如何更改 LiveData 的值而不通知观察者

Question

在某些情况下，我想更改 LiveData 的值，并且不希望观察者通知更改。我该怎么做？

Answer 1

我只会使用 Pair<T, Boolean> 作为值类型，观察者可以检查pair.second

这个想法很简单。将MutableLiveData<String>替换为MutableLiveData<Pair<String, Boolean>>

当您想用值s通知观察者时，只需调用liveData.setValue(new Pair(s, true))并调用liveData.setValue(new Pair(s, false))否则

在观察者方面，只需检查pair.second即可区分这两种情况

编辑：我正在使用电话来回答这个问题，似乎堆栈溢出隐藏了用尖括号包围的字符。

Answer 2

我想我的自定义类会帮助你

import android.arch.lifecycle.Lifecycle;
import android.arch.lifecycle.LifecycleOwner;
import android.arch.lifecycle.MutableLiveData;
import android.arch.lifecycle.Observer;
import android.support.annotation.MainThread;
import android.support.annotation.NonNull;
import android.util.Pair;
import java.util.ArrayList;
import java.util.List;
public class SSBLiveData<T> extends MutableLiveData<T> {
    private List<Pair<LifecycleOwner, Observer>> observers = new ArrayList<>();
    @MainThread
    @Override
    public void observe(@NonNull LifecycleOwner owner, @NonNull final Observer observer) {
        observers.add(new Pair<>(owner, observer));
        super.observe(owner, observer);
    }
    @MainThread
    private void resetAllObservers() {
        for (Pair<LifecycleOwner, Observer> observer : observers) {
            super.observe(observer.first, observer.second);
        }
    }
    @MainThread
    private void removeAllObservers() {
        for (Pair<LifecycleOwner, Observer> observer : observers) {
            removeObservers(observer.first);
        }
    }
    @MainThread
    void setValueWithoutNotify(T value) {
        removeAllObservers();
        super.setValue(value);
        System.out.println("Not notifying value " + getValue());
    }
   // For androidx use this method
   /*@MainThread
   @Override
   public void removeObserver(@NonNull Observer<? super T> observer) {
    for (Pair<LifecycleOwner, Observer> observerItem : observers) {
        if (observerItem.second.equals(observer) && observerItem.first.getLifecycle().getCurrentState() == Lifecycle.State.DESTROYED) {
            observers.remove(observerItem);
        }
    }
    super.removeObserver(observer);
   }*/
    @MainThread
    @Override
    public void removeObserver(@NonNull Observer<T> observer) {
        for (Pair<LifecycleOwner, Observer> observerItem : observers) {
            if (observerItem.second.equals(observer) && observerItem.first.getLifecycle().getCurrentState() == Lifecycle.State.DESTROYED) {
                observers.remove(observerItem);
            }
        }
        super.removeObserver(observer);
    }
    @Override
    public void setValue(T value) {
        super.setValue(value);
        if (!hasObservers()) {
            resetAllObservers();
        }
    }
}

以这种方式使用它

SSBLiveData<String> liveData = new SSBLiveData<>();
liveData.setValueWithoutNotify("It won't get notified");
liveData.setValue("It will get notified");

Answer 3

在接受的答案中描述的相同类，但在 Kotlin 和AndroidX 导入中被覆盖：

import android.util.Pair
import androidx.annotation.MainThread
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleOwner
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.Observer
import java.util.*
class ExtendedLiveData<T>() : MutableLiveData<T>() {
    private val observers: MutableList<Pair<LifecycleOwner, Observer<in T>>> = ArrayList()
    @MainThread
    override fun observe(owner: LifecycleOwner, observer: Observer<in T>) {
        observers.add(Pair(owner, observer))
        super.observe(owner, observer)
    }
    @MainThread
    private fun resetAllObservers() {
        for (observer in observers) {
            super.observe(observer.first, observer.second)
        }
    }
    @MainThread
    private fun removeAllObservers() {
        for (observer in observers) {
            removeObservers(observer.first)
        }
    }
    @MainThread
    fun setValueWithoutNotify(value: T) {
        removeAllObservers()
        super.setValue(value)
        println("Not notifying value " + getValue())
    }
    @MainThread
    override fun removeObserver(observer: Observer<in T>) {
        for (observerItem in observers) {
            if (observerItem.second == observer && observerItem.first.lifecycle
                    .currentState == Lifecycle.State.DESTROYED
            ) {
                observers.remove(observerItem)
            }
        }
        super.removeObserver(observer)
    }
    override fun setValue(value: T) {
        super.setValue(value)
        if (!hasObservers()) {
            resetAllObservers()
        }
    }
    fun resetValue() {
        super.setValue(null)
    }
}