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：