首次呼叫后无法观察LiveData [英] LiveData not Observing after first Call
问题描述
我实现了LiveData&模仿AsyncTaskLoader的ViewModel.
我从DCIM的摄影机目录中加载文件名，然后在删除文件(图片)时将fileObserver附加到Observe，然后回调告诉LiveData在发生delete事件时重新获取文件名
问题:
下面的代码应借助LiveData异步地从DCIM/Pictures中获取文件名，然后将FileObserver附加到目录(DCIM/Pictures)，以监视何时删除文件并使用如代码所示，LiveData子类可以重新加载文件.
好的，它是第一次工作，也就是说，第一次加载文件，调用setValue()并传递触发onChange的文件名，以便在观察活动/片段中调用.但是，当删除文件时，回调函数将再次调用loadFiles()函数以重新加载文件，但是这次调用setValue并传入FileNames不会触发观察活动/片段中的OnChange.
您必须调用setValue(T)方法来更新LiveData对象 从主线程开始.
我很想知道为什么LiveData在第一次调用后没有更新其值.
代码
MyLiveData
class MyLiveData() : MutableLiveData<MutableList<String>>(), PictureDelete {
override fun onPicDelete() {
loadFileNames()
}
val TAG = "MyLiveData"
val fileNamesList: MutableList<String> = ArrayList()
val fileWatcher : MyFileWatcher
init {
loadFileNames()
val pathToWatch = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DCIM + "/Camera").getAbsolutePath()
fileWatcher = MyFileWatcher(pathToWatch, this)
fileWatcher.startWatching()
}
private fun loadFileNames() {
val fileDir: File
try {
fileDir = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DCIM + "/Camera")
} catch (e: Exception) {
Log.e(TAG, e.message)
return
}
Log.d(TAG, "Actively Loading Files in Status LiveData")
val arrayOfFiles = fileDir.listFiles()
if (arrayOfFiles == null || arrayOfFiles.size < 1) return
Log.d(TAG, "Actively Loading Files. Size: ${arrayOfFiles.size}")
setValue(fileNamesList)
}
}
MyViewModel
class MyViewModel() : ViewModel() {
val myLiveData: MyLiveData
val TAG = "WhatsAppFragment-VModel"
init {
myLiveData = MyLiveData()
}
}
MyFragment
class MyFragment : Fragment() {
private val TAG = "MyFragment"
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
val view = inflater.inflate(R.layout.fragment_layout, container, false)
return view
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
val viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)
viewModel.myLiveData.observe(this, androidx.lifecycle.Observer { fileNames ->
Log.d(TAG, "New Live Data Dispatch")
for ((index, name) in fileNames.withIndex()) {
Log.d(TAG, "the element at $index is $name")
}
})
}
}
MyFileObserver
class MyFileWatcher(pathToWatch: String, val picDelete: PictureDelete) : FileObserver(pathToWatch, DELETE) {
val TAG = "MyFileWatcher"
init {
Log.d(TAG, "Initialization")
}
override fun onEvent(event: Int, path: String?) {
if (event = FileObserver.DELETE) { // EventCode 512 == Delete
Log.d(TAG, "OnEvent. Event: $event Path: $path")
picDelete.onPicDelete()
}
}
}
PictureDelete界面
interface PictureDelete {
fun onPicDelete()
}
我的实施有什么问题?
我在这里有一个@Micklo_Nerd示例，但是它没有使用删除文件的问题，但是可以使您了解需要执行的操作. 在我的示例中，用户输入名称，然后单击按钮后，列表将更改.
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<Button
android:text="Add"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/buttonAdd"
app:layout_constraintStart_toStartOf="@+id/filename"
app:layout_constraintEnd_toEndOf="@+id/filename"
android:layout_marginTop="24dp"
app:layout_constraintTop_toBottomOf="@+id/filename"/>
<EditText
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:inputType="textPersonName"
android:ems="10"
android:id="@+id/filename"
android:layout_marginStart="8dp"
app:layout_constraintStart_toStartOf="parent"
android:layout_marginEnd="8dp"
app:layout_constraintEnd_toEndOf="parent"
android:layout_marginTop="32dp"
app:layout_constraintTop_toTopOf="parent"/>
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/textView"
app:layout_constraintStart_toStartOf="parent"
android:layout_marginStart="8dp"
app:layout_constraintEnd_toEndOf="parent"
android:layout_marginEnd="8dp"
android:layout_marginTop="16dp"
app:layout_constraintTop_toBottomOf="@+id/buttonAdd"/>
</android.support.constraint.ConstraintLayout>
MyRepository(在您的示例中是MyLiveData)
在这里，您必须完成获取文件夹中文件名的工作，并将其放入MutableLiveData中.
