In the fragement I display a Date from a Calendar object. This date is got from the viewModel of the fragement and it's a live date :
private val _theDate = MutableLiveData<Calendar>()
val theDate: LiveData<Calendar>
get() = _theDate
In the viewModel also I have a function that add 1 day to _theDate
fun goToDayAfter() {
_theDate.value!!.add(Calendar.DATE, 1)
}
This function is called after clicking on a button of the same fragment, and it does not trigger the observer :
viewModel.theDate.observe(viewLifecycleOwner, androidx.lifecycle.Observer { newDate ->
displayDate(newDate)
})
For more details and after debugging, I believe that _theDate is well changed but the observer is not triggered, If I change to another fragement and come back the new _theDate is changed.
Also if I change the method goToDayAfter() to :
fun goToDayAfter() {
val tmp = _theDate.value!!
tmp.add(Calendar.DATE, 1)
_theDate.value = tmp
}
It works!
Why cahnging _theDate directly do not trigger the observer ? is it because it s an object and not a premitive ? Is there any better solution that to pass by a tmp variable ?
For the update to trigger, you have to set the value of the LiveData, like you have discovered, i.e. by using:
_theDate.value = ...
Othwerwise, if you mutate (i.e. change) the value that the LiveData points to (_theDate.value!!.add(Calendar.DATE, 1), it has no way of knowing that the value was changed! That's why no update is triggered.
Prefer immutable objects
I recommend using immutable objects and re-creating them every time you want to change them. With immutable objects it's more difficult to run into this kind of bugs. An added bonus is that the code that observes _theDate cannot change it.
So instead of Calendar, I would use java.time.LocalDateTime. You can use it if you add a dependency to desugar_jdk_libs in your build.gradle file:
coreLibraryDesugaring 'com.android.tools:desugar_jdk_libs:1.1.1'
in which case your code would like like this:
fun goToDayAfter() {
_theDate.value = _theValue!!.plusDays(1)
}
Related
I am updating the old mutableStateOf() object data by modifying it but new data is not reflecting on it.
variable: as
val offTime = mutableStateOf<List<OffTime>>(emptyList())
update code
fun updateOffTime(newOffTime: OffTime, index: Int){
val updatedOffTime = offTime.value.mapIndexed { i, offTime ->
var result = offTime
if(index == i) result = newOffTime
result
}
offTime.value = updatedOffTime
Log.d(TAG, "updateOffTime: $updatedOffTime")
Log.d(TAG, "updateOffTime: ${offTime.value}")
}
Note: It works when the object OffTime is without id
i.e. OffTime(fromTime, toTime) :- works
OffTime(id,fromTime, toTime) :- doesn't works
I don't know exactly your use-case, neither your entire code implementation, but if I may ask why are you using an ordinary collection list instead of a SnapshotStateList or an extension of its new instance, mutableStateListOf when its part of your requirement to perform list operations?
Have you tried converting your offtime as a SnapshotStateList like this?
val offTime = mutableStateListOf<OffTime>(mutableStateListOf())
and performing updates liks this?
offTime.add( ... )
//or
offtime.remove(...)
//or
offTime[index] = offtime copy
//or
var offTimeIterator = offTime.lisIterator() // where you can safely modify indeces
SnapshotStateList is created exactly for such use-cases in compose, where you can perform normal list operations such as (add, remove, update, or batch updates) and guarantees re-composition.
Your call
offTime.value = updatedOffTime // if this is a new instance of a list
will trigger an entire re-composition as the entire list reference had been changed, but with SnapshotStateList, any changes to the structure is guaranteed to match a specific re-composition, say if you modify an item at index 7, and if this is observed by say a LazyColumn only LazyColumn's 7th index will re-compose
Also I don't know if your Offtime is a data class or a standard class, I would recommend it to be a data-class so you can easily copy() a certain instance of it, pass a new value to a certain property of it and re-assign it in a SnapshotStateList.
I'm working on a simple calorie counter app using two fragments and a ViewModel. I'm a beginner and this is a modification of an app I just created for a course (this app is not a homework assignment). It uses ViewModel and has a fragment that collects user input and a fragment that displays the input as a MutableList of MutableLiveData. I would like for the list screen to initially be empty except for a TextView with instructions, and I'd like the instructions to disappear once an entry has been added to the list. My class instructor told me to use an if-else statement in the fragment with the list to achieve this, but it's not working. He didn't tell me exactly where to put it. I tried a bunch of different spots but none of them worked. I don't get errors - just no change to the visibility of the TextView.
