How to use Single Live Event to show toast in Kotlin

问题

I want to use single live event class to show toast (like flag) Here is my code what I tried. I want to not use peding like flag. how do I fix it?

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)
                }
            }
        })
}

回答1:

Solution

Step 1. Copy the SingleLiveEvent.kt to your app

/*
 *  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);
    }
}

Step 2. Use from your code.

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)
        }
    })
}

回答2:

SingleLiveEvent extends MutableLiveData. So, you can use it just like a normal MutableLiveData.

First, you need to include 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). Copy this class file and add it to your project.

You can set it like this in your ViewModel when you want to show toast,

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

Make a function in your ViewModel to observe this SingleLiveEvent toastMsg and observe it just like you observe your regular LiveData in your Activity

In ViewModel:

SingleLiveEvent getToastSLE() {
    return toastMsg
}

In Activity:

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

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

来源：https://stackoverflow.com/questions/65266324/how-to-use-single-live-event-to-show-toast-in-kotlin

标签

android

kotlin

mutablelivedata