KVO conditionally fire method in observeValueForKeyPath

Question

I have a countdown that counts down from 5. I use KVO on a float property that represents the countdown. It fires observeValueForKeyPath once or not at all. I want it fire observeValueForKeyPath until the property being observed matches a particular value, so it can fire a method in observeValueForKeyPath.

Please see code

EDIT

 __block float progress;
    self.countdown.labelVCBlock = ^(KAProgressLabel *label) {
        progress = 5-(label.progress *5);
        label.text = [NSString stringWithFormat:@"%.f", (progress)];


    };

     self.progressFloat = 5.0f; //setting the initial value
     [self addObserver:self forKeyPath:@"progressFloat" options:NSKeyValueObservingOptionNew context:NULL];
    self.progressFloat -= progress;//progress is a float that counts down from 5

}

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context{

    if ([keyPath isEqualToString:@"progressFloat"]) {
          NSLog(@"KVO called");
        if (self.progressFloat < 0.1f) {//when self.progressFloat is less than 0.1 second, then fire the method below. 

            [self takePhoto:nil];
        }
        }
    }

The method either fires immediately or observeValueForKeyPath never gets called.

The code you posted only shows `self.progressFloat` being changed once. How does the count down happen? — Phillip Mills, Jan 26 '16 at 01:36
Not sure what I'm supposed to see in the edit...`self.progressFloat` is still only being changed once after the observer is set. — Phillip Mills, Jan 26 '16 at 01:49
The block updates `progress` which is subtracted for `self.progressFloat` — noobsmcgoobs, Jan 26 '16 at 02:19
You don't show where it's being subtracted unless you mean that you're repeatedly calling the method that sets `self.progressFloat` to 5 and adds the observer. — Phillip Mills, Jan 26 '16 at 14:00
@PhillipMills no, the block runs over and over again, updating progress. But I get what you mean. `self.progressFloat` only gets updated once. @vienvu's answer below gets to the heart of it but I can't get it to work quite yet. Thanks for pointing that out, I got lost in the block. — noobsmcgoobs, Jan 26 '16 at 21:54
Side note: if you notice yourself adding an object as an observer to itself (e.g, `self` observing a property on `self` as you have it), then you could just override the setter for the property — A O, Jan 27 '16 at 05:38

score 0 · Accepted Answer · answered Jan 26 '16 at 03:33

0

I see that you don't update your variable progressFloat in block. Try change code to:

__unsafe_unretained typeof(self) weakSelf = self;
self.countdown.labelVCBlock = ^(KAProgressLabel *label) {
    progress = 5-(label.progress *5);
    label.text = [NSString stringWithFormat:@"%.f", (progress)];
    weakSelf.progressFloat -= progress;
};

answered Jan 26 '16 at 03:33

vien vu

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I've never seen weakSelf before, I'll give it a try – noobsmcgoobs Jan 26 '16 at 04:15
@noobsmcgoobs It just protect not retain cycle when using block. – vien vu Jan 26 '16 at 04:16
I tried it but got red error. DO I have to declare it as weak instead of strong in the .h file? – noobsmcgoobs Jan 26 '16 at 05:38
weakSelf - sorry first time I saw that. I got the `weakSelf` working but it just freezes on the start of the block now. Hmmm... – noobsmcgoobs Jan 26 '16 at 22:13
@noobsmcgoobs what about free? App free UI? – vien vu Jan 26 '16 at 23:16
@vien_vu I got it, had to remove observer. Thanks! – noobsmcgoobs Jan 27 '16 at 00:07

KVO conditionally fire method in observeValueForKeyPath

1 Answers1