返回UIViewController时UILabel不更新

Question

我有一个简单的应用程序，它在appDelegate中有一个NSTimer对象，可供所有视图访问。应用程序的结构是使用UINavigationController。当我触发NSTimer对象时，我的UILabel正在使用正确的倒计时功能进行更新，但当我返回到rootViewController并返回到倒数计时器视图时，我的UILabel正在使用当前的倒计时时间进行更新，但没有后续更新UILabel发生了。我错过了什么？我已经做过研究确保UILabel对象不是nil，我在viewDidAppear方法上调用该函数，似乎没有任何工作！这是代码：

AppDelegate.h

@interface AppDelegate : UIResponder <UIApplicationDelegate> {
}
@property (nonatomic, retain) NSTimer *countdownTimer;

AppDelegate.m

@implementation AppDelegate

@synthesize countdownTimer;

CountdownTimerViewController.h

#import "AppDelegate.h"
enter code here
@interface CountdownTimerViewController : UIViewController {
enter code here
AppDelegate *appdelegate;

}

@property (strong, nonatomic) IBOutlet UILabel *labelCountdownTimer;
@property (strong, nonatomic) IBOutlet UIButton *buttonStartTimer;
@property (strong, nonatomic) IBOutlet UIButton *buttonStopTimer;

- (IBAction)startTimer:(id)sender;
- (IBAction)stopTimer:(id)sender;

CountdownTimerViewController.m

@implementation CountdownTimerViewController

@synthesize labelCountdownTimer;

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.

    //Instatiating Appdelegate
    if(!appdelegate)
        appdelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];

}

- (void) viewDidAppear:(BOOL)animated {

    if ([appdelegate.countdownTimer isValid]) {
        [self countDown];
    }

}

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

#pragma mark - Button Action Methods

- (IBAction)startTimer:(id)sender {

    [self updateCounter];

}

- (IBAction)stopTimer:(id)sender {

    [appdelegate.countdownTimer invalidate];
    labelCountdownTimer.text = @"00:00:00";

}

int countLimit=30; //seconds
NSDate *startDate;

- (void)countDown {

    if([[NSDate date] timeIntervalSinceDate:startDate] >= countLimit) {
        [appdelegate.countdownTimer invalidate];
        return;
    }
    else {            
    NSDate *currentDate = [NSDate date];
    NSTimeInterval timeInterval = -([currentDate timeIntervalSinceDate:startDate]);
    NSDate *timerDate = [NSDate dateWithTimeIntervalSince1970:timeInterval];
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"mm:ss"];
    [dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0.0]];
    NSString *timeString = [dateFormatter stringFromDate:timerDate];
        NSLog(@"timeString: %@",timeString);
        NSLog(@"labelCountdownTimer: %@",labelCountdownTimer);
    labelCountdownTimer.text = timeString;
    }

}

- (void)updateCounter {

    labelCountdownTimer.text = @"00:00:00";
    startDate = [NSDate date];

    appdelegate.countdownTimer = [NSTimer scheduledTimerWithTimeInterval:1.0/10.0
                                                                  target:self
                                                                selector:@selector(countDown)
                                                                userInfo:nil
                                                                 repeats:YES];

}

Answer 1

感谢大家的意见。我实际上通过执行一个方法解决了它，它将检索NSTimer在我的AppDelegate中更新的值，因为当我离开视图并返回时，触发NSTimer的方法不再出现在主线程中。只要我的NSTimer有效，此方法就会循环。我还放置了一个延迟，允许UI更新值，然后再次执行该方法。以下是代码，以防有人遇到类似的问题。我从chandan提供的建议中得到了这个想法，谢谢!!

AppDelegate.h

@interface AppDelegate : UIResponder <UIApplicationDelegate> {

}

@property (nonatomic, retain) NSTimer *countdownTimer;
@property (nonatomic, retain) NSString *timeString;

AppDelegate.m

@interface AppDelegate : UIResponder <UIApplicationDelegate> {

}

@property (nonatomic, retain) NSTimer *countdownTimer;
@property (nonatomic, retain) NSString *timeString;

CountdownTimerViewController.h

@interface CountdownTimerViewController : UIViewController {

AppDelegate *appdelegate;

}

@property (strong, nonatomic) IBOutlet UILabel *labelCountdownTimer;

@property (strong, nonatomic) IBOutlet UIButton *buttonStartTimer;
@property (strong, nonatomic) IBOutlet UIButton *buttonStopTimer;

- (IBAction)startTimer:(id)sender;
- (IBAction)stopTimer:(id)sender;

CountdownTimerViewController.m

@implementation CountdownTimerViewController

@synthesize labelCountdownTimer;

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
        // Do any additional setup after loading the view.

