If I'm understanding the documentation correctly a LaunchedEffect should not run again if the rememberUpdatedState has not changed.
If I run something like this code below then it's not working as expected and the value is getting updated again on rotation.
Without the LaunchedEffect the rememberSaveable is remembered on config change and the text in the input is correct (if I type something it's still there). This leads me to believe that the rememberUpdatedState should also not have changed but yet it gets triggered. Why?
What am I doing wrong or is this a bug? Alternatively is there a better way to do this?
Thanks :)
#Composable
fun ThingView(
thingViewModel: ThingViewModel,
id: String?
) {
var thingName by rememberSaveable { mutableStateOf("") }
val scope = rememberCoroutineScope()
LaunchedEffect(rememberUpdatedState(newValue = thingName)) {
scope.launch {
id?.let {
val thing = thingViewModel.getThing(id)
thingName = thing.name
}
}
}
OutlinedTextField(
value = thingName,
onValueChange = { thingName = it },
label = { Text("Name") }
)
}
Edit:
To clarify, the goal is to allow the user enter text in a textField and not have that text cleared on rotation. That would be super annoying for a user and probably not what they expect.
I think what you are seeing is that the LaunchedEffect state gets cleared on rotation when the configuration changes. Consequently I don't think there is any value that you can provide as a key to keep it from running.
There are two options that come to mind. In the style of your original code, you might consider only declaring the LaunchedEffect when you determine the state has changed. For example, you might maintain the last seen id in a saveable, say lastId, and test for changes on entry. It requires you have a sentinel value for lastId initially of course.
#Composable
fun ThingView(
thingViewModel: ThingViewModel,
id: String?
) {
var thingName by rememberSaveable { mutableStateOf("") }
var lastId: String? by rememberSaveable { mutableStateOf(null) }
val scope = rememberCoroutineScope()
if (lastId != id) {
lastId = id
LaunchedEffect(id) {
scope.launch {
id?.let {
val thing = thingViewModel.getThing(id)
thingName = thing.name
}
}
}
}
OutlinedTextField(
value = thingName,
onValueChange = { thingName = it },
label = { Text("Name") }
)
}
Another option that might be more traditional is to use the ViewModel to persist the state across configuration changes. You might update the ViewModel on each onValueChange and have the ViewModel provide the current value back to the OutlinedTextField. In this style you might not need the effect or the coroutine.
Just a last word regarding my understanding about rememberUpdatedState. I believe it is typically intended to allow a long-lived coroutine or callback to reference the current value of a variable in the composable scope without requiring that the coroutine or callback be recreated on each state change. The closure seems to be scoped to the effect and it capture the values at the time of creation. I am still trying to understand it better.
Related
I have a StateFlow from which my List composable collects any changes as a State.
private val _people = MutableStateFlow(personDataList())
val people = _people.asStateFlow()
And inside my viewModel, I perform modifications on _people and I verify that people as a read-only StateFlow is also getting updated. I also have to make a copy of the original _people as an ordinary kotlin map to use for some verifications use-cases.
val copyAsMap : StateFlow<MutableMap<Int, Person>> = people.map {
it.associateBy( { it.id }, { it } )
.toMutableMap()
}.stateIn(viewModelScope, SharingStarted.Eagerly, mutableMapOf())
however, with my attempt above, it (the copyAsMap) doesn't get updated when I try to modify the list (e.g delete) an item from the _people StateFlow
Any ideas..? Thanks!
Edit:
Nothing is collecting from the copyAsMap, I just display the values everytime an object is removed from _person state flow
delete function (triggered by an action somewhere)
private fun delete(personModel: Person) {
_person.update { list ->
list.toMutableStateList().apply {
removeIf { it.id == personModel.id }
}
}
copyAsMap.values.forEach {
Log.e("MapCopy", "$it")
}
}
So based on your comment how you delete the item, that's the problem:
_people.update { list ->
list.removeIf { it.id == person.id }
list
}
You get an instance of MutableList here, do the modification and you "update" the flow with the same instance. And, as StateFlow documentation says:
Values in state flow are conflated using Any.equals comparison in a similar way to distinctUntilChanged operator. It is used to conflate incoming updates to value in MutableStateFlow and to suppress emission of the values to collectors when new value is equal to the previously emitted one.
Which means that your updated list is actually never emitted, because it is equal to the previous value.
