Update UILabel continuously — Objective-c

Question

I'm trying to display the count of a timer in a UILabel . The timer works fine but I'm not able to update the label. The NSTimer is set up like this in Class1 .

- (void)startTimer {
    int timerCount = 5;
    timer = [NSTimer scheduledTimerWithTimeInterval:4.0 
                                             target:self 
                                           selector:@selector(timerDecrement) 
                                           userInfo:nil 
                                            repeats:YES];
    [timer fire];
}

- (void)timerDecrement {
    Class2 *cls2 = [[Class2 alloc] init];
    timerCount = timerCount-1;
    [cls2 updateTimeLeftLabel];
}

+ (int)getTimerCount {
    return timerCount;
}

In Class2 :

- (void)viewDidLoad {
    if (timeLeftLabel == nil) {

        timeLeftLabel = [[UILabel alloc] initWithFrame:CGRectMake(200, 70, 120, 30)];
        timeLeftLabel.text = @"Time left";
        [self.view addSubview:timeLeftLabel];          
    }

- (void)updateTimeLeftLabel {
    NSLog(@"%@", [NSString stringWithFormat:@"%i", 
                  [Class1 getTimerCount]]);
    timeLeftLabel.text = [NSString stringWithFormat:@"Time left: %i", 
                          [Class1 getTimerCount]];
}

The correct timer value is logged but the UILabel isn't updated at all. What's wrong?

Answer 1

尽量让timerCount为@property类或static 。

Answer 2

You create a new instance of the Class2 and do not present it. You should store instance of Class2 somewhere and work with it.

Answer 3

You can simply pass the time left to the method updating the UILabel as a parameter:

- (void)updateTimeLeftLabel:(NSInteger)timeLeft {
    timeLeftLabel.text = [NSString stringWithFormat:@"Time left: %i",
                          timeLeft];
}

- (void)timerDecrement {
    Class2 *cls2 = [[Class2 alloc] init];
    timerCount = timerCount-1;
    [cls2 updateTimeLeftLabel:timerCount];
}

Answer 4

I needed to update count on my UIButton so i have done like this

    -(void)viewDidLoad {
       self.myLeagueTimer = [NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:@selector(updateCountdown) userInfo:nil repeats: YES];
    }

- (int)timeIndervalRemain {
    NSDate *currentTime = [NSDate date];
    return (TIME_INTERVAL - [currentTime timeIntervalSinceDate:self.startTime]);
}

-(void) updateCountdown {

    if ([self timeIndervalRemain] > 0) {
        self.title = [NSString stringWithFormat:@"%d", [self timeIndervalRemain]];
    }  else {
        [self timeExpire];
    }
}


-(void) timeExpire {

    if (_myLeagueTimer) {
        [_myLeagueTimer invalidate];
        _myLeagueTimer = nil;
    }
}