmentedControl.selectedSegmentIndex;
        [[NSUserDefaults standardUserDefaults] setInteger:selectedIndex
                                                   forKey:@"selectedIndex"];
        
        //模擬一個25秒的過程
        [NSThread sleepForTimeInterval:25];
        
        NSLog(@"Finishing background task with %f seconds remaining",
              app.backgroundTimeRemaining);
        //告訴系統，我們已經完成
        [app endBackgroundTask:taskId];
    });
}

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

運行效果：