Here is the code for the ViewModel with the list:
val entryList: MutableLiveData<MutableList<Entry>>
get() = _entryList
init {
_entry = MutableLiveData<Entry>()
_entryList.value = mutableListOf()
}
fun addEntry(entryInfo: Entry){
_entry.value = entryInfo
_entryList.value?.add(_entry.value!!)
}
}
And this is the code for the observer in the list fragment:
Observer { entryList ->
val entryListView: View = inflater.inflate(R.layout.fragment_entry_list, null, false)
if (entryList.isNullOrEmpty()) {
entryListView.instructions_text_view.visibility = View.VISIBLE
} else {
entryListView.instructions_text_view.visibility = View.GONE
}
entryList.forEach {entry ->
val view: View = inflater.inflate(R.layout.entry_list_item, null, false)
view.date_entry_text_view.text = String.format(getString(R.string.date), entry.date)
view.calories_entry_text_view.text =
view.line_divider
binding.entryList.addView(view)
}
Thanks for any help.
I guess you are expecting your observer to get notified of the event when you are adding entryInfo to your event list (_entryList.value?.add(_entry.value!!).
But this won't happen as you are just adding an element to the same mutable list, and as the list reference hasn't changed, live data won't emit any update.
To solve this, you have two options.
Create a new boolean live data which controls when to show and hide the info text. Set its initial value to false, and update it to true in addEntry() function.
Instead of updating the same mutable list, create of copy of it, add the element and set the entryList.value equal to this new list. This way your observer will be notified of the new list.
Additionally, its generally not a good practice to expose mutable data unless there is no alternative. Here you are exposing a mutable list of Entry and that too in the form of a mutable live data. Ideally, your should be exposing LiveData<List<Entry>>.
This is one possible implementation of all the points that I mentioned:
private val _entryList = MutableLiveData(listOf<Entry>()) // Create a private mutable live data holding an empty entry list, to avoid the initial null value.
val entryList: LiveData<List<Entry>> = _entryList // Expose an immutable version of _entryList
fun addEntry(entryInfo: Entry) {
_entryList.value = entryList.value!! + entryInfo
}
I haven't used the _entry live data here, but you can implement it the same way.
set your viewModel to observe on entry added.
I think you have gotten your visibility toggle in the your if else blocks wrong.
if (entryList.isNullOrEmpty()) {
entryListView.instructions_text_view.visibility = View.GONE // OR View.INVISIBLE
} else {
entryListView.instructions_text_view.visibility = View.VISIBLE
}
Your Observer should get notified of changes to entryList when _entryList has changed. Make sure you are calling addEntry() function to trigger the notification.
I am trying to display several download progress bars at once via a list of data objects containing the download ID and the progress value. The values of this list of objects is being updated fine (shown via logging) but the UI components WILL NOT update after their initial value change from null to the first progress value. Please help!
I see there are similar questions to this, but their solutions are not working for me, including attaching an observer.
class DownLoadViewModel() : ViewModel() {
...
private var _progressList = MutableLiveData<MutableList<DownloadObject>>()
val progressList = _progressList // Exposed to the UI.
...
//Update download progress values during download, this is called
// every time the progress updates.
val temp = _progressList.value
temp?.forEach { item ->
if (item.id.equals(download.id)) item.progress = download.progress
}
_progressList.postValue(temp)
...
}
UI Component
#Composable
fun ExampleComposable(downloadViewModel: DownloadViewModel) {
val progressList by courseViewModel.progressList.observeAsState()
val currentProgress = progressList.find { item -> item.id == local.id }
...
LinearProgressIndicator(
progress = currentProgress.progress
)
...
}
I searched a lot of text to solve the problem that List in ViewModel does not update Composable. I tried three ways to no avail, such as: LiveData, MutableLiveData, mutableStateListOf, MutableStateFlow
According to the test, I found that the value has changed, but the interface is not updated. The document says that the page will only be updated when the value of State changes. The fundamental problem is the data problem. If it is not updated, it means that State has not monitored the data update.
The above methods are effective for adding and deleting, but the alone update does not work, because I update the element in T, but the object has not changed.
The solution is to deep copy.
fun agreeGreet(greet: Greet) {
val g = greet.copy(agree = true) // This way is invalid
favourites[0] = g
}
fun agreeGreet(greet: Greet) {
val g = greet.copy() // This way works
g.agree = true
favourites[0] = g
}
Very weird, wasted a lot of time, I hope it will be helpful to those who need to update.
As far as possible, consider using mutableStateOf(...) in JC instead of LiveData and Flow. So, inside your viewmodel,
class DownLoadViewModel() : ViewModel() {
...
private var progressList by mutableStateOf(listOf<DownloadObject>()) //Using an immutable list is recommended
...