    //Instatiating Appdelegate
    if(!appdelegate)
        appdelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];

}

- (void) viewDidAppear:(BOOL)animated {

    if ([appdelegate.countdownTimer isValid]) {
        [self updateLabel];
    } else {
        labelCountdownTimer.text = @"00:00:00";
    }

}

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

#pragma mark - Button Action Methods

- (IBAction)startTimer:(id)sender {

    [self updateCounter];

}

- (IBAction)stopTimer:(id)sender {

    [appdelegate.countdownTimer invalidate];
    labelCountdownTimer.text = @"00:00:00";

}

int countLimit=30; //seconds
NSDate *startDate;

- (void)updateCounter {

    labelCountdownTimer.text = @"00:00:00";
    startDate = [NSDate date];

    appdelegate.countdownTimer = [NSTimer scheduledTimerWithTimeInterval:1.0/10.0
                                                                  target:self
                                                                selector:@selector(countDown)
                                                                userInfo:nil
                                                                 repeats:YES];

}

  - (void)countDown {

    if([[NSDate date] timeIntervalSinceDate:startDate] >= countLimit) {
        [appdelegate.countdownTimer invalidate];
        return;
    }
    else {            
    NSDate *currentDate = [NSDate date];
    NSTimeInterval timeInterval = -([currentDate timeIntervalSinceDate:startDate]);
    NSDate *timerDate = [NSDate dateWithTimeIntervalSince1970:timeInterval];
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"mm:ss"];
    [dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0.0]];
    appdelegate.timeString = [dateFormatter stringFromDate:timerDate];
    labelCountdownTimer.text = appdelegate.timeString;
    }

} 


- (void) updateLabel {

    if ([appdelegate.countdownTimer isValid]) {
        labelCountdownTimer.text = appdelegate.timeString;
        [self performSelector:@selector(updateLabel) withObject:nil afterDelay:0.05];
    } 

}

Answer 2

像这样输入：

appdelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];

实际上不会将UIApplicationDelegate放入AppDelegate类并添加额外的参数。因此，此变量中没有指向计时器的指针。

您需要一种不同的方法来存储计时器指针。

Answer 3

尝试在主线程上更新UILabel上的文本。有时UILabel中的更新不适用于backgound线程。

- (void) viewDidAppear:(BOOL)animated 
{
     if ([appdelegate.countdownTimer isValid]) 
     {
          [self performSelectorOnMainThread:@selector(countDown) withObject:nil waitUntilDone:NO];
     } 
}

如果您的appdelegate对象工作正常并且UILabel正在使用当前倒计时时间进行更新，但是没有对UILabel进行后续更新，那么在主线程上应用UI更改

- (void)countDown {

 [self performSelectorOnMainThread:@selector(changeCountDownValue) withObject:nil waitUntilDone:NO];
}

- (void)changeCountDownValue
{
    if([[NSDate date] timeIntervalSinceDate:startDate] >= countLimit) {
        [appdelegate.countdownTimer invalidate];
        return;
    }
    else {            
    NSDate *currentDate = [NSDate date];
    NSTimeInterval timeInterval = -([currentDate timeIntervalSinceDate:startDate]);
    NSDate *timerDate = [NSDate dateWithTimeIntervalSince1970:timeInterval];
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"mm:ss"];
    [dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0.0]];
    NSString *timeString = [dateFormatter stringFromDate:timerDate];
        NSLog(@"timeString: %@",timeString);
        NSLog(@"labelCountdownTimer: %@",labelCountdownTimer);
    labelCountdownTimer.text = timeString;
    }

}

请仔细检查NSTimer对象。它应该适用于UILabel更新。如果仍有问题，请告诉我。