LiveData没法调用Transformations

Question

#学习到LiveData的map和switchMap用法，但是无法正常调用Transformations方法，倒是有Transformation。请教下大家哪里出问题了。

MainActivity.kt

package com.example.jetpacktest

import android.content.Context
import android.content.SharedPreferences
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Button
import android.widget.TextView
import androidx.core.content.edit
import androidx.lifecycle.ViewModelProvider

class MainActivity : AppCompatActivity() {

    private lateinit var viewModel: MainViewModel
    private lateinit var sp: SharedPreferences
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val textView: TextView = findViewById(R.id.textView)

        sp = getPreferences(Context.MODE_PRIVATE)
        val countReserved = sp.getInt("count_reserved", 0)
        viewModel = ViewModelProvider(this, MainViewModelFactory(countReserved))[MainViewModel::class.java]
        val button: Button = findViewById(R.id.button)
        button.setOnClickListener {
            viewModel.plusOne()
        }

        val clearBtn: Button = findViewById(R.id.clearBtn)
        clearBtn.setOnClickListener {
            viewModel.clear()
        }

        val getUserBtn: Button = findViewById(R.id.getUserBtn)
        getUserBtn.setOnClickListener {
            val userId = (0..10000).random().toString()
            viewModel.getUser(userId)
        }

        viewModel.user.observe(this) { user ->
            textView.text = user.firstName
        }

     /*   viewModel.counter.observe(this) {
            count -> textView.text = count.toString()
        }  */
    }

    override fun onPause() {
        super.onPause()
        sp.edit {
            putInt("count_reserved", viewModel.counter.value ?: 0)
        }
    }
}

MainViewModel.kt

package com.example.jetpacktest

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel

class MainViewModel(countReserved: Int): ViewModel() {

    private val userIdLiveData = MutableLiveData<String>()
    val user: LiveData<User> = Transformations.switchMap(userIdLiveData) {
        userId -> Repository.getUser(userId)
    }
    val counter: LiveData<Int>get() = _counter
    private val _counter = MutableLiveData<Int>()
    init {
        _counter.value = countReserved
    }

    fun plusOne() {
        val count = counter.value ?: 0
        _counter.value = count + 1
    }

    fun clear() {
        _counter.value = 0
    }

    fun getUser(userId: String) {
        userIdLiveData.value = userId
    }
}

MainViewModelFactory.kt

package com.example.jetpacktest

import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProvider

class MainViewModelFactory(private val countReserved: Int): ViewModelProvider.Factory {
    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        return MainViewModel(countReserved) as T
    }
}

MyObserver.kt

package com.example.jetpacktest

import android.util.Log
import androidx.lifecycle.DefaultLifecycleObserver
import androidx.lifecycle.LifecycleOwner

class MyObserver: DefaultLifecycleObserver {
    override fun onStart(owner: LifecycleOwner) {
        super.onStart(owner)
        Log.d("MyObserver", "onStart")
    }

    override fun onStop(owner: LifecycleOwner) {
        super.onStop(owner)
        Log.d("MyObserver", "onStop")
    }
}

Repository.kt

package com.example.jetpacktest

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData

object Repository {
    fun getUser(userId: String): LiveData<User> {
        val liveData = MutableLiveData<User>()
        liveData.value = User(userId, userId, 0)
        return liveData
    }
}

User.kt

package com.example.jetpacktest

data class User(var firstName: String, var lastName: String, var age: Int)

activity_mail

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="16dp"
        android:textSize="32sp" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="16dp"
        android:text="@string/button" />

    <Button
        android:id="@+id/clearBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal"
        android:text="@string/clear"/>

    <Button
        android:id="@+id/getUserBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:text="get user"/>

</LinearLayout>

依赖库

implementation 'androidx.core:core-ktx:1.8.0'
    implementation 'androidx.appcompat:appcompat:1.5.0'
    implementation 'com.google.android.material:material:1.8.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.1.3'
    implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'
    implementation 'androidx.lifecycle:lifecycle-livedata-ktx:2.6.1'
    testImplementation 'junit:junit:4.13.2'
    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'

Answer 1

都没导包，Transformations是android.lifecycle包中一个工具类