TextEditingController.text has the following docs:
Setting this will notify all the listeners of this [TextEditingController] that they need to update (it calls [notifyListeners]). For this reason, this value should only be set between frames, e.g. in response to user actions, not during the build, layout, or paint phases.
This property can be set from a listener added to this [TextEditingController]; however, one should not also set [selection] in a separate statement. To change both the [text] and the [selection] change the controller's [value].
How am I supposed to update the TextEditingController.text inside a build method if I want then?
Also, is this limitation regarding performance issues, as to no lock up the build method with the listeners execution? Or is it something else?
Related
I have an entity object, for simplicity lets call it now Todo. I want to have a widget where I can edit multiple of these todos simultaneously, something like EditableTodoList (and this would show a list of EditableTodos).
This widget would recieve a List<Todo> and also something like Function onTodoEdited(Todo).
I am trying to use immutable data, so when an EditableTodo is being edited (and the event gets propagated to the widget which holds the EditableTodoList), I'd create a new List<Todo> which contais the updated Todo, but then EditableTodoList would rebuild with all its EditableTodos inside.
How could I optimize this, that only the relevant EditableTodo is rebuilt? I don't want to hack around with mutating the prop original List<Todo> because that hides greatly an important detail.
Background: currently the state is inside a cubit and I am context.select-ing for every EditableTodo list item the corresponding Todo in the state, this way only the edited item rebuilds. I'd like to decouple this EditableTodoList from the cubit.
Theoretically I assume its not possible, as EditableTodoList is given new properties, so it has to rebuild, and that means all subtree gets rebuilt. EditableTodo cannot be const as its prop is not a constant.
But still, what would be the most elegant way of separating EditableTodoList from the cubit? As it only needs a List<Todo> to show, it should be possible somehow to optimize the rendering of not changed Todos
You should consider how widget, element and render trees work in order to be concerned. There is a great video about Flutter's rendering mechanism. When rebuilding a tree, framework compares the two versions of widgets by using only the key and runtime type values. This means that if you change string value of the Text widget, the same render object with mutated data will be used again. New render object won't be created, which means no unnecessary rebuilds will happen on the render tree. Rebuilding the widget tree is not expensive compared to the render tree.
There is a limit on minimizing the the build scope in the ListView. ListView itself should be rebuilt in order to update an item. So you can't target a spesific item to rebuild. But items can reactively rebuild themselves (eg. an item that listens to a stream can rebuild without effecting the others). Therefore immutable collections doesn't matter in this context since the framework doesn't care the value but key and runtime type. You can either pass a completely modified list or a list with just one element changed, it will have the same effect.
Maybe my question is a nooby one, but i really try to understand how to implement the following operations:
How to add item for a list without the need of rebuilding all list.
How to update a class instance property value.
Does BLoCProvider used only once in a class?
===> I'm asking those question because i see that BLoC working with 'Equatable' and that in a the state class there is always a need to bring default value to the state we provide to the view class.
Another Question (what change the view and operate the state):
Does i need to bring always a list in order for example to update an item from the list or only changing the state+emit(in cubit) will change the state of the application, maybe it realted to BLocListener?
I would like my toggle buttons to be layout in a table, and not as a single row.
The number of toggle buttons is not static -
that is upon init I load a resource which contains a list of all the texts that should become the toggle buttons.
Looked at a number of approaches, each has its issues:
Create a list of ToggleButtons and a list of lists of bools to store the appropriate selected state as a data structure to divide the toggle buttons into a number of rows. The problem with this approach is in the implementation of the onPressed method - how to get a reference to the appropriate element in the list of lists of bools? Or in other words - how to get a reference to ToggleButtons object from within the onPressed method?
Use key property to pass the index of the current ToggleButtons. It is not intended for this purpose, so it is a bad practice, also again, there seems to be no straightforward way to access the key property from the onPressed method.
