如何使用 Single Live Event 在 Kotlin 中显示 toast

Question

我想使用单个直播活动 class 来展示祝酒词（如旗帜）这是我尝试过的代码。我不想像旗帜一样使用 peding。我该如何解决？

MainViewModel

class MainViewModel(private val movieRepository: MovieRepository) : ViewModel() {
    val keyword = MutableLiveData<String>()
    val movieList = MutableLiveData<List<Movie>>()
    val msg = MutableLiveData<String>()
    val pending: AtomicBoolean = AtomicBoolean(false)

    fun findMovie() {
        val keywordValue = keyword.value ?: return
        pending.set(true)
        if (keywordValue.isNullOrBlank()) {
            msg.value = "emptyKeyword"
            return
        }
        movieRepository.getMovieData(keyword = keywordValue, 30,
            onSuccess = {
                if (it.items!!.isEmpty()) {
                    msg.value = "emptyResult"
                } else {
                    msg.value = "success"
                    movieList.value = it.items
                }
            },
            onFailure = {
                msg.value = "fail"
            }
        )
    }
}

MainActivity

 private fun viewModelCallback() {
        mainViewModel.msg.observe(this, {
            if (mainViewModel.pending.compareAndSet(true, false)) {
                when (it) {
                    "success" -> toast(R.string.network_success)
                    "emptyKeyword" -> toast(R.string.keyword_empty)
                    "fail" -> toast(R.string.network_error)
                    "emptyResult" -> toast(R.string.keyword_result_empty)
                }
            }
        })
}

Answer 1

SingleLiveEvent 扩展 MutableLiveData。所以，你可以像普通的 MutableLiveData.

一样使用它

首先，您需要包含 SingleLiveEvent.java class (https://github.com/android/architecture-samples/blob/dev-todo-mvvm-live/todoapp/app/src/main/java/com/example/android/architecture/blueprints/todoapp/SingleLiveEvent.java)。复制此 class 文件并将其添加到您的项目中。

当你想显示 toast 时，你可以在你的 ViewModel 中这样设置，

SingleLiveEvent<String> toastMsg = new SingleLiveEvent<>(); //this goes in ViewModel constructor
toastMsg.setValue("hello"); //when you want to show toast

在您的 ViewModel 中创建一个函数来观察此 SingleLiveEvent toastMsg 并像观察您的 Activity

中的常规 LiveData 一样观察它

在视图模型中：

SingleLiveEvent getToastSLE() {
    return toastMsg
}

在Activity中：

viewmodel.getToastSLE().observe(this, toastString -> {
    Toast.makeText(this, toastString, Toast.LENGTH_LONG).show() //this will display toast "hello"
})

原文：https://medium.com/androiddevelopers/livedata-with-snackbar-navigation-and-other-events-the-singleliveevent-case-ac2622673150

Answer 2

解决方案

步骤 1. 将 SingleLiveEvent.kt 复制到您的应用程序

/*
 *  Copyright 2017 Google Inc.
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */

package com.example.myapp;

import android.util.Log;

import androidx.annotation.MainThread;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.lifecycle.LifecycleOwner;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.Observer;

import java.util.concurrent.atomic.AtomicBoolean;

/**
 * A lifecycle-aware observable that sends only new updates after subscription, used for events like
 * navigation and Snackbar messages.
 * <p>
 * This avoids a common problem with events: on configuration change (like rotation) an update
 * can be emitted if the observer is active. This LiveData only calls the observable if there's an
 * explicit call to setValue() or call().
 * <p>
 * Note that only one observer is going to be notified of changes.
 */
public class SingleLiveEvent<T> extends MutableLiveData<T> {

    private static final String TAG = "SingleLiveEvent";

    private final AtomicBoolean mPending = new AtomicBoolean(false);

    @MainThread
    public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {
        if (hasActiveObservers()) {
            Log.w(TAG, "Multiple observers registered but only one will be notified of changes.");
        }
        // Observe the internal MutableLiveData
        super.observe(owner, t -> {
            if (mPending.compareAndSet(true, false)) {
                observer.onChanged(t);
            }
        });
    }

