I'm discovering Flutter and I really love it.
I'm building a simple app where I have few instances of Statefull custom widget "Counter" on a Scafold's body, and I have a "Reset" button on the drawer.
What's the best way to call a "resetCounter" method on all "Counter" instances when tap on the "Reset" button ?
I've managed the other way (calling a callback method of a parent from a child), but I can't find the other way. The only solution I've found is by using GlobalKeys, but it doesn't seems appropriate.
Thanks
Indeed for your case callback function might be the right one. But when your app will grow the widget hierarchy will become more and more complex. For now you have Parent —> Child relationship and it’s quite easy to pass callback function, but when you’ll get Parent —> Widget A—> Widget B—> Child situation in order to pass callback from Parent to Child you’ll need to pass it through Widget A and Widget B as well.
For me, your problem sounds like a perfect candidate for reactive streams. You may create a Sink which will be used when user tap on Reset button and your Counter will observe this event and handle it with resetCounter method. You may learn more about reactive programming (and other ways to solve your problem) from this presentation.
There is another approach to extend ChangeNotifier, and add child as listeners. Once parent perform an action, you can call notifyChange to trigger child widgets callbacks.
Related
Flutter widget update pattern is a little bit confusing when coming from "old/classic" win32.
For instance :
I have a widget "button" => I click on it => I update a cell in a widget "datatable".
With classic API (VCL, Net Forms, ...) I get the address of button (by name, id, ...) and call directly
datatable_address.cell[x,y] = new_value;
I understand that I have to use setState() but do I need to create an "event" in Datatable (to run its setState()) and fire up this event from my button ?
(BloC seems pretty close to Qt signals)
The difference is composition vs aggregation. The widgets themselves are immutable so you can't change the value inside a widget.
What you can do is create a new object with latest values. Flutter framework will take care of the rest. For example,
when you do setState in parent widget of both button as well as datatable, you build method of said widget is called. Now, you will create a new datatable object with updated cell values. Internally, flutter will handle this.
You can only do setState in State class which is not immutable which is linked to Stateful widget which themselves are immutable.
This architecture comes with its own set of problems like if you need to update some UI based on some button press, you will have to have the logic inside the common widget of both the affected widget and button. That's where Stream/Bloc comes into picture.
I have one ParentWidget having two child widget.
I want to pass some callback action from FirstChild() to SecondChild() Widget without rebuild ParentWidget()like setState((){}).
I want to do because ParentWidget() has many widget. And this callback action continuously happen.
And continuously setState((){}) is not viable option for ParentWidget()
For example
setState((){}) call from FirstChild() than I want to rebuild SecondChild() without rebuild ParentWidget()
StatefulWidget is only useful in very simple business logic cases. I highly recommend that you have a look at Riverpod, where you can access a "state" from any widget, regardless of who its parent widget is. And because of how Riverpod is designed, it will not trigger any unnecessary rebuilds.
Sticking with StatefulWidget, you will need to use something like InheritedWidget as a parent of both widgets, and then have them both access it.
You can get help with state management solutions like Provider or Riverpod.
I may write an example code if you use Riverpod, but generally speaking, in such case, you need to place the state outside the Parent widget into some external variable. Then you can access and share it between both Children without rebuilding Parent.
I am new to Flutter and reactive programming is also new thing for me.
Let's say I want to build a timer with Flutter.
I add a Scaffold with all the necessary stuff in it and I add a IconButton which starts the Stopwatch and Text which displays elapsed time. I also add Timer.periodic to periodically (every 0.5 second) update the text.
Text Widget controls it's own state by checking if Stopwatch is running and updating it's values.
So now let's say I want to have more complicated logic that changes the text based on some actions with other buttons which are the siblings of Text. However it is not possible to call setState of Text widget directly from sibling widgets. As I understand the point of reactive paradigm is that the state can be passed down the Tree. However if I make my Scaffold as StatefulWidget and update the state of the parent every 0.5 second it will redraw my entire Scaffold with all it's children. So eventually when the Scaffold gets big enough it will have to update everything instead of single Text widget.
Am I correct? And is there any solution to this. I read something about Streams and Sinks however it looks very complicated and I think that there should be another solution.
You don't need to rebuild the whole tree, if the state only changed in a sub widget, ideally you want to call set state in that widget so only that part of the tree (the one whose state changed) is rebuilt.
Streams aren't really that complicated, it's a good way for you to send messages between different components in your app, which is what you're trying to do here.
In your case you can also use a ValueNotifier to store state in the parent widget, or maybe an AnimationController, and send its listener down to the sub widget that needs be updated on change.
In any case, the state is lifted to a parent widget, which then becomes accessible to the sub widget through a listener, or a stream. When the listener triggers a signal, you rebuild the sub widget only.
Extract out widget and call setState() form that widget and it's don't render all the widget again
Is it possible to do inter widget communication via something like a notification/event bus?
I need to be able to tell one widget to react to something that happened in another and didn't want to create a hard link.
The notification listener will only fire is if it is higher in the widget tree than both of the widgets so that isn't probably a viable solution.
There are lots of ways to do this depending on your use case.
You could have them be AnimatedWidgets that are passed a ValueNotifier or ChangeNotifier as the listenable. You can see this pattern in the Gallery's animation example.
You could use StreamBuilder to have your widgets rebuild automatically when new events come in on a Stream. There aren't a lot of examples of this in the main Flutter repo, but it's something that you're likely to need once you start using plugins or doing network I/O.
You could use a GlobalKey to get currentState and have one State call methods on the other. This is how snackbars work (example).
You can also extend InheritedWidget to provide widgets with information that wasn't passed as a constructor argument, and they'll automatically be marked for rebuild when that information changes. This is how Themes work, for example.
If you can provide more details on what your widgets do / what their relationship is, or ideally a code snippet I can help you decide which approach would make the most sense for your situation.
I've tried doing this but only one panel gets the Widget shown. What is the approach to add an instance of a Widget into many panels? Or is it even possible?
If by inject, you mean use some DI tool (like Guice), then yes, you can inject the instance into many other instances as a singleton (just like any other singleton).
However, a given Widget can only be rendered in one place at a time. Elements can be cloned, but there is no general method of copying a widget to draw it in more than one place, mostly due to all of the event handlers that have to be added again.
How would then this method work?
Widget getParent()
Gets this widget's parent panel.
So you have to create multiple instances. Btw what's your use case?