CAEmitterCell不支持birthRate更改

Question

我想创建一个粒子效果，它只在用户触摸屏幕时发射，但是一旦设置为非零值，我就不能更改CAEmitterCell birthRate属性。

我有一个UIView的子类，它可以按照我想要的方式设置我的CAEmitterLayer和CAEmitterCell。我在该类上定义了两个属性：

@property (strong, nonatomic) CAEmitterLayer *emitterLayer;
@property (strong, nonatomic) CAEmitterCell *emitterCell;

然后，在我的视图控制器中，我跟踪触摸，设置emitterLayer的位置和emitterCell出生速率：

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint tappedPt = [touch locationInView:touch.view];
    NSLog(@"began x:%f y:%f",tappedPt.x, tappedPt.y);
    emitterView.emitterCell.birthRate = 42;
    emitterView.emitterLayer.emitterPosition = tappedPt;
}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint tappedPt = [touch locationInView:touch.view];
    NSLog(@"moved x:%f y:%f",tappedPt.x, tappedPt.y);
    emitterView.emitterLayer.emitterPosition = tappedPt;
}

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event{
    NSLog(@"ending %f", emitterView.emitterCell.birthRate);
    emitterView.emitterCell.birthRate = 0.00;
    NSLog(@"ended %f", emitterView.emitterCell.birthRate);
}

日志报告emitterView.emitterCell.birthRate发生了变化：

began x:402.000000 y:398.500000
ending 42.000000
ended 0.000000

当我触摸屏幕时，发射器按预期启动，层跟随触摸，但当我结束触摸时，发射器单元愉快地发射初始设置的任何值(在touchesBegan中设置的值)。无论我做什么，一旦将出生率值设置为非零值，似乎就无法更改出生率值。日志报告这些值设置正确，但发射器仍在发射。

但是，如果我更改touchesEnded方法以更改层的位置，则在emitterCell上设置birthRate之后，一切都会按预期运行：

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event{
    UITouch *touch = [touches anyObject];
    CGPoint tappedPt = [touch locationInView:touch.view];
    NSLog(@"began x:%f y:%f",tappedPt.x, tappedPt.y);

    NSLog(@"ending %f", emitterView.emitterCell.birthRate);
    emitterView.emitterCell.birthRate = 0.0;
    NSLog(@"ended %f", emitterView.emitterCell.birthRate);
    emitterView.emitterLayer.emitterPosition = tappedPt;
}

有人能解释一下为什么吗？

Answer 1

要停止发射粒子，您必须将CAEmitterLayer实例的birthRate属性设置为0，尽管它最初是在CAEmitterCell实例上设置的...不知道为什么，但它很有效。

Swift 3示例：

func emitParticles() {
    let particlesEmitter = CAEmitterLayer()
    particlesEmitter.emitterPosition = center
    particlesEmitter.emitterShape = kCAEmitterLayerCircle
    particlesEmitter.emitterSize = CGSize(width: 50, height: 50)
    particlesEmitter.renderMode = kCAEmitterLayerAdditive

    let cell = CAEmitterCell()
    cell.birthRate = 15
    cell.lifetime = 1.0
    cell.color = bubble.color.cgColor
    cell.velocity = 150
    cell.velocityRange = 50
    cell.emissionRange = .pi
    cell.scale = 0.1
    cell.scaleSpeed = -0.1

    cell.contents = UIImage(named: "particle")?.cgImage
    particlesEmitter.emitterCells = [cell]

    layer.addSublayer(particlesEmitter)

    DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
        particlesEmitter.birthRate = 0
    }
}

Answer 2

我不知道为什么它的行为有点奇怪，但我发现使用键值编码解决了这个问题，细胞停止发射：

假设您的CAEmitterLayer是"yourEmitterLayer“，并且您有一个名为"yourEmitterCell”的CAEmitterCell，如果放在touchesEnded中，这将停止发射：

[yourEmitterLayer setValue:[NSNumber numberWithInt:0] forKeyPath:@"emitterCells.yourEmitterCell.birthRate"];