Completing a Long-Running Task in the Background（在后台完成长时间运行的任务）

当app运行中，按下home键，则此app进入后台，所有操作都将会被中止。若此时有一个需长时间运行的任务尚未结束，则可以向ios借用时间让其在后台继续完成
方法
- (void)applicationDidEnterBackground:(UIApplication *)application
- (UIBackgroundTaskIdentifier)beginBackgroundTaskWithExpirationHandler:(void (^)(void))handler
- (void)endBackgroundTask:(UIBackgroundTaskIdentifier)identifier

e.g. －－AppDelegate.m文件中

@property (nonatomic, unsafe_unretained) UIBackgroundTaskIdentifier backgroundTaskIdentifier;//请求在后台运行的任务标识（唯一）
@property (nonatomic, strong) NSTimer *myTimer; //用来模拟需长时间运行的操作

- (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;
}
//是否支持多任务处理

- (BOOL) isMultitaskingSupported{
BOOL result = NO;
if ([[UIDevice currentDevice]respondsToSelector:@selector(isMultitaskingSupported)]){
result = [[UIDevice currentDevice] isMultitaskingSupported];
}
return result;
}
//进入后台运行，如按home
- (void)applicationDidEnterBackground:(UIApplication *)application{
if ([self isMultitaskingSupported] == NO){
return;
}
self.myTimer = [NSTimer scheduledTimerWithTimeInterval:1.0f
target:self
selector:@selector(timerMethod:)
userInfo:nil
repeats:YES];
//向IOS借时间来执行需长时间运行的任务
self.backgroundTaskIdentifier = [application beginBackgroundTaskWithExpirationHandler:^(void) {
[self endBackgroundTask]; //必须匹配一个结束任务的操作，来标识此任务运行结束，应用完全进入后台休眠状态
}];
}
- (void) endBackgroundTask{
dispatch_queue_t mainQueue = dispatch_get_main_queue();
__weak AppDelegate *weakSelf = self;
dispatch_async(mainQueue, ^(void) {
AppDelegate *strongSelf = weakSelf;
if (strongSelf != nil){
[strongSelf.myTimer invalidate];
[[UIApplication sharedApplication] endBackgroundTask:self.backgroundTaskIdentifier];
strongSelf.backgroundTaskIdentifier =UIBackgroundTaskInvalid;
}
});
}
- (void) timerMethod:(NSTimer *)paramSender{
NSTimeInterval backgroundTimeRemaining = [[UIApplication sharedApplication] backgroundTimeRemaining]; //后台运行时间
if (backgroundTimeRemaining ==DBL_MAX){
NSLog(@"Background Time Remaining = Undetermined");
} else {
NSLog(@"Background Time Remaining = %.02f Seconds",
backgroundTimeRemaining);
}
}
//重新点击app应用图标,进入前台
- (void)applicationWillEnterForeground:(UIApplication *)application{
if (self.backgroundTaskIdentifier != UIBackgroundTaskInvalid){
[self endBackgroundTask];
}
}