class MyRepository {
fun loadFileNames(liveData : MutableLiveData<MutableList<String>>, filename: String){
var fileList : MutableList<String>? = liveData.value
if(test == null)
fileList = MutableList(1){ filename }
else
fileList.add(filename)
liveData.value = fileList
}
}
MyViewModel
在这里，我有两种方法:一种是在单击按钮时更新列表，另一种是获取文件名列表.您可能只需要获取列表的那个
class MyViewModel : ViewModel() {
val repo: MyRepository
var mutableLiveData : MutableLiveData<MutableList<String>>
init {
repo = MyRepository()
mutableLiveData = MutableLiveData()
}
fun changeList(filename: String){
repo.loadFileNames(mutableLiveData, filename)
}
fun getFileList() : MutableLiveData<MutableList<String>>{
return mutableLiveData
}
}
MainActivity
在这里，您看到我正在观察返回文件名列表的方法，这是您需要做的，因为这将要更改.
class MainActivity : AppCompatActivity(), View.OnClickListener {
private val TAG = "MyFragment"
private lateinit var viewModel: MyViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)
viewModel.getFileList().observe(this, Observer<MutableList<String>> { fileNames ->
Log.d(TAG, "New Live Data Dispatch")
textView.text = ""
for ((index, name) in fileNames!!.withIndex()) {
textView.append("the element at $index is $name\n")
}
})
buttonAdd.setOnClickListener(this)
}
override fun onClick(v: View?) {
when(v!!.id){
R.id.buttonAdd -> {
viewModel.changeList(filename.text.toString())
filename.text.clear()
}
}
}
}
希望这会有所帮助.
I implemented LiveData & ViewModel to mimic AsyncTaskLoader.
I load file names from the camera directory in DCIM, and then i attach a fileObserver to Observe when a File (picture) is deleted, and then a callback tells the LiveData to re-fetch the fileNames when delete event occurs
The Problem:
The code below should fetch the file Names from DCIM/Pictures asynchronously with the help of LiveData and then a FileObserver is attached to the directory (DCIM/Pictures), to monitor when a file is deleted and a callback is implemented with the LiveData sub-class to reload the files, as demonstrated in code.
okay, it works the first time, that is, the files are loaded the first time, calling setValue() and passing the fileNames triggered onChange to be called in the observing Activity/Fragment. But when a file is deleted, the callback function calls the loadFiles() function to re-load the files again but calling the setValue and passing in the FileNames does not trigger OnChange in the observing Activity/Fragment this time around.
According to the official documentation of LiveData
You must call the setValue(T) method to update the LiveData object from the main thread.
I am curious to know why LiveData is not updating its value after the first call.
