I'm using MVVM pattern in my project and i show the spinner inside the viewmodel by passing the build context to the view model method
for example :
void getSomeData(BuildContext context,int someDataID){
showSpinner(context);
}
the mentioned method getSomeData(....) is called from screen file
it works fine but i just wanna know will that make any problem?
Thanks
I think it is an ok pattern to use but it may cause problems in the future when your app is much bigger. You may either go to debug your viewmodel and view or you want to extend some functionality based on or closely related to the spinner and find it hard to navigate through your code to find what you want as the viewmodel and view are very intertwined.
Strictly speaking the viewmodel should direct the view via state variables held within the viewmodel, rather than allowing the viewmodel to make direct changes to the view via context. This helps keep all the build code in the view and all the logic that controls that in the viewmodel. Below is an example of what I mean using a ChangeNotifier as a viewmodel (using the provider package) e.g.
class ViewModel extends ChangeNotifier {
bool _showSpinner = false;
bool get shouldShowSpinner => _showSpinner;
void showSpinner() {
_showSpinner = true;
notifyListeners() // (Method for ChangeNotifiers) Or equivalent call to rebuild the view
}
void hideSpinner() {
_showSpinner = false;
notifyListeners() // (Method for ChangeNotifiers) Or equivalent call to rebuild the view
}
}
class View extends StatelessWidget {
#override
Widget build(BuildContext context) {
return ChangeNotifierProvider<ViewModel>(
create: (context) => ViewModel(),
child: Consumer<ViewModel>(
build: (context, model, child) {
return Stack(
children: [
SomePageContents(),
if (model.shouldShowSpinner) Spinner(),
],
);
}
)
);
}
}
Just to reiterate, I think this helps maintainability in the long run but if your method works for you, i.e. the scope of your project is relatively small and your original way doesn't cause too many dependency issues then I'd say do it your original way as the MVVM method should help not hinder development.
Related
I want to bind a controller to view 1 but I don't want to go to that view 1 via using Get.to(Page());.
Instead I want to use view 1 directly inside view 2 by creating an object.
Simplified code (BTW I'm using Veiw1Controller variables inside View1 itself)
class Veiw2 extends GetView<Veiw2Controller>{
return View1();
}
When I'm doing the above code, it throws an error saying
"View1Controller" not found. You need to call
"Get.put(View1Controller())" or
"Get.lazyPut(()=>View1Controller())"
I did call Get.put(...) in the binding but I think since we are not calling Get.to()therefore GetX does not realize when we are using that view and it does not bind the dependencies
Here is what I've done
class View1 extends Bindings {
#override
void dependencies() {
Get.put<View1Controller>(
View1Controller(),
);
}
}
What is the best way to do that?
well, the Bindings API was made to work together with the Getx navigation features, without actually Get.to(), Get.toNamed()..., you should not expect the automatic dependency injection of Getx.
However, you still could inject those dependencies manually: like this:
class BindingsOne extends Bindings {
#override
void dependencies() {
Get.put<View1Controller>(
View1Controller(),
);
}
}
Now when you want to get a Widget:
Widget getViewWidget() {
BindingsOne().dependencies(); // this will inject all your dependencies from the bindings.
return YourWidget();
}
you could also inject them before you use Navigator routing features.
// ...
BindingsOne().dependencies(); // this will inject all your dependencies from the bindings.
Navigator.of(context).push(MaterialPageRoute(builder: (context) => YourWidget()));
}
For a simple Email login with OTP code I have a structure as follows.
View
await _signUpCntrl.signUp(email, password);
Controller
_showOtpDialog(email);
_showOtpDialog func
return Get.dialog(
AlertDialog(
So the thing is _showOtpDialog function is inside a controller file. ie. /Controllers/controller_file.dart
I want do something like a blocListener, call the _showOtpDialog from a screen(view) file on signup success. (also relocate the _showOtpDialog to a view file)
Using GetX I have to use one of the builders either obs or getbuilder. Which is I think not a good approach to show a dialog box.
On internet it says Workers are the alternative to BlocListener. However Workers function resides on Controller file and with that the dialog is still being called on the controller file.
As OTP dialog will have its own state and a controller I wanted to put it inside a /view/viewfile.dart
How do I obtain this?
I tried using StateMixin but when I call Get.dialog() it throw an error.
visitChildElements() called during build
Unlike BLoC there's no BlocListener or BlocConsumer in GetX.