You have to do something like this:
_people.update { list ->
list.toMutableList().apply { removeIf { ... } }
}
Also, you should define your state as val _people: MutableStateFlow<List<T>> = .... This would prevent some mistakes you can make.
I am trying to show a list of Orders in a list using LazyColumn. Here is the code:
#Composable
private fun MyOrders(
orders: List<Order>?,
onClick: (String, OrderStatus) -> Unit
) {
orders?.let {
LazyColumn {
items(
items = it,
key = { it.id }
) {
OrderDetails(it, onClick)
}
}
}
}
#Composable
private fun OrderDetails(
order: Order,
onClick: (String, OrderStatus) -> Unit
) {
println("Composing Order Item")
// Item Code Here
}
Here is the way, I call the composable:
orderVm.fetchOrders()
val state by orderVm.state.collectAsState(OrderState.Empty)
if (state.orders.isNotEmpty()) {
MyOrders(state.orders) {
// Handle status change click listener
}
}
I fetch all my orders and show in the LazyColumn. However, when a single order is updated, the entire LazyColumn gets rrecomposed. Here is my ViewModel looks like:
class OrderViewModel(
fetchrderUseCase: FetechOrdersUseCase,
updateStatusUseCase: UpdateorderUseCase
) {
val state = MutableStateFlow(OrderState.Empty)
fun fetchOrders() {
fetchrderUseCase().collect {
state.value = state.value.copy(orders = it.data)
}
}
fun updateStatus(newStatus: OrderStatus) {
updateStatusUseCase(newStatus).collect {
val oldOrders = status.value.orders
status.value = status.value.copy(orders = finalizeOrders(oldOrders))
}
}
}
NOTE: The finalizeOrders() does some list manipulation based on orderId to update one order with the updated one.
This is how my state looks like:
data class OrderState(
val orders: List<Order> = listOf(),
val isLoading: Boolean = false,
val error: String = ""
) {
companion object {
val Empty = FetchOrdersState()
}
}
If I have 10 orders in my DB and I update one's status (let's say 5th item), then OrderDetails gets called for 20 times. Not sure why. Caan I optimize it to make sure only the 5th indexed item will be recomposed and the OrderDetals gets called only with the new order.
Is the Orderclasss stable? If not it could be the reason why all the items get recomposed:
Compose skips the recomposition of a composable if all the inputs are stable and haven't changed. The comparison uses the equals method
This section in the compose's doc explains what are stable types and how to skip recomposition.
Note: If you scroll a lazy list, all invisible items will be destroyed. That means if you scroll back they will be recreated not recomposed (you can't skip recreation even if the input is stable).
I am fairly new with using ReactiveSwift and ReactiveCocoa and I seem to have hit a road block regarding the proper way of initializing a Property that has a dependencies.
For instance in the following code, I try to initialize a Property but i get a compiler error which is expected. My question is how/ what is the "correct" way to do this.
class SomeViewModel {
// illustration purposes, in reality the property (dependency) we will observe will change over time
let dependency = Property(value: true)
let dependency2 = Property(value: "dependency2")
let dependency3 = Property(value: 12345)
let weightLabel: Property<String>
// private(set) var weightLabel: Property<String>!
// using private(set) var weightLabel: Property<String>! works,
// however this changes the meaning behind using let, because we could
// reinitalize weightLabel again which is not similar to using a let so not a good alternative
// let weightLabel: Property<String> = Property(value: "")
// another solution that will work but will result in a wrong value
// upon initalization then, changed into the "correct value" thus, i
// am discrading this as well
init() {
weightLabel = dependency.map {
// compiler error, 'self' captured by closure before all members were initalized.
// My question is if there is a way to handle this scenario properly
if $0 && self.dependency2.value == "dependency2" && self.dependency3.value == 12345 {
return ""
}
return ""
}
}
}
So as you might have noticed above in the comments I am wondering if there is a way to handle this scenario with ReactiveSwift other then the ones i mentioned above that are not really ideal solutions.
The instrument that fits the scenario is combineLatest, which provides a combined version of all these properties (streams) whenever any of them has been updated.
weightLabel = Property.combineLatest(dependency, dependency2, dependency3)
.map { d1, d2, d3 in
return "Hello World! \(d1) \(d2) \(d3)"
}
Regarding the compiler error, the issue is that you are capturing/referring to self in a closure before every stored property has been initialised. Depending on the intention, you may use a capture list to capture directly the values and objects you are interested w/o self.
let title: String
let action: () -> Void
init() {
title = "Hello World!"
