[LiveData - SingleEvent单次消费]

[LiveData - SingleEvent单次消费]

一、LiveData单次消费

在整理Jetpack Hilt依赖注入时，发现了架构组件项目中对LiveData事件做单次处理时从设计模式层面给出了相应的处理方式，记录一下。后期还是建议切换到Flow上面。

/*
 * Copyright (C) 2019 The Android Open Source Project
 *
 * 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.android.architecture.blueprints.todoapp

import androidx.lifecycle.Observer

/**
 * Used as a wrapper for data that is exposed via a LiveData that represents an event.
 */
open class Event<out T>(private val content: T) {

    @Suppress("MemberVisibilityCanBePrivate")
    var hasBeenHandled = false
        private set // Allow external read but not write

    /**
     * Returns the content and prevents its use again.
     */
    fun getContentIfNotHandled(): T? {
        return if (hasBeenHandled) {
            null
        } else {
            hasBeenHandled = true
            content
        }
    }

    /**
     * Returns the content, even if it's already been handled.
     */
    fun peekContent(): T = content
}

/**
 * An [Observer] for [Event]s, simplifying the pattern of checking if the [Event]'s content has
 * already been handled.
 *
 * [onEventUnhandledContent] is *only* called if the [Event]'s contents has not been handled.
 */
class EventObserver<T>(private val onEventUnhandledContent: (T) -> Unit) : Observer<Event<T>> {
    override fun onChanged(event: Event<T>?) {
        event?.getContentIfNotHandled()?.let {
            onEventUnhandledContent(it)
        }
    }
}

二、出处

architecture-samples，切换到 dev-hilt 分之下。

LiveData with SnackBar, Navigation and other events (the SingleLiveEvent case)需要楼梯🪜，作者是同一个人Jose Alcérreca。