swift 使用 CAEmitterLayer实现小雨、中雨、暴雨、下雪、沙尘暴、高温的动画

使用 `CAEmitterLayer` 可以很方便地实现各种粒子效果，下面是使用 `CAEmitterLayer` 实现不同天气效果的示例代码：

### 小雨效果

let emitterLayer = CAEmitterLayer()
emitterLayer.emitterShape = .line
emitterLayer.emitterSize = CGSize(width: view.bounds.width, height: 1)

let rain = makeRainCell()
emitterLayer.emitterCells = [rain]

view.layer.addSublayer(emitterLayer)

func makeRainCell() -> CAEmitterCell {
    let cell = CAEmitterCell()
    cell.contents = UIImage(named: "raindrop")?.cgImage
    cell.birthRate = 50
    cell.lifetime = 2
    cell.velocity = 100
    cell.velocityRange = 50
    cell.yAcceleration = 500
    cell.scale = 0.2
    cell.scaleRange = 0.1
    cell.emissionLongitude = -.pi
    cell.emissionRange = .pi / 4
    return cell
}

### 中雨效果

let emitterLayer = CAEmitterLayer()
emitterLayer.emitterShape = .line
emitterLayer.emitterSize = CGSize(width: view.bounds.width, height: 1)

let rain = makeRainCell()
emitterLayer.emitterCells = [rain]

view.layer.addSublayer(emitterLayer)

func makeRainCell() -> CAEmitterCell {
    let cell = CAEmitterCell()
    cell.contents = UIImage(named: "raindrop")?.cgImage
    cell.birthRate = 100
    cell.lifetime = 2
    cell.velocity = 150
    cell.velocityRange = 50
    cell.yAcceleration = 500
    cell.scale = 0.3
    cell.scaleRange = 0.1
    cell.emissionLongitude = -.pi
    cell.emissionRange = .pi / 4
    return cell
}

### 暴雨效果

let emitterLayer = CAEmitterLayer()
emitterLayer.emitterShape = .line
emitterLayer.emitterSize = CGSize(width: view.bounds.width, height: 1)

let rain = makeRainCell()
emitterLayer.emitterCells = [rain]

view.layer.addSublayer(emitterLayer)

func makeRainCell() -> CAEmitterCell {
    let cell = CAEmitterCell()
    cell.contents = UIImage(named: "raindrop")?.cgImage
    cell.birthRate = 200
    cell.lifetime = 2
    cell.velocity = 200
    cell.velocityRange = 50
    cell.yAcceleration = 500
    cell.scale = 0.4
    cell.scaleRange = 0.1
    cell.emissionLongitude = -.pi
    cell.emissionRange = .pi / 4
    return cell
}

### 下雪效果

let emitterLayer = CAEmitterLayer()
emitterLayer.emitterShape = .line
emitterLayer.emitterSize = CGSize(width: view.bounds.width, height: 1)

let snow = makeSnowCell()
emitterLayer.emitterCells = [snow]

view.layer.addSublayer(emitterLayer)

func makeSnowCell() -> CAEmitterCell {
    let cell = CAEmitterCell()
    cell.contents = UIImage(named: "snowflake")?.cgImage
    cell.birthRate = 50
    cell.lifetime = 10
    cell.velocity = 50
    cell.velocityRange = 20
    cell.yAcceleration = 10
    cell.scale = 0.2
    cell.scaleRange = 0.1
    cell.emissionLongitude = -.pi
    cell.emissionRange = .pi / 4
    return cell
}

### 沙尘暴效果

let emitterLayer = CAEmitterLayer()
emitterLayer.emitterShape = .line
emitterLayer.emitterSize = CGSize(width: view.bounds.width, height: 1)

let dust = makeDustCell()
emitterLayer.emitterCells = [dust]

view.layer.addSublayer(emitterLayer)

func makeDustCell() -> CAEmitterCell {
    let cell = CAEmitterCell()
    cell.contents = UIImage(named: "dust")?.cgImage
    cell.birthRate = 100
    cell.lifetime = 5
    cell.velocity = 100
    cell.velocityRange = 50
    cell.yAcceleration = 10
    cell.scale = 0.2
    cell.scaleRange = 0.1
    cell.emissionLongitude = -.pi
    cell.emissionRange = .pi / 4
    return cell
}

### 高温效果

let emitterLayer = CAEmitterLayer()
emitterLayer.emitterShape = .line
emitterLayer.emitterSize = CGSize(width: view.bounds.width, height: 1)

let fire = makeFireCell()
emitterLayer.emitterCells = [fire]

view.layer.addSublayer(emitterLayer)

func makeFireCell() -> CAEmitterCell {
    let cell = CAEmitterCell()
    cell.contents = UIImage(named: "fire")?.cgImage
    cell.birthRate = 100
    cell.lifetime = 1
    cell.velocity = 50
    cell.velocityRange = 20
    cell.yAcceleration = -50
    cell.scale = 0.2
    cell.scaleRange = 0.1
    cell.color = UIColor.orange.cgColor
    cell.emissionLongitude = .pi
    cell.emissionRange = .pi / 4
    return cell
}

以上代码中，每个天气效果都是通过创建一个 `CAEmitterLayer` 和一个或多个 `CAEmitterCell` 来实现的。在每个 `CAEmitterCell` 中，可以设置粒子的属性，例如：出生率、寿命、速度、加速度、缩放比例、颜色等等。可以根据具体需求自由调整每个属性的值，以达到想要的效果。