// 🚫 `action` has not been initialised when `self` is
// being captured.
action = { print(self.title) }
// ✅ Capture `title` directly. Now the compiler is happy.
action = { [title] in print(title) }
}
The code is on DartPad if you need a complete example (see the while loop towards the end.)
I have a loop,
Place place = places[0];
while (places.isNotEmpty) {
// Get a list of places within distance (we can travel to)
List reachables = place.getReachables();
// Get the closest reachable place
Place closest = place.getClosest(reachables);
// Remove the current place (ultimately should terminate the loop)
places.remove(place);
// Iterate
place = closest;
}
But it's not removing place on the second-to-last line. i.e., the length of the places list remains the same, making it an infinite loop. What's wrong?
This could be because the object in the list has a different hashCode from the object you are trying to remove.
Try using this code instead, to find the correct object by comparing the objects properties, before removing it:
var item = list.firstWhere((x) => x.property1== myObj.property1 && x.property2== myObj.property2, orElse: () => null);
list.remove(item);
Another option is to override the == operator and hashCode in your class.
class Class1 {
#override
bool operator==(other) {
if(other is! Class1) {
return false;
}
return property1 == (other as Class1).property1;
}
int _hashCode;
#override
int get hashCode {
if(_hashCode == null) {
_hashCode = property1.hashCode
}
return _hashCode;
}
}
I have faced the very same issue. Unfortunately I haven't found the root cause, but in the same situation I replaced
places.remove[place]
with
places.removeWhere(p => p.hachCode == place.hashCode)
as a workaround. One more approach was helpful too:
// Get the place from your set:
final place = places.first;
// Replace the place in the set:
places.add(place);
// Remove the place from the set:
places.remove(place);
Most likely place is not in the list for some reason. It's hard to debug without knowing the exact data used, the problem doesn't reproduce with the three-place sample you have in the linked DartPad.
Try figuring out which element is causing the problem. For example you can
try adding an if (!places.contains(place)) print("!!! $place not in $places"); before the remove, or something similar that detects the state when the problem occurs.
This way you can remove object from dynamic list
List data = [
{
"name":"stack"
},
{
"name":"overflow"
}
];
data.removeWhere((item) => item["name"]=="stack");
print(data);
Output
[{name: overflow}]
Use the plugin Equatable
class Place extends Equatable {
...
}
https://pub.dev/packages/equatable
I was having the same issue. I did something like this using removeWhere.
myList.removeWhere((item) => item.id == yourItemId.id)
I would like to pass a primitive (int, bool, ...) by reference. I found a discussion about it (paragraph "Passing value types by reference") here: value types in Dart, but I still wonder if there is a way to do it in Dart (except using an object wrapper) ? Any development ?
The Dart language does not support this and I doubt it ever will, but the future will tell.
Primitives will be passed by value, and as already mentioned here, the only way to 'pass primitives by reference' is by wrapping them like:
class PrimitiveWrapper {
var value;
PrimitiveWrapper(this.value);
}
void alter(PrimitiveWrapper data) {
data.value++;
}
main() {
var data = new PrimitiveWrapper(5);
print(data.value); // 5
alter(data);
print(data.value); // 6
}
If you don't want to do that, then you need to find another way around your problem.
One case where I see people needing to pass by reference is that they have some sort of value they want to pass to functions in a class:
class Foo {
void doFoo() {
var i = 0;
...
doBar(i); // We want to alter i in doBar().
...
i++;
}
void doBar(i) {
i++;
}
}
In this case you could just make i a class member instead.
No, wrappers are the only way.
They are passed by reference. It just doesn't matter because the "primitive" types don't have methods to change their internal value.
Correct me if I'm wrong, but maybe you are misunderstanding what "passing by reference" means? I'm assuming you want to do something like param1 = 10 and want this value to still be 10 when you return from your method. But references aren't pointers. When you assign the parameter a new value (with = operator), this change won't be reflected in the calling method. This is still true with non-primitive types (classes).
