[JAVA] [Gang of Four] Design pattern learning --Observer

Observer-Observer

** Table of Contents ** This pattern is to let other objects monitoring it do something at the same time when the properties of one object are updated.

It's an image that notifies the monitored object that the updated object has been updated. This is the Live Data of Android Architecture Components.

Purpose

Define a one-to-many dependency between objects so that when an object changes state, all objects that depend on it are automatically notified and updated.

Component

· Subject Observed abstract class -Observer Abstract class you want to monitor -ConcreteSubject A monitored concrete class -ConcreteObserver The concrete class you want to monitor

Implementation

We will implement a sample program that is automatically updated when the application installed on the smartphone is updated. The play store is the subject and the smartphone is the observer. I don't implement an abstract class because it's annoying.

ConcreteSubject The concrete class to be monitored

It is a play store.

PlayStore.kt

package observer

class PlayStore {
    interface UpdateListener {
        fun update(latestVersion: Int)
    }

    private val listeners = ArrayList<UpdateListener>()

    fun update(latestVersion: Int) {
        println("The sample app${latestVersion}It has been updated to.")
        listeners.forEach {
            it.update(latestVersion)
        }
    }

    fun addListener(listener: UpdateListener) {
        listeners.add(listener)
    }
}

ConcreteObserver The concrete class you want to monitor

Implement two classes to observe so that you can easily feel the effect of this pattern.

First user who has the sample app installed

SnakeSmartPhone.kt

package observer

class SnakeSmartPhone(playStore: PlayStore) {
    init {
        playStore.addListener(object: PlayStore.UpdateListener {
            override fun update(latestVersion: Int) {
                println("Snake's smartphone app${latestVersion}The sample was updated to.")
            }
        })
    }
}

Second user who has the sample app installed

Raiden.kt

package observer

class RaidenSmartPhone(playStore: PlayStore) {
    init {
        playStore.addListener(object: PlayStore.UpdateListener {
            override fun update(latestVersion: Int) {
                println("Raiden's smartphone sample app${latestVersion}It has been updated to.")
            }
        })
    }
}

People who use

Client.kt

package observer

class Client {
    init {
        val playStore = PlayStore()
        SnakeSmartPhone(playStore)
        RaidenSmartPhone(playStore)

        playStore.update(1)

        playStore.update(2)
    }
}

Output result

[out-put]
The sample app has been updated to 1.
Snake's smartphone sample app has been updated to 1.
Raiden's smartphone sample app has been updated to 1.
The sample app has been updated to 2.
Snake's smartphone sample app has been updated to 2.
Raiden's smartphone sample app has been updated to 2.

Every time you update the play store app, the smartphone on which it is installed is automatically updated.

that's all