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November 19, 2020 20:38
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LiveDataViewModel
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/* | |
* 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.android.example.livedatabuilder | |
import androidx.lifecycle.LiveData | |
import androidx.lifecycle.ViewModel | |
import androidx.lifecycle.ViewModelProvider | |
import androidx.lifecycle.liveData | |
import androidx.lifecycle.switchMap | |
import androidx.lifecycle.viewModelScope | |
import kotlinx.coroutines.Dispatchers | |
import kotlinx.coroutines.delay | |
import kotlinx.coroutines.launch | |
import java.util.Date | |
/** | |
* Showcases different patterns using the liveData coroutines builder. | |
*/ | |
class LiveDataViewModel( | |
private val dataSource: DataSource | |
) : ViewModel() { | |
// Exposed LiveData from a function that returns a LiveData generated with a liveData builder | |
val currentTime = dataSource.getCurrentTime() | |
// Coroutines inside a transformation | |
val currentTimeTransformed = currentTime.switchMap { | |
// timeStampToTime is a suspend function so we need to call it from a coroutine. | |
liveData { emit(timeStampToTime(it)) } | |
} | |
// Exposed liveData that emits and single value and subsequent values from another source. | |
val currentWeather: LiveData<String> = liveData { | |
emit(LOADING_STRING) | |
emitSource(dataSource.fetchWeather()) | |
} | |
// Exposed cached value in the data source that can be updated later on | |
val cachedValue = dataSource.cachedData | |
// Called when the user clicks on the "FETCH NEW DATA" button. Updates value in data source. | |
fun onRefresh() { | |
// Launch a coroutine that reads from a remote data source and updates cache | |
viewModelScope.launch { | |
dataSource.fetchNewData() | |
} | |
} | |
// Simulates a long-running computation in a background thread | |
private suspend fun timeStampToTime(timestamp: Long): String { | |
delay(500) // Simulate long operation | |
val date = Date(timestamp) | |
return date.toString() | |
} | |
companion object { | |
// Real apps would use a wrapper on the result type to handle this. | |
const val LOADING_STRING = "Loading..." | |
} | |
} | |
/** | |
* Factory for [LiveDataViewModel]. | |
*/ | |
object LiveDataVMFactory : ViewModelProvider.Factory { | |
private val dataSource = DefaultDataSource(Dispatchers.IO) | |
override fun <T : ViewModel?> create(modelClass: Class<T>): T { | |
@Suppress("UNCHECKED_CAST") | |
return LiveDataViewModel(dataSource) as T | |
} | |
} |
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