Instead GetX has RxWorkers. You can store your response object in a Rx variable:
class SomeController extends GetxController{
final response= Rxn<SomeResponse>();
Future<void> someMethod()async{
response.value = await someApiCall();
}
}
And then right before the return of your widget's build method:
class SomeWidget extends StatelessWidget{
final controller = Get.put(SomeController());
#override
Widget build(BuildContext context){
ever(controller.response, (SomeResponse res){
if(res.success){
return Get.dialog(SuccessDialog()); //Or snackbar, or navigate to another page
}
....
});
return UI();
}
First thing, you will need to enhance the quality of your question by making things more clearly. Add the code block and the number list, highlight those and making emphasize texts are bold. Use the code block instead of quote.
Seconds things, Depends on the state management you are using, we will have different approaches:
Bloc (As you already added to the question tag). By using this state management, you controller ( business logic handler) will act like the view model in the MVVM architecture. In terms of that, You will need to emit a state (e.g: Sent success event). Afterward, the UI will listen to the changes and update it value according to the event you have emitted. See this Bloc example
GetX (As your code and question pointed out): GetX will acts a little bit different. you have multiple ways to implement this:
Using callbacks (passed at the start when calling the send otp function)
Declare a general dialog for your application ( this is the most used when it comes to realization) and calling show Dialog from Bloc
Using Rx. You will define a Reactive Variable for e.g final success = RxBool(true). Then the view will listen and update whenever the success changes.
controller.dart
class MyController extends GetxController {
final success = RxBool(false);
void sendOtp() async {
final result = await repository.sendOTP();
success.update((val) => {true});
}
}
view.dart
class MyUI extends GetView<MyController> {
#override
Widget build(BuildContext context) {
ever(controller.success, (bool success) {
// This will update things whenever success is updated
if (success) {
Get.dialog(AlertDialog());
}
});
return Container();
}
}
Context: I'll be having a couple of scrollable lists in my app and I always want to scroll them to the latest item whenever an item is added.
Problem: My ListView.builders and the places where items are added are going to be quite far apart in my widget tree. Passing around all those scroll controllers via constructors seems to be super awkward.
My Solution:As I'm practising with Provider at the moment, I came up with a working solution using Provider:
class ScrollControllerProvider with ChangeNotifier {
ScrollController _paneController = ScrollController();
//setting up all other controllers here later
get paneController {
return _paneController;
}
void scrollHistory() {
WidgetsBinding.instance?.addPostFrameCallback((_) {
if (_paneController.hasClients) {
_paneController.jumpTo(_paneController.position.maxScrollExtent);
}
});
}
}
I'll add all scroll controllers to that provider and grab what I need, where I need it. It already works with one, but someone on reddit told me it's not a good idea, as scroll controllers should be disposed. Im not super knowledgeable on the topic of life cycle yet and find it difficult to assess this.
Questions: Is it really a bad idea to use Provider here? Can you help me to understand why? If yes, what is the best approach to solve this issue?
Provider is not the problem, using a disposable item inside a provider is. ScrollController is a disposable item related to its main Widget, or better to say its State.
If you want to notify your widgets about newly added items, create a variable inside the provider and listen to that variable in your widgets, then use your ScrollController to change the position.
To find out more about your question take a look at ScrollController class and Disposable class
For posterity, Payam Asefi pointed me in the right direction.
How I'm doing it now.
tldr; Provider contains a value that can be toggled and a method to toggle it. I provide the value where I can also access the scroll controler. If it is toggled, the scroll conroler is used. I provide the method to toggle the value where I add new items to the list.
item is added > value in provider is triggered > listeners realized the value has changed calling the build method > scroll controller is used to go to maxscrollextend.
Long answer with code:
Provider with a) a bool that can be toggled b) a method to toggle the bool c) a getter for the bool
Code:
class ScrollControllerToggles with ChangeNotifier {
bool _historyPaneSwitch = true;
get getTogglePaneSwitch {
return _historyPaneSwitch;
}
void toggleHistoryPane() {
_historyPaneSwitch = !_historyPaneSwitch;
notifyListeners();
}
}
In the widget I'm using the Listview.builder: a) I define a scroll controller, b) I use a function dependent on the _historyPaneSwitch inside that Provider. That funtion also uses the scroll controller to scroll the list to the end.
void triggerScrollController() {
bool scrollHistoryPane =
Provider.of<ScrollControllerToggles>(context).getTogglePaneSwitch;
WidgetsBinding.instance?.addPostFrameCallback((_) {
if (paneController.hasClients) {
paneController.jumpTo(paneController.position.maxScrollExtent);
}
});
}
In the widget adding new items to the list, I access the Provider again and grab the method to toggle "_historyPaneSwitch".
