HeadFrist 观察者模式（Kotlin版）

HeadFrist 观察者模式（Kotlin版）

封装变化

多用组合，少用继承

针对接口编程，不针对实现编程

为了交互对象之间的松耦合设计而努力

气象站会通过设备获取一些气候信息，每隔一段时间发送到各个需要展示的面板上，面板显示：温度，湿度，压强

Observable(被观察者)

interface Observable {

    public fun addObserver(o : Observer)

    public fun deleteObserver(o : Observer)

    public fun notifyObserver()

    public fun setChanged()
}

气象站就是一个被观察者，通知各个面板数据的更新

WeatherData 实现Observable接口

package degisn.chapter2_observer

class WeatherData : Observable {

    private var observers = ArrayList<Observer>()
        get() = field
    private var temperature: Float = 0f  //温度
        get() = field
    private var humidity: Float = 0f //湿度
        get() = field
    private var pressure: Float = 0f //气压
        get() = field
    private var changed: Boolean = false

    /**
     * 注册观察者
     */
    override fun addObserver(o: Observer) {
        observers.add(o)
    }

    /**
     * 解注册
     */
    override fun deleteObserver(o: Observer) {
        var i: Int = observers.indexOf(o)
        if (i >= 0) {
            observers.remove(o)
        }
    }

    /**
     * 通知
     */
    override fun notifyObserver() {
        if（changed） {
            for (i in 0 until observers.size step 1) {
                var observer: Observer = observers.get(i)
                observer.update(temperature, humidity, pressure)
            }   
            changed = false 
        }
    }

    override fun setChanged() {
        changed = true
    }

    private fun measurementsChanged() {
        setChanged()
        notifyObserver()
    }

    public fun setMeasurementsChanged(temp: Float, humidity: Float, pressure: Float) {
        this.temperature = temp
        this.humidity = humidity
        this.pressure = pressure
        measurementsChanged()
    }
}

Observer(观察者)

interface Observer {
    public fun update(temp : Float, humidity : Float, pressure : Float)
}

DisplayElement(面板显示接口)

每一个面板显示的格式不同，采用原则：多用组合，少用继承

interface DisplayElement {
    public fun display()
}

各个面板需要接收到订阅的消息，由Observable来通知

实现一个面板 CurrentConditionDisplay

class CurrentConditionDisplay : Observer, DisplayElement {

    var temperature : Float = 0f
    var humidity : Float = 0f
    var pressure : Float = 0f
    var weatherData : Observable ?

    constructor(weatherData: Observable){
        this.weatherData = weatherData
        weatherData.addObserver(this)
    }

    override fun update(temp: Float, humidity: Float, pressure: Float) {
        this.temperature = temp
        this.humidity = humidity
        this.pressure = pressure
        display()
    }

    override fun display() {
        println("Current conditions: " + temperature + " F degree  " + humidity + " % humidity")
    }
}

构成气象站，测试

class WeatherStation {

    companion object { //静态代码块

        /**
         * 程序的主入口
         */
        @JvmStatic
        fun main(args: Array<String>){

            var weatherData : WeatherData = WeatherData()

            var currentConditionDisplay : CurrentConditionDisplay = CurrentConditionDisplay(weatherData)

            weatherData.setMeasurementsChanged(12f, 13f, 14f)

        }
    }
}