Changing CAEmitterCell properties of CAEmitterLayer after emission starts

Question

When I first set up the emitter I can do this:

self.cell = [CAEmitterCell emitterCell];
self.cell.yAcceleration = 20;
...
self.emitter.emitterCells = [NSArray arrayWithObjects:self.cell,nil];

But say I create a timer that fires 5 seconds later, and I do this:

- (void)timerFired
{
    self.cell.yAcceleration = -10;

}

The timer fires, but the yAcceleration of the CAEmitterCell does not get changed. Or at least nothing changes in the particle emission on screen. How can I get a CAEmitterCell to respect changes I make to its properties?

Answer 1

This is not real obvious, but here's the solution:

[self.emitter setValue:[NSNumber numberWithFloat:-10.0]
               forKeyPath:@"emitterCells.cell.yAcceleration"];

Where "cell" is the name given here:

[self.cell setName:@"cell"];

Answer 2

When you init the self.emitter with a new cell,the object will be retained,so ..when you change the cell.yAcceleration with a timer,the cell of self.emitter can not be changed,self.cell.yAcceleration has been changed already.So, you should use the key path of self.emitter.