后台处理

　　用户按下主屏幕按钮将调用applicationWillResignActive：,如果谈们稍后将应用切换回前台，将调用applicationDidBecomeActive：。如果用户接听电话，也会发生相同序列的事件。最神奇的是应用启动时也会调用applicationDidBecomeActive：。这两个方法可以用来启动或禁用任何动画、应用内的音频或者其他用于向用户展示的应用内元素。由于使用applicationDidBecomeActive：较多，我们可能在其中添加一些应用初始化代码，而不是在application：didFinishLaunchingWithOptions:中。

　　当设备显示收到信息的提醒时，将调用applicationWillResignActive：，应用没有被发送到后台，而是出于不活跃状态，如果此应用是一个游戏或正在进行着视频、音频或动画，那么这是需要暂停他们。点击关闭按钮将调用applicationDidBecomeActive：，点击回复按钮将依次调用applicationDidBecomeActive：、applicationWillResignActive：和applicationDidEnterBackground：三个方法。

　　applicationDidEnterBackground：中释放所有可在以后重新创建的资源，保存所用用户数据，关闭网络连接等。如果需要还可以在这里请求在后台运行更长的时间用来保存数据。

　　applicationWillEnterForeground：中重新创建在applicationDidEnterBackground：中销毁的内容，比如重新加载用户数据、重新建立网络连接等。

#import "BIDViewController.h"

@interface BIDViewController ()

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

@end

@implementation BIDViewController
{
    BOOL animate;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    //创建Label
    CGRect bounds = self.view.bounds;
    CGRect labelFrame = CGRectMake(bounds.origin.x, CGRectGetMidY(bounds) - 50,
                                   bounds.size.width, 100);
    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];
    
    // 创建ImageView, smiley.png is 84 x 84
    CGRect smileyFrame = CGRectMake(CGRectGetMidX(bounds) - 42,
                                    CGRectGetMidY(bounds)/2 - 42,
                                    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;
    
    
    //在哪里恢复分段选择的索引并用于配置分段控件，applicationWillEnterForeground方法并不是我们想要的。这个方法被调用的时候应用已经处于运行状态，但设置还是最原始的状态。应该在应用重新运行之后就进行设置，应该在viewDidLoad方法中进行处理。
    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;
    }
    
    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];
}

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

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

- (void)applicationWillResignActive
{
    NSLog(@"VC: %@", NSStringFromSelector(_cmd));
    animate = NO;
}

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

- (void)applicationDidEnterBackground
{
    NSLog(@"VC: %@", NSStringFromSelector(_cmd));
    UIApplication *app = [UIApplication sharedApplication];
    
    __block UIBackgroundTaskIdentifier taskId;
    taskId = [app beginBackgroundTaskWithExpirationHandler:^{
        NSLog(@"Background task ran out of time and was terminated.");
        [app endBackgroundTask:taskId];
    }];
    
    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;
        NSInteger selectedIndex = self.segmentedControl.selectedSegmentIndex;
        [[NSUserDefaults standardUserDefaults] setInteger:selectedIndex
                                                   forKey:@"selectedIndex"];
        
        // simulate a lengthy (25 seconds) procedure
        [NSThread sleepForTimeInterval:25];
        
        NSLog(@"Finishing background task with %f seconds remaining",
              app.backgroundTimeRemaining);
        [app endBackgroundTask:taskId];
    });
}

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

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}