Problems invalidating & re-creating NSTimer(s)

Question

I'm having problems starting & stopping NSTimers. The docs say that a timer is stopped by [timer invalidate]; I have a timer object declared as such ``` .h NSTimer *incrementTimer; @property (nonatomic, retain) NSTimer *incrementTimer; .m @synthesize incrementTimer; -(void)dealloc { [incrementTimer release]; [super dealloc]; } ``` -The usual. When it's needed, my method does the following: ``` -(void)setGenCount { if(!condition1 && condition2) { incrementTimer = [NSTimer scheduledTimerWithTimeInterval: 2.0 target: self selector:@selector(incrementBatteryVoltage:) userInfo: nil repeats: YES]; } } ``` Everything above works fine. However, once that timer does it's job, I want it to invalidate itself. I invalidate the timer because there is an equal decrement method that could be called and would fight against the incrementTimer if it was still active. (Previously, I noticed that my two timers, if active, were acting on the same ivar by increasing & decreasing the value (a sort of fight)…

Accepted Answer

UPDATE: See Darren's answer. The problem is that you are not using your property accessor when setting the timers. Instead of:

```
incrementTimer = [NSTimer ...
```

You should have:

```
self.incrementTimer = [NSTimer ...
```

The `self.propertyName = ...` syntax will call your accessor method, and thereby automatically retain the object that you send to it (since your property is set up as `retain`). Simply calling `propertyName = ...` does not use the property accessor. You are simply changing the value of your ivar directly.

UPDATE #2: After an enlightening conversation with Peter Hosey (see comments), I have removed my earlier suggestion to "never retain or release" your timer object. I have also completely re-written my earlier code because I think the following is a better approach:

Controller.h:

```
NSTimer *voltageTimer;
float targetBatteryVoltage;
...
@property (nonatomic, retain) NSTimer *voltageTimer;
```

Controller.m:

```
@implementation Controller
@synthesize voltageTimer;

- (void)stopVoltageTimer {
    [voltageTimer invalidate];
    self.voltageTimer = nil;
}

- (void)setTargetBatteryVoltage:(float)target {
    [voltageTimer invalidate];
    targetBatteryVoltage = target;
    self.voltageTimer = [NSTimer scheduledTimerWithTimeInterval: 2.0
                                target: self
                              selector: @selector(updateBatteryVoltage:)
                              userInfo: nil
                               repeats: YES];
}

- (void)updateBatteryVoltage:(NSTimer *)timer {
    const float increment = 0.1;
    if (abs(battVoltage - targetBatteryVoltage) < increment) {
        [timer invalidate];
    }
    else if (battVoltage < targetBatteryVoltage) {
        generatorDisplay.battVoltage += increment;
    }
    else if (battVoltage > targetBatteryVoltage) {
        generatorDisplay.battVoltage -= increment;
    }
}
```

Now, you can simply set a target battery voltage, and the timer magic will happen behind the scenes:

```
[self setTargetBatteryVoltage:24.0];
```

Your power-off method would look as follows:

```
- (void)deEnergizeDisplays {
    [self stopVoltageTimer];
}
```

Problems invalidating & re-creating NSTimer(s)

Solution

Problem