Example:
class Test {
int val;
Test(this.val);
}
void main() {
Test t = new Test(1);
fn1(t);
print(t.val); // 2
fn2(t);
print(t.val); // still 2, because "t" has been assigned a new instance in fn2()
}
void fn1(Test t) {
print(t.val); // 1
t.val = 2;
}
void fn2(Test t) {
t = new Test(10);
print(t.val); // 10
}
EDIT
I tried to make my answer more clear, based on the comments, but somehow I can't seem to phrase it right without causing more confusion. Basically, when someone coming from Java says "parameters are passed by reference", they mean what a C/C++ developer would mean by saying "parameters are passed as pointers".
As dart is compiled into JavaScript, I tried something that works for JS, and guess what!? It worked for dart!
Basically, what you can do is put your value inside an object, and then any changes made on that field value inside that function will change the value outside that function as well.
Code (You can run this on dartpad.dev)
main() {
var a = {"b": false};
print("Before passing: " + a["b"].toString());
trial(a);
print("After passing: " + a["b"].toString());
}
trial(param) {
param["b"] = true;
}
Output
Before passing: false
After passing: true
One of the way to pass the variables by reference by using the values in List. As arrays or lists are Pass by reference by default.
void main() {
List<String> name=['ali' ,'fana'];
updatename(name);
print(name);
}
updatename(List<String> name){
name[0]='gufran';
}
Try this one, This one of the simplest way to pass by reference.
You can use ValueNotifier
And, you can pass it as ValueListenable to classes or methods that needs to know up-to-date value, but should not edit it:
class Owner {
final theValue = ValueNotifier(true);
final user = User(theValue);
...
}
class User {
final ValueListeneble<bool> theValue;
User(this.theValue);
...
}
It provides more functionality than actually needed, but solves the problem.
If ValueNotifier + ValueListenable do not work for you (you want to make sure the client does not listen to every change of the value, or your package is pure Dart package and thus cannot reference Flutter libraries), use a function:
class Owner {
int _value = 0;
int getValue() => _value;
void increase() => _value++;
}
void main() {
final owner = Owner();
int Function() obtainer = owner.getValue;
print(obtainer());
owner.increase();
print(obtainer());
}
Output will be:
0
1
This approach has memory usage related downside: the obtainer will hold the reference to the owner, and this, even if owner is already not referenced, but obtainer is still reachable, owner will be also reachable
and thus will not be garbage collected.
If you do not want the downside, pass the smaller container than the entire owner:
import 'package:flutter/foundation.dart';
class ListenableAsObtainer<T> implements ValueObtainer<T> {
ListenableAsObtainer(this._listenable);
final ValueListenable<T> _listenable;
#override
T get value => _listenable.value;
}
class FunctionAsObtainer<T> implements ValueObtainer<T> {
FunctionAsObtainer(this._function);
final T Function() _function;
#override
T get value => _function();
}
class ValueAsObtainer<T> implements ValueObtainer<T> {
ValueAsObtainer(this.value);
#override
T value;
}
/// Use this interface when the client needs
/// access to the current value, but does not need the value to be listenable,
/// i.e. [ValueListenable] would be too strong requirement.
abstract class ValueObtainer<T> {
T get value;
}
The usage of FunctionAsObtainer will still result in holding the owner from garbage collection, but two other options will not.
Just to make it clear:
void main() {
var list1 = [0,1,2];
var modifiedList1 = addMutable(list1, 3);
var list2 = [0,1,2];
var modifiedList2 = addImmutable(list2, 3);
print(list1);
print(modifiedList1);
print(list2);
print(modifiedList2);
}
List<int> addMutable(List<int> list, int element){
return list..add(element);
}
List<int> addImmutable(List<int> list, int element){
return [...list, element];
}
Output:
[0, 1, 2, 3]
[0, 1, 2, 3]
[0, 1, 2]
[0, 1, 2, 3]
All variables are passed by value. If a variable contains a primitive (int, bool, etc.), that's it. You got its value. You can do with it whatever you want, it won't affect the source value. If a variable contains an object, what it really contains is a reference to that object.
The reference itself is also passed by value, but the object it references is not passed at all. It just stayed where it was. This means that you can actually make changes to this very object.
Therefore, if you pass a List and if you .add() something to it, you have internally changed it, like it is passed by reference. But if you use the spread operator [...list], you are creating a fresh new copy of it. In most cases that is what you really want to do.
Sounds complicated. Isn't really. Dart is cool.