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DefaultDataSource
/*
* 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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