The Code
MyLiveData
class MyLiveData() : MutableLiveData<MutableList<String>>(), PictureDelete {
override fun onPicDelete() {
loadFileNames()
}
val TAG = "MyLiveData"
val fileNamesList: MutableList<String> = ArrayList()
val fileWatcher : MyFileWatcher
init {
loadFileNames()
val pathToWatch = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DCIM + "/Camera").getAbsolutePath()
fileWatcher = MyFileWatcher(pathToWatch, this)
fileWatcher.startWatching()
}
private fun loadFileNames() {
val fileDir: File
try {
fileDir = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DCIM + "/Camera")
} catch (e: Exception) {
Log.e(TAG, e.message)
return
}
Log.d(TAG, "Actively Loading Files in Status LiveData")
val arrayOfFiles = fileDir.listFiles()
if (arrayOfFiles == null || arrayOfFiles.size < 1) return
Log.d(TAG, "Actively Loading Files. Size: ${arrayOfFiles.size}")
setValue(fileNamesList)
}
}
MyViewModel
class MyViewModel() : ViewModel() {
val myLiveData: MyLiveData
val TAG = "WhatsAppFragment-VModel"
init {
myLiveData = MyLiveData()
}
}
MyFragment
class MyFragment : Fragment() {
private val TAG = "MyFragment"
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
val view = inflater.inflate(R.layout.fragment_layout, container, false)
return view
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
val viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)
viewModel.myLiveData.observe(this, androidx.lifecycle.Observer { fileNames ->
Log.d(TAG, "New Live Data Dispatch")
for ((index, name) in fileNames.withIndex()) {
Log.d(TAG, "the element at $index is $name")
}
})
}
}
MyFileObserver
class MyFileWatcher(pathToWatch: String, val picDelete: PictureDelete) : FileObserver(pathToWatch, DELETE) {
val TAG = "MyFileWatcher"
init {
Log.d(TAG, "Initialization")
}
override fun onEvent(event: Int, path: String?) {
if (event = FileObserver.DELETE) { // EventCode 512 == Delete
Log.d(TAG, "OnEvent. Event: $event Path: $path")
picDelete.onPicDelete()
}
}
}
PictureDelete Interface
interface PictureDelete {
fun onPicDelete()
}
What is wrong with my Implementation?
I have here an example @Micklo_Nerd but it does not use your problem of getting the files deleted but it gives you the idea for what you need to do. In my example the user insert a name and after clicking a button, the list is changed.
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<Button
android:text="Add"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/buttonAdd"
app:layout_constraintStart_toStartOf="@+id/filename"
app:layout_constraintEnd_toEndOf="@+id/filename"
android:layout_marginTop="24dp"
app:layout_constraintTop_toBottomOf="@+id/filename"/>
<EditText
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:inputType="textPersonName"
android:ems="10"
android:id="@+id/filename"
android:layout_marginStart="8dp"
app:layout_constraintStart_toStartOf="parent"
android:layout_marginEnd="8dp"
app:layout_constraintEnd_toEndOf="parent"
android:layout_marginTop="32dp"
app:layout_constraintTop_toTopOf="parent"/>
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/textView"
app:layout_constraintStart_toStartOf="parent"
android:layout_marginStart="8dp"
app:layout_constraintEnd_toEndOf="parent"
android:layout_marginEnd="8dp"
android:layout_marginTop="16dp"
app:layout_constraintTop_toBottomOf="@+id/buttonAdd"/>
</android.support.constraint.ConstraintLayout>
MyRepository (In your example is the MyLiveData)
In here you must do the work of getting the filenames in the folder, and put the in the MutableLiveData.
class MyRepository {
fun loadFileNames(liveData : MutableLiveData<MutableList<String>>, filename: String){
var fileList : MutableList<String>? = liveData.value
if(test == null)
fileList = MutableList(1){ filename }
else
fileList.add(filename)
liveData.value = fileList
}
}
MyViewModel
In here, I have two methods: one to update the list as I click the button and another to get the list of file names. You should probably only need the one that gets the list
class MyViewModel : ViewModel() {
val repo: MyRepository
var mutableLiveData : MutableLiveData<MutableList<String>>
init {
repo = MyRepository()
mutableLiveData = MutableLiveData()
}
fun changeList(filename: String){
repo.loadFileNames(mutableLiveData, filename)
}
fun getFileList() : MutableLiveData<MutableList<String>>{
return mutableLiveData
}
}
MainActivity
In here, you see I am observing the method that returns the list of filenames, which is what you need to do, because that is what is going to change.
class MainActivity : AppCompatActivity(), View.OnClickListener {
private val TAG = "MyFragment"
private lateinit var viewModel: MyViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)
viewModel.getFileList().observe(this, Observer<MutableList<String>> { fileNames ->
Log.d(TAG, "New Live Data Dispatch")
textView.text = ""
for ((index, name) in fileNames!!.withIndex()) {
textView.append("the element at $index is $name\n")
}
})
buttonAdd.setOnClickListener(this)
}
override fun onClick(v: View?) {
when(v!!.id){
R.id.buttonAdd -> {
viewModel.changeList(filename.text.toString())
filename.text.clear()
}
}
}
}
Hope this helps.
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