Question

我是Kotlin和ViewModel使用的新手。我正在尝试创建一个简单的ViewModel，它具有字符串列表。想法是在此ViewModel中更新图片路径列表（字符串）。

我的问题是我的ViewModel没有更新。它只会添加1个条目。

class PicturesViewModel : ViewModel() {

    var pictureListLive: MutableLiveData<MutableList<String>> = MutableLiveData()

    var list = ArrayList<String>()

    fun addPictureToList(picture: String) {
        list.add(picture)
        pictureListLive.value = list
    }
}

在我的活动中，我初始化viewModel并像这样观察：

 private fun configureViewModel(){
        this.picturesViewModel = ViewModelProviders.of(this).get(PicturesViewModel::class.java!!)
        this.picturesViewModel.pictureListLive.observe(this, Observer{
            if (it != null) {
                Log.i("Pictures",it.size.toString())
            }
        })
    }

然后我在ViewModel中添加一个字符串：

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        if (resultCode == Activity.RESULT_OK && requestCode == REQUEST_IMAGE_CAPTURE) {
            picturesViewModel.addPictureToList(currentPhotoPath)
            val intentPreview = Intent(this,PreviewActivity::class.java)
            intentPreview.putExtra("Picture",currentPhotoPath)
            startActivity(intentPreview)}

        if (resultCode == Activity.RESULT_OK && requestCode == REQUEST_CHOOSE_PHOTO){
            this.pictureSelectedPath = data?.data.toString()
            Log.i("Photo",pictureSelectedPath)
        }
    }

ActivityOnResult来自相机拍照后的结果。我想更新ViewModel的字符串列表，以在拍摄完几张照片后添加照片相机路径。

今天当我像这样使用它时，ViewModel列表的大小保持为1。我认为其中只有最后一个。

感谢您的帮助。

Answer 1

这是初始化和使用简单ViewModel的用例

//in your pictures fragment
private lateinit var picturesViewModel: PicturesViewModel

比您的onCreate

picturesViewModel= ViewModelProviders.of(this).get(PicturesViewModel::class.java)

初始化视图模型后，您可能会在初始化以下的过程中观察在那里发生的数据更改：

picturesViewModel.pictureListLive.observe(this, Observer {
            //list changes are handled here 
        })

如果我要重写ViewModelClass，它将像这样：

class PictureViewModel : ViewModel() {

    private var pictureListLive: MutableLiveData<MutableList<String>> = MutableLiveData()


    fun setPictureList(picture: String) {
       val list = ArrayList<String>()
       list.add(picture)
       pictureListLive.value = list 
    }
}

不需要返回列表，因为MutableLiveData以发布-订阅逻辑（也称为Observable Pattern）工作。

这类似于EventBus或RxJava operators

注意

我需要有关该setPictureList方法的更多信息，因为那样的话，它就没有太多意义了。或者，也许您想在其他地方或idk称呼它。

Answer 2

2021 年 4 月 10 日在 Android Studio 4.1.3 Kotlin 1.4.32 compileSdkVersion 30.0.3 可能的解决方案：

dependencies {
implementation 'androidx.core:core-ktx:1.3.2'
implementation 'androidx.appcompat:appcompat:1.2.0'
implementation "androidx.lifecycle:lifecycle-livedata-ktx:2.3.1"
implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:2.3.1"
implementation "androidx.fragment:fragment-ktx:1.3.2"
}

AppCompatActivity 中的代码：创建 ViewModel：

import android.app.Activity
import android.content.Intent
import android.os.Bundle
import android.util.Log
import android.view.View
import androidx.activity.result.contract.ActivityResultContracts
import androidx.activity.viewModels
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.LifecycleOwner

class MainActivity : AppCompatActivity() {
private val picturesViewModel: PicturesViewModel by viewModels()
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    picturesViewModel.pictureListLive.observe(this, {
       it?.let { Log.i("Pictures", "size =  ${it.size} ${if (it.size>0) it[it.size-1] 
       else "empty"}") }
    })
}

调用应用相机并返回结果

private var resultLauncher = registerForActivityResult(
    ActivityResultContracts.StartActivityForResult()) { result ->
if (result.resultCode == Activity.RESULT_OK) {
    // There are no request codes
    val data: Intent? = result.data
    val path = result?.data?.getStringExtra(PATH) ?:"sssss"
    picturesViewModel.addPictureToList(path)
  }
}

fun onClick(view: View) {
    val questionIntent = Intent(this, Camera::class.java)
    // new call for startActivityForResult:
    resultLauncher.launch(questionIntent)    }
 }

PicturesViewModel.kt：

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

class PicturesViewModel : ViewModel() {

var pictureListLive: MutableLiveData<MutableList<String>> =
    MutableLiveData(mutableListOf())

    fun addPictureToList(picture: String) {
     pictureListLive.value?.add(picture)
     // so that the observer can see that the list has changed
     pictureListLive.value = pictureListLive.value
    }
  }

结果：

2021-04-10 com.example.kotlinviewmodellistofstring I/Pictures: size =  0 empty
2021-04-10 com.example.kotlinviewmodellistofstring I/Pictures: size =  1 123Path
2021-04-10 com.example.kotlinviewmodellistofstring I/Pictures: size =  2 234Path
2021-04-10 com.example.kotlinviewmodellistofstring I/Pictures: size =  3 Path345
2021-04-10 com.example.kotlinviewmodellistofstring I/Pictures: size =  4 Pa456th
2021-04-10 com.example.kotlinviewmodellistofstring I/Pictures: size =  5 Pa567th
2021-04-10 com.example.kotlinviewmodellistofstring I/Pictures: size =  6 Path678
2021-04-10 com.example.kotlinviewmodellistofstring I/Pictures: size =  7 cam123456

Kotlin ViewModel字符串列表

2 个答案: