如何从 MutableLiveData 发出不同的值?

How to emit distinct values from MutableLiveData?

我观察到 MutableLiveData 会触发观察者 onChanged,即使向其 setValue 方法提供了相同的对象实例也是如此。

//Fragment#onCreateView - scenario1
val newValue = "newValue"
mutableLiveData.setValue(newValue) //triggers observer
mutableLiveData.setValue(newValue) //triggers observer

//Fragment#onCreateView - scenario2
val newValue = "newValue"
mutableLiveData.postValue(newValue) //triggers observer
mutableLiveData.postValue(newValue) //does not trigger observer

如果向 setValue()/postValue()

提供相同或等效的实例,是否有办法避免观察者被通知两次

我尝试扩展 MutableLiveData 但没有成功。我可能在这里遗漏了一些东西

class DistinctLiveData<T> : MutableLiveData<T>() {

    private var cached: T? = null

    @Synchronized override fun setValue(value: T) {
        if(value != cached) {
            cached = value
            super.setValue(value)
        }
    }

    @Synchronized override fun postValue(value: T) {
        if(value != cached) {
            cached = value
            super.postValue(value)
        }
    }
}

您可以使用以下魔术来消耗“相同的物品”:

fun <T> LiveData<T>.distinctUntilChanged(): LiveData<T> = MediatorLiveData<T>().also { mediator ->
    mediator.addSource(this, object : Observer<T> {
        private var isInitialized = false
        private var previousValue: T? = null

        override fun onChanged(newValue: T?) {
            val wasInitialized = isInitialized
            if (!isInitialized) {
                isInitialized = true
            }
            if(!wasInitialized || newValue != previousValue) {
                previousValue = newValue
                mediator.postValue(newValue)
            }
        }
    })
}

如果要检查引用相等性,那就是!==


但它已被添加到 Transformations.distinctUntilChanged

API 中已有:Transformations.distinctUntilChanged()

distinctUntilChanged

public static LiveData<X> distinctUntilChanged (LiveData<X> source)

Creates a new LiveData object does not emit a value until the source LiveData value has been changed. The value is considered changed if equals() yields false.

<<snip remainder>>

如果我们谈论 MutableLiveData,您可以创建一个 class 并覆盖 setValue,然后仅在 new value != old value

时通过 super 调用
class DistinctUntilChangedMutableLiveData<T> : MutableLiveData<T>() {
    override fun setValue(value: T?) {
        if (value != this.value) {
            super.setValue(value)
        }
    }
}

在我的例子中,我有相当复杂的对象,我必须按某些字段进行比较。为此,我更改了 EpicPandaForce 的回答:

fun <T> LiveData<T>.distinctUntilChanged(compare: T?.(T?) -> Boolean = { this == it }): LiveData<T> = MediatorLiveData<T>().also { mediator ->
    mediator.addSource(this) { newValue ->
        if(!newValue.compare(value)) {
            mediator.postValue(newValue)
        }
    }
}

默认情况下它使用标准 equals 方法,但如果您需要 - 您可以更改区分逻辑