//Update download progress values during download, this is called
// every time the progress updates.
val temp = progress.value
temp?.forEach { item ->
if (item.id.equals(download.id)) item.progress = download.progress
}
progress.postValue(temp)
...
}
Now, if you wish to add an element to the progressList, you could do something like:-
progressList = progressList + listOf(/*item*/)
In your activity,
#Composable
fun ExampleComposable(downloadViewModel: DownloadViewModel) {
val progressList by courseViewModel.progressList
val currentProgress = progressList.find { item -> item.id == local.id }
...
LinearProgressIndicator(
progress = currentProgress.progress
)
...
}
EDIT,
For the specific use case, you can also use mutableStateListOf(...)instead of mutableStateOf(...). This allows for easy modification and addition of items to the list. It means you can just use it like a regular List and it will work just fine, triggering recompositions upon modification, for the Composables reading it.
It is completely fine to work with LiveData/Flow together with Jetpack Compose. In fact, they are explicitly named in the docs.
Those same docs also describe your error a few lines below in the red box:
Caution: Using mutable objects such as ArrayList or mutableListOf() as state in Compose will cause your users to see incorrect or stale data in your app.
Mutable objects that are not observable, such as ArrayList or a mutable data class, cannot be observed by Compose to trigger recomposition when they change.
Instead of using non-observable mutable objects, we recommend you use an observable data holder such as State<List> and the immutable listOf().
So the solution is very simple:
make your progressList immutable
while updating create a new list, which is a copy of the old, but with your new progress values
When setting a value to MediatorLiveData that reacts to a source added in the constructor of a viewModel or activity onCreate observer in the ViewModel , like this for example:
showingMethodLiveData.addSource(stateChangeLiveData) {
when (it) {
ConfigurationState.CURRENT -> showingMethodLiveData.value = commMethod[it]
ConfigurationState.PENDING -> showingMethodLiveData.value = commMethod[it]
}
}
The value isn't set to the observing view, although the set method is called.
I can work around this by either adding the source in onStart (which creates other problems of registering observer more than once), or using postValue instead of setValue.
The debug of setValue method leads me to following code, where there is an interesting comment that tells the story, the method returns without setting the value to the binded view.
in androidx.databinding package of lifecycle dependency:
class ViewDataBinding:
private void handleFieldChange(int mLocalFieldId, Object object, int fieldId) {
if (mInLiveDataRegisterObserver) {
// We're in LiveData registration, which always results in a field change
// that we can ignore. The value will be read immediately after anyway, so
// there is no need to be dirty.
return;
}
boolean result = onFieldChange(mLocalFieldId, object, fieldId);
if (result) {
requestRebind();
}
}
The value is not set afterwards either, but only when the mediatorlivedata is invoked again by change in it's source.
Why this situation occurs?
Thank you for the help
PS
I think it may be an android library bug
The use of Mediatorlivedata is to compare two values and then provide a result.
If you want to change the value of a variable, you can simply use MutableLiveData and to assign a new value, write variableName.value = newValue
Should be even easier to achieve like this:
val showingMethodLiveData = Transformations.map(stateChangeLiveData) { commMethod[it] }
I have a LiveData named navigationArgs:
private val _navigationArgs = MutableLiveData<Item>()
val navigationArgs: LiveData<Item>; get() = _navigationArgs
which stores the arguments to be passed to the next fragment. It is attached to an observer, to navigate when the value is changed:
viewModel.navigationArgs.observe(this, Observer{
//navigation code
viewModel.finishedNavigating()
})
in which in finishedNavigating(), value of _navigationArgs is set to null:
fun finishedNavigating(){
_navigationArgs.value = null
}
When finishedNavigating() is included in the observer, the app hangs without even navigating, when the _navigationArgs value is changed.
Why does this happen? I am using Android Studio 4.0 Canary. Thank you.
When you assigning something to _navigationArgs.value, code will be automatically called in Observer, that used in viewModel.navigationArgs.observe (navigationArgs and _navigationArgs are same objects because navigationArgs has getter, that returns _navigationArgs ).
In your case you assigning null to _navigationArgs.value in finishedNavigating(), which calls code in Observer, which calls finishedNavigating() again etc...
You just have recursion here.
You should add recursion exit condition. For example:
viewModel.navigationArgs.observe(this, Observer{
//navigation code
if (it != null) //don't call finishedNavigating, when null passed in to _navigationArgs.value
viewModel.finishedNavigating()
})
mmm, it looks like an endless loop of values sent to the observer, every time you set a value null is sent and then again and again.