I am using ChangeNotifier and i change their value and notify like below,
// Declare
ValueNotifier<bool> isDisplay = new ValueNotifier(false);
// Change value
isDisplay.value = false;
isDisplay.notifyListeners();
It's working fine. but it's raise warning like below,
Can we resolve this using proper implementation or other things ?
I was getting these errors too, more specifically, those 2:
The member 'notifyListeners' can only be used within instance members of subclasses of 'package:flutter/src/foundation/change_notifier.dart'.dart(invalid_use_of_protected_member)
The member 'notifyListeners' can only be used within 'package:flutter/src/foundation/change_notifier.dart' or a test.dart(invalid_use_of_visible_for_testing_member)
After a bit of research, I've come to some conclusions and a solution. Thought I should share it with people who encounter this problem.
The problem
What it says actually is quite obvious, “If you wanna call notifyListeners(), you have to do it in a subclass of ChangeNotifier or in a test file”
“But why is that?” Understandably, this was my second question. But before that, a little bit about where the heck that notifyListeners method is coming from
notifyListeners method
If you look at the implementation of ValueNotifier, it extends ChangeNotifier class:
and ChangeNotifier has a notifyListeners() method (which is why, in the first place, we can use this method) but not in a usual way:
It's protected.
#protected annotation in Dart | doc
[#protected annotation] Indicates that the annotated instance member (method, getter, setter, operator, or field) m in a class or mixin C should only be referenced in specific locations.
A reference from within the library in which C is declared is valid. Additionally, a reference from within an instance member in C, or a class that extends, implements, or mixes in C (either directly or indirectly), or a mixin that uses C as a superclass constraint is valid. Additionally, a reference from within an instance member in an extension that applies to C is valid.”
In a more worldly explanation: it can be used in subclasses that directly extends (or mixins) the ChangeNotifier class or in test files. Since you want to use it in your logic code, you have to do the first section in this paragraph: “extend or mixin ChangeNotifier where you want to call notifyListeners method.”
Where to extend (or mixin) the ChangeNotifier class?
You may be tempted to use it in your Stateful or Stateless Widget, and, since Dart is single-inheritance, you'd have to “mixin” the ChangeNotifier class to your class, but you'd be wrong.
1. Why you must not mixin to StatefulWidget
ChangeNotifier class defines its own dispose method. And since the class or mixin that mixins to your class supersedes the previous inherited methods, by looking at the methods of ChangeNotifier, you'll notice the dispose method (which is used to remove the listeners). So, if you mix ChangeNotifier to StatefulWidget, now your dispose would be ChangeNotifier's dispose rather than the State's dispose.
Remember, you have to release the resources used by State class no matter what. Normally, if you do not override the dispose method in your State, the framework does this for you and releases the resources by itself, which is cool; but since you just changed the dispose method's ancestor, framework will not trigger the required dispose method. And that will raise a runtime error.
2. Why you must not mixin to StatelessWidget
And on the StatelessWidget's case, now your widget would have an instance variable _listener that's not final which comes from the mixed ChangeNotifier. So, you couldn't define a const constructor, which is the most important part of a StatelessWidget (talking about widget rebuild and performance-related situations).
What to do?
In the end, a solution to this would be, to create a new class to manipulate value changes and extend or mixin ChangeNotifier to this class. Then you can call notifyListeners() method wherever and whenever you like! (but of course, don't call notifyListeners unless you use complex objects or send the same value)
Also, these classes are preferably called as 'Controller', 'ViewModel', 'BLoC', or any other name to state that they're mutating/controlling your values and trigger UI to rebuild when needed. (Which what you should do in the first place: separate your UI from your Logic layer).
Further readings
State management for minimalists
You do not need to call notifyListeners, as this is automatically called when a new value is set.
ChangeNotifier implementations like ValueListener should never require notifyListeners to be called manually.
what is the correct way to write a meaningful test for extending a class with the super keyword?
class FooBase<T> {
final T data;
const FooBase(this.data);
}
class Foo<T> extends FooBase<T> {
const Foo(T data)
: super(data);
}
I suppose super(data) ist passing the constructor parameter to the constructor of the base class (I do not know a lot about dart).
You can only test if super was called by observing its behavior. You have to check the base classes constructor for what it is doing with the parameter and then see if you can observe if that happened (if possible). Example: If the base class constructor is assigning the parameter to a field that you can access from the outside, you can assert on that parameter being set. This only works if
You can access the field from your test code. Access in this context does not mean to access it directly or via a getter. Any means of observing that the field has been set should be sufficient.
The class under test is not setting the field itself (there is probably no way to decide which constructor set the field).
How to select a specific class from set of classes which are implementing same interface class ?
You don't call a method directly from an interface, you call it on a reference that points to an instance of a class. Whichever class that is determines which method gets called.
How do I get a class object of a certain class in GObject / Gtk? For example, if my class is GtkSpinButton, I want to get the instance of GtkSpinButtonClass that represents the class. It is the parameter "class" in
gtk_spin_button_class_init (GtkSpinButtonClass *class)
and it is the object where virtual functions are stored. When I have an instance of GtkSpinButton, I can call
GtkSpinButtonClass *class = GTK_SPIN_BUTTON_GET_CLASS (instance)
however I don't have an instance around. GTK_TYPE_SPIN_BUTTON gives me the type id, a number, and not the class object. Any idea how to get the actual instance?
You want to use g_type_class_ref
GtkSpinButtonClass *klass = g_type_class_ref(GTK_TYPE_SPIN_BUTTON);
and when you're done with it
g_type_class_unref(klass);
I created a Class A in which all the methods are class methods (+). Another Class B is a singleton.
I want to know if I can check if Class A [A someoperation] Is like class B in that only one instance of A exists and I do not need to instantiate it.
How can I accomplish this?
When you call a class method, the class is not necessarily instantiated, unless the class method actually creates a class.
Also - class methods do not make a class a Singleton. It just means that the method is called on the class instead of an objet of the class.