LiveData 「粘性事件」问题

问题描述

dialog里监听了liveData数据变化，当dialog打开关闭再次打开时候，会发现旧数据会再次灌入。等到网络请求回来，新数据才会展示。

现象

页面首先展示老数据，过一会又展示新的数据。又称为数据倒灌。

解决方案

1. 使用LiveEvent替换原来的MutableLiveData,解决
   val liveData = LiveEvent()

/**
 * @desc fix 粘性事件引发的问题
 */
open class LiveEvent<T> : MediatorLiveData<T>() {

    private val observers = ArraySet<ObserverWrapper<in T>>()

    @MainThread
    override fun observe(owner: LifecycleOwner, observer: Observer<in T>) {
        observers.find { it.observer === observer }?.let { _ -> // existing
            return
        }
        val wrapper = ObserverWrapper(observer)
        observers.add(wrapper)
        super.observe(owner, wrapper)
    }

    @MainThread
    override fun observeForever(observer: Observer<in T>) {
        observers.find { it.observer === observer }?.let { _ -> // existing
            return
        }
        val wrapper = ObserverWrapper(observer)
        observers.add(wrapper)
        super.observeForever(wrapper)
    }

    @MainThread
    override fun removeObserver(observer: Observer<in T>) {
        if (observer is ObserverWrapper && observers.remove(observer)) {
            super.removeObserver(observer)
            return
        }
        val iterator = observers.iterator()
        while (iterator.hasNext()) {
            val wrapper = iterator.next()
            if (wrapper.observer == observer) {
                iterator.remove()
                super.removeObserver(wrapper)
                break
            }
        }
    }

    @MainThread
    override fun setValue(t: T?) {
        observers.forEach { it.newValue() }
        super.setValue(t)
    }

    private class ObserverWrapper<T>(val observer: Observer<T>) : Observer<T> {

        private var pending = false

        override fun onChanged(t: T?) {
            if (pending) {
                pending = false
                observer.onChanged(t)
            }
        }

        fun newValue() {
            pending = true
        }
    }
}

2. 使用SharedFlow

参考文章

Jetpack MVVM 中正确处理 Events 链接