Function scrollHistoryPane =
Provider.of<ScrollControllerToggles>(context).toggleHistoryPane;
void dayChange(Function scrollHistoryPane) {
mainElementList.insert(0, MainElement(false, DateTime.now().toString()));
scrollHistoryPane;
}
I am using get package.
Here's what my code looks like,
class MyController extends GetXController{
//code to fetch data from firebase
}
class SecondScreen extends GetView<MyController>{
#override
Widget build(BuildContext context) {
return GetBuilder(
init: MyController(),
builder: (controller) {
return Scaffold(
//code...
);
},
);
}
}
Doubt:
I have a button, using which I am navigating to the secondScreen from homePage, and everytime I tap on the button the controller MyController is initialized again and so the data is fetched again. But I want to do something that will keep that controller that is initizlized the first time in memory permanently. How can I do that?
I know that, we can do something like this,
Get.put(Controller(), permanent: true);
But, in my code, I haven't used Get.put method anywhere as when the class extending GetView is called the controller is initialized automatically.
Well, actually you are putting/initializing MyController. Just not inside the GetX dependency container.
Because you are doing:
GetBuilder(
init: MyController(),
....
)
What you should do instead is:
GetBuilders(
init: Get.put(MyController()),
....
)
That way you are letting GetX dependency manager to manage your dependencies. And it's smart enough to know that that route is on the backstack so doesn't remove from memory.
Adding "permanent" to where you use "get.put" will fix the problem
Get.put(Controller(), permanent: true);
my widgets were not staying permanently, this is how I solved it
I'm new to dart/flutter and having a little trouble getting my head around communication patterns.
One reoccurring problem is that I keep looking to expose a public method on a widget so it can be called by other widgets.
The problem is with stateful widgets. In these cases, I need to call down to the widgets state to do the actual work.
The problem is that the widget doesn't have a copy of the state.
I have been saving a copy of the state in the widget but of course this throws a warning as it makes the widget mutable.
Let me give a specific example:
I have a specialised menu which can have a set of menu items.
Each are stateful.
When the menu is closing it needs to iterate over the list of menu items that it owns and tell each one to hide (the menu items are not visually contained within the menu so hiding the menu doesn't work).
So the menu has the following code:
class Menu{
closeMenu() {
for (var menuItem in menuItems) {
menuItem.close();
}
}
So that works fine, but of course in the MenuItem class I need to:
class MenuItem {
MenuItemState state;
close()
{
state.close();
}
But of course having the state object stored In the MenuItem is a problem given that MenuItem is meant to be immutable. (It is only a warning so the code works, but its clearly not the intended design pattern).
I could do with seeing more of your code to get a better idea of how to solve your specific issue but it appears that the Flutter documentation will help you in some regard, specifically the section on Lifting state up:
In Flutter, it makes sense to keep the state above the widgets that use it.
Why? In declarative frameworks like Flutter, if you want to change the UI, you have to rebuild it.
…it’s hard to imperatively change a widget from outside, by calling a method on it. And even if you could make this work, you would be fighting the framework instead of letting it help you.
It appears you're trying to fight the framework in your example and that you were correct to be apprehensive about adding public methods to your Widgets. What you need to do is something closer to what's detailed in the documentation (which details all of the new classes etc you'll see below). I've put a quick example together based on this and the use of Provider which makes this approach to state management easy. Here's a Google I/O talk from this year encouraging its use.
void main() {
runApp(
ChangeNotifierProvider(
builder: (context) => MenuModel(),
child: MyApp(),
),
);
}
class MyApp extends StatelessWidget {
…
// call this when the menu is closed
void onMyMenuClosed(BuildContext context) {
var menuModel = getMyMenuModel(context);
menuModel.hideMenuItems();
}
}
class MenuModel extends ChangeNotifier {
bool _displayItems = false;
void hideMenuItems() {
_displayItems = false;
notifyListeners();
}
void showMenuItems() {
_displayItems = true;
notifyListeners();
}
}
Calling hideMenuItems() makes a call to notifyListeners() that'll do just that; notify any listeners of a change which in turn prompts a rebuild of the Widget/s you wrap in a Consumer<MenuModel> Now, when the Widget that displays the menu is rebuilt, it just grabs the appropriate detail from the MenuModel class - the one source of truth for the state. This reduces the number of code paths you'd otherwise have to deal with to one and makes it far easier to see what's happening when you make further changes.
#override
Widget build(BuildContext context) {
return Consumer<MenuModel>(
builder: (context, menuModel, child) {
return menuModel._displayItems() ? MenuItemsWidget() : Container();
},
);
}
I recommend you read the entire page on state management.