iOS 幕後處理，ios幕後處理

iOS 幕後處理，ios幕後處理

iOS 幕後處理的常見用途

1、進入後台時候刪除資源：應用處於掛起狀態的時候所佔用的資源越少，該應用被iOS終止的風險就越低。通過從記憶體中清理那些易於重新建立的資源，可以增加應用駐留記憶體的機會，因此可以大幅加快重啟速度。

2、進入後台時候儲存狀態:儲存與使用者執行的操作相關的所有資訊，這樣的話，使用者下次回來的時候，依然可以恢複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，應用可能會從記憶體中移除，如果應用進行中檔案傳輸，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，然後啟動一個程式塊，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));    //先擷取共用的UIApplication 執行個體。    UIApplication *app = [UIApplication sharedApplication];        //聲明了taskId變數、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式塊    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餘的時間。    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;                //儲存segmentedControl的位置        NSInteger selectedIndex = self.segmentedControl.selectedSegmentIndex;        [[NSUserDefaults standardUserDefaults] setInteger:selectedIndex                                                   forKey:@"selectedIndex"];                //類比一個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：