[移动开发]基于LiveData实现事件总线

前言：提及事件总线常常使用org.greenrobot:eventbus框架，而本文通过LiveData即可轻松实现Eventbus。
实现：这里只定义了1个LiveData，通过TAG作事件的区分。
1、定义事件基类

data class BaseEvent<T>(
    val tag: Int,
    val msg: T
)

2、定义事件操作类

object EventBus{
    val onEventLiveData = MutableLiveData<BaseEvent<*>>()

    const val TAG_CONNECT_PRINTER_SUCCESS = 1
}

3、注册事件

EventBus.onEventLiveData.observe(this, Observer {
    if (it.tag == EventBus.TAG_CONNECT_PRINTER_SUCCESS) {
        mViewModel.showMsgToastLiveData.value = "连接打印机成功"
    }
})

4、发送事件

EventBus.onEventLiveData.setValue(BaseEvent(EventBus.TAG_CONNECT_PRINTER_SUCCESS, true))

问题：由于LiveData本身是具有数据粘性的（即页面A、B都注册了同一事件，而在A中更新事件后进入B页面则立即会收到更新事件，然而实际场景我们只关注页面注册事件的更新事件，而之前的不需要）
原因：这是LiveData框架本身的设计原因（即注册时给订阅者和LiveData记录初始版本号，更新LiveData时其版本号加1，当订阅者的版本比LiveData小时则更新，而当有新的注册时，订阅者版本为初始值，LiveData为更新后的值，大于初始值所有就收到更新）
方案：注册的时候给订阅者版本号为LiveData的版本号即可。

/**
 * 非粘性LiveData
 */
class UnPeekMutableLiveData<T> : MutableLiveData<T>() {
    private val START_VERSION = -1
    private val mCurrentVersion = AtomicInteger(START_VERSION)

    override fun observe(owner: LifecycleOwner, observer: Observer<in T>) {
        super.observe(owner, createObserverWrapper(observer, mCurrentVersion.get()))
    }

    override fun observeForever(observer: Observer<in T>) {
        super.observeForever(createObserverForeverWrapper(observer, mCurrentVersion.get()))
    }


    fun observeSticky(owner: LifecycleOwner, observer: Observer<T>) {
        super.observe(owner, createObserverWrapper(observer, START_VERSION))
    }


    fun observeStickyForever(observer: Observer<in T>) {
        super.observeForever(createObserverForeverWrapper(observer, START_VERSION))
    }

    override fun setValue(value: T) {
        mCurrentVersion.getAndIncrement()
        super.setValue(value)
    }

    override fun removeObserver(observer: Observer<in T>) {
        if (TextUtils.isEmpty(observer.toString())) {
            super.removeObserver(observer)
        } else {
            super.removeObserver(createObserverWrapper(observer, START_VERSION))
        }
    }

    private fun createObserverForeverWrapper(
        observer: Observer<in T>,
        version: Int
    ): MyObserverWrapper<T> {
        return MyObserverWrapper(observer, mCurrentVersion, version, true)
    }

    private fun createObserverWrapper(
        observer: Observer<in T>,
        version: Int
    ): MyObserverWrapper<T> {
        return MyObserverWrapper(observer, mCurrentVersion, version, false)
    }

    fun clear() {
        super.setValue(null)
    }
}

class MyObserverWrapper<T> constructor(
    private val mObserver: Observer<in T>, private val mLiveDataVersion: AtomicInteger,
    private val mVersion: Int, private val mIsForever: Boolean): Observer<T> {

    override fun onChanged(t: T?) {
        if (mLiveDataVersion.get() > mVersion) {
            mObserver.onChanged(t)
        }
    }

    override fun equals(o: Any?): Boolean {
        if (this === o) {
            return true
        }
        if (o == null || javaClass != o.javaClass) {
            return false
        }
        val that = o as MyObserverWrapper<*>
        return Objects.equals(mObserver, that.mObserver)
    }

    override fun hashCode(): Int {
        return Objects.hash(mObserver)
    }

    override fun toString(): String {
        return if (mIsForever) "IS_FOREVER" else ""
    }
}

只需将EventBus里的MutableLiveData替换为UnPeekMutableLiveData即可

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