    @MainThread
    public void setValue(@Nullable T t) {
        mPending.set(true);
        super.setValue(t);
    }

    /**
     * Used for cases where T is Void, to make calls cleaner.
     */
    @MainThread
    public void call() {
        setValue(null);
    }
}

第 2 步。 从您的代码中使用。

MainViewModel

class MainViewModel(private val movieRepository: MovieRepository) : ViewModel() {
    val keyword = MutableLiveData<String>()
    val movieList = MutableLiveData<List<Movie>>()
    val msg = SingleLiveEvent<String>()

    fun findMovie() {
        val keywordValue = keyword.value ?: return
        if (keywordValue.isNullOrBlank()) {
            msg.value = "emptyKeyword"
            return
        }
        movieRepository.getMovieData(keyword = keywordValue, 30,
            onSuccess = {
                if (it.items!!.isEmpty()) {
                    msg.value = "emptyResult"
                } else {
                    msg.value = "success"
                    movieList.value = it.items
                }
            },
            onFailure = {
                msg.value = "fail"
            }
        )
    }
}

MainActivity

private fun viewModelCallback() {
    mainViewModel.msg.observe(this, {
        when (it) {
            "success" -> toast(R.string.network_success)
            "emptyKeyword" -> toast(R.string.keyword_empty)
            "fail" -> toast(R.string.network_error)
            "emptyResult" -> toast(R.string.keyword_result_empty)
        }
    })
}

Answer 3

而不是 SingleLiveEvent，如果您使用的是 Kotlin 并且仅一次性触发 data/event，请使用 MutableSharedFlow

示例：

// init
val data = MutableSharedFlow<String>()

// set value
data.emit("hello world)

lifecycleScope.launchWhenStarted {
      data.collectLatest { 
          // value only collect once unless a new trigger come
      }
}

MutableSharedFlow 不会触发方向更改或返回上一个片段等

Answer 4

如 here 在 2021 年 12 月编辑 最后所述，您应该让视图告诉您的 viewModel 您的事件已被处理。它不是一个漂亮的解决方案，但它绝对是最容易理解和实施的解决方案之一。

基本上你是在你的 viewModel 中添加一个 StateFlow 来保存你的事件然后在你的视图收集它之后，你再次将该状态重置为空：
在你的 viewModel ->

private val _loadingPostVisibilityEvent = MutableStateFlow<Boolean?>(null)
val loadingPostVisibilityEvent: StateFlow<Boolean?> = _loadingPostVisibilityEvent
fun setLoadingPostVisibilityEvent(isVisible: Boolean?) {
    _loadingPostVisibilityEvent.value = isVisible
}

那么在你看来->

viewLifecycleOwner.lifecycleScope.launch {
    repeatOnLifecycle(Lifecycle.State.CREATED) {
        launch {
           actionsViewModel.loadingPostVisibilityEvent.filterNotNull().collect {
                  // do your magic with the value $it
                  //then don't forget to reset the state.
                  actionsViewModel.setLoadingPostVisibilityEvent(null)
           }
        }
    }
}

注意如果您没有将事件 stateFlow 重置为 null，如果再次重新创建视图，它可能会再次被收集。

如果你想使用扩展功能收集一次然后添加这个 ->

suspend fun <T> StateFlow<T?>.collectOnce(reset: (T?) -> Unit, action: (value: T) -> Unit) {
    this.filterNotNull().onEach { reset.invoke(null) }.collect {
        action.invoke(it)
    }
}

并像这样使用它

viewLifecycleOwner.lifecycleScope.launch {
    repeatOnLifecycle(Lifecycle.State.CREATED) {
        launch {
           actionsViewModel.loadingPostVisibilityEvent.collectOnce(actionsViewModel::setLoadingPostVisibilityEvent) {
                  // do your magic with the value $it
           }
        }
    }
}

如何使用 Single Live Event 在 Kotlin 中显示 toast

How to use Single Live Event to show toast in Kotlin

android

kotlin

mutablelivedata