Extend the ToggleButtons class, and specifically override its build method. This is considered an anti-pattern in Flutter in general. Specifically In this approach, as we want all the functionality to remain the same, and change only the internal Row -> Table widget generation in the build method, it looks like we would have to duplicate all the code of this method, which is a bad idea as it might brake things as it changes in future versions of this widget
Create a table of checkbox / switch widgets as an alternative, which should work easily, but I want the look & feel of toggle buttons, not checkboxes or switches :)
I must be missing something simple!
After posting this I have a new idea :)
A table of FlatButtons! Will be probably possible to achieve similar UI to ToggleButtons. Will try it in a bit.
I would still love to hear other suggestions regarding ToggleButtons.
Try using SwitchListTile.
The entire list tile is interactive: tapping anywhere in the tile toggles the switch. Tapping and dragging the Switch also triggers the onChanged callback.
To ensure that onChanged correctly triggers, the state passed into value must be properly managed. This is typically done by invoking State.setState in onChanged to toggle the state value.
The value, onChanged, activeColor, activeThumbImage, and inactiveThumbImage properties of this widget are identical to the similarly-named properties on the Switch widget.
The title, subtitle, isThreeLine, and dense properties are like those of the same name on ListTile.
The selected property on this widget is similar to the ListTile.selected property, but the color used is that described by activeColor, if any, defaulting to the accent color of the current Theme. No effort is made to coordinate the selected state and the value state; to have the list tile appear selected when the switch is on, pass the same value to both.
The switch is shown on the right by default in left-to-right languages (i.e. in the ListTile.trailing slot) which can be changed using controlAffinity. The secondary widget is placed in the ListTile.leading slot.
To show the SwitchListTile as disabled, pass null as the onChanged callback.
I have an issue with the provider pattern, I have a solution to my problem i just wanted to check its a good method and if there a better alternates.
I am trying to update the text controllers text when they first load the widget to have the value stored in my provider state. Originally, I just created the text controller in the init and just loaded the state in the build method and set the controllers text.
This was working until i wanted to have some validation of the content that would prevent certain things being passed into the state ( for example preventing empty strings).
textController.addListener(() {
if( RULE ABOUT WHETHER TO SET STATE BASED ON TEXT){
state.textField = textController.text;
}
});
The problem arose here, as they user would delete their word and this would fail the rule. The rebuild would occur (as they had pressed a key) and the text.controller would then rebuild to what was in the state (a single letter as it failed the rule). Effectively this prevented them from deleting, which is something i want them to be able to do but for this not to update the state.
I found this library https://pub.dev/packages/after_layout which allows a method to be overridden which is called once immediately after the build.
My solution uses this by creating the text controller in the initstate.
Assigning it to the TextField in the build.
Then creating a local field that would maintain the textField widget string content.
Then in the new afterbuild method i would assign whats on the state to the variable and update the text controllers text to have this value.
Then in the text.controllers text changed listener I used my rule to only update this state if it met the rule. Therefore if they deleted content they shouldn't be able to and closed/reopened the widget the state would rebuild to still have a valid value.
Is using this library a good solution to this or is there a better technique?
---------------- EDIT
Sorry for my wordy first part. I think my main question is just how am i meant to init Controllers if the data i need has to come from the state in the build.
For example.
// This must go in the build as it requires state
myTabsController = TabController(length: myState.list.length, vsync: this);
I'm initialisng the controller every time it builds now... How am i meant to put this in the init but still access the state variables.
(I've tried using the afterFirstLayout() callback from the AfterLayoutMixin but that just causes more problems.
Feel free to ask me to clarify more.
Thanks for your help.
How is the PropertyChanged event used?
I'm wanting to evaluated the next state of a state machine whenever any property of the class is changed.
Regards Steve
PropertyChanged is part of the standard IPropertyNotifyChanged signaling.
If you want to create side effects if any change is done - consider setting "HasUserCode" on each attribute, Generate code, now you will find a partial method available for you to fill for Set and Get of attributes.
In a setter you can have side effects like running a trigger to step a state machine.