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。

  1 #import "BIDViewController.h"
  2 
  3 @interface BIDViewController ()
  4 
  5 @property (strong, nonatomic) UILabel * label;
  6 @property (strong, nonatomic) UIImage * smiley;
  7 @property (strong, nonatomic) UIImageView * smileyView;
  8 @property (strong, nonatomic) UISegmentedControl * segmentedControl;
  9 
 10 @end
 11 
 12 @implementation BIDViewController
 13 {
 14     BOOL animate;
 15 }
 16 
 17 - (void)viewDidLoad
 18 {
 19     [super viewDidLoad];
 20     
 21     //创建Label
 22     CGRect bounds = self.view.bounds;
 23     CGRect labelFrame = CGRectMake(bounds.origin.x, CGRectGetMidY(bounds) - 50,
 24                                    bounds.size.width, 100);
 25     self.label = [[UILabel alloc] initWithFrame:labelFrame];
 26     self.label.font = [UIFont fontWithName:@"Helvetica" size:70];
 27     self.label.text = @"Bazinga!";
 28     self.label.textAlignment = NSTextAlignmentCenter;
 29     self.label.backgroundColor = [UIColor clearColor];
 30     
 31     // 创建ImageView, smiley.png is 84 x 84
 32     CGRect smileyFrame = CGRectMake(CGRectGetMidX(bounds) - 42,
 33                                     CGRectGetMidY(bounds)/2 - 42,
 34                                     84, 84);
 35     self.smileyView = [[UIImageView alloc] initWithFrame:smileyFrame];
 36     self.smileyView.contentMode = UIViewContentModeCenter;
 37     NSString *smileyPath = [[NSBundle mainBundle] pathForResource:@"smiley"
 38                                                            ofType:@"png"];
 39     self.smiley = [UIImage imageWithContentsOfFile:smileyPath];
 40     self.smileyView.image = self.smiley;
 41     
 42     
 43     //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。
 44     self.segmentedControl = [[UISegmentedControl alloc] initWithItems:
 45                              [NSArray arrayWithObjects:
 46                               @"One", @"Two", @"Three", @"Four", nil]] ;
 47     self.segmentedControl.frame = CGRectMake(bounds.origin.x + 20,
 48                                              50,
 49                                              bounds.size.width - 40, 30);
 50     
 51     
 52     [self.view addSubview:self.segmentedControl];
 53     [self.view addSubview:self.smileyView];
 54     [self.view addSubview:self.label];
 55     
 56     NSNumber *indexNumber = [[NSUserDefaults standardUserDefaults]
 57                              objectForKey:@"selectedIndex"];
 58     if (indexNumber) {
 59         NSInteger selectedIndex = [indexNumber intValue];
 60         self.segmentedControl.selectedSegmentIndex = selectedIndex;
 61     }
 62     
 63     NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
 64     [center addObserver:self
 65                selector:@selector(applicationWillResignActive)
 66                    name:UIApplicationWillResignActiveNotification
 67                  object:nil];
 68     [center addObserver:self
 69                selector:@selector(applicationDidBecomeActive)
 70                    name:UIApplicationDidBecomeActiveNotification
 71                  object:nil];
 72     
 73     [center addObserver:self
 74                selector:@selector(applicationDidEnterBackground)
 75                    name:UIApplicationDidEnterBackgroundNotification
 76                  object:nil];
 77     [center addObserver:self
 78                selector:@selector(applicationWillEnterForeground)
 79                    name:UIApplicationWillEnterForegroundNotification
 80                  object:nil];
 81 }
 82 
 83 - (void)rotateLabelDown
 84 {
 85     [UIView animateWithDuration:0.5
 86                      animations:^{
 87                          self.label.transform = CGAffineTransformMakeRotation(M_PI);
 88                      }
 89                      completion:^(BOOL finished){
 90                          [self rotateLabelUp];
 91                      }];
 92 }
 93 
 94 - (void)rotateLabelUp
 95 {
 96     [UIView animateWithDuration:0.5
 97                      animations:^{
 98                          self.label.transform = CGAffineTransformMakeRotation(0);
 99                      }
100                      completion:^(BOOL finished){
101                          if (animate) {
102                              [self rotateLabelDown];
103                          }
104                      }];
105 }
106 
107 - (void)applicationWillResignActive
108 {
109     NSLog(@"VC: %@", NSStringFromSelector(_cmd));
110     animate = NO;
111 }
112 
113 - (void)applicationDidBecomeActive
114 {
115     NSLog(@"VC: %@", NSStringFromSelector(_cmd));
116     animate = YES;
117     [self rotateLabelDown];
118 }
119 
120 - (void)applicationDidEnterBackground
121 {
122     NSLog(@"VC: %@", NSStringFromSelector(_cmd));
123     UIApplication *app = [UIApplication sharedApplication];
124     
125     __block UIBackgroundTaskIdentifier taskId;
126     taskId = [app beginBackgroundTaskWithExpirationHandler:^{
127         NSLog(@"Background task ran out of time and was terminated.");
128         [app endBackgroundTask:taskId];
129     }];
130     
131     if (taskId == UIBackgroundTaskInvalid) {
132         NSLog(@"Failed to start background task!");
133         return;
134     }
135     
136     dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
137     ^{
138         NSLog(@"Starting background task with %f seconds remaining",
139               app.backgroundTimeRemaining);
140         
141         self.smiley = nil;
142         self.smileyView.image = nil;
143         NSInteger selectedIndex = self.segmentedControl.selectedSegmentIndex;
144         [[NSUserDefaults standardUserDefaults] setInteger:selectedIndex
145                                                    forKey:@"selectedIndex"];
146         
147         // simulate a lengthy (25 seconds) procedure
148         [NSThread sleepForTimeInterval:25];
149         
150         NSLog(@"Finishing background task with %f seconds remaining",
151               app.backgroundTimeRemaining);
152         [app endBackgroundTask:taskId];
153     });
154 }
155 
156 - (void)applicationWillEnterForeground
157 {
158     NSLog(@"VC: %@", NSStringFromSelector(_cmd));
159     NSString * smileyPath = [[NSBundle mainBundle] pathForResource:@"smiley"
160                                                            ofType:@"png"];
161     self.smiley = [UIImage imageWithContentsOfFile:smileyPath];
162     self.smileyView.image = self.smiley;
163 }
164 
165 - (void)didReceiveMemoryWarning
166 {
167     [super didReceiveMemoryWarning];
168     // Dispose of any resources that can be recreated.
169 }