The code, here as a Gist, will print e
. If I remove the override, i.e. remove output
from Baz
, it will print w
from Bar
.
This leads me to the conclusion that the method "priority" is own class
->
mixin
->
super class
.
If I add more mixins, e.g. like this:
mixin Zoo {
output() {
print('j');
}
}
class Baz extends Foo with Bar, Zoo {
// ...
Now, the output is j
. If I swap around Bar
and Zoo
:
class Baz extends Foo with Zoo, Bar {
// ...
Now, the outpt is w
again.
Consequently, I would define the priority like this: own class
->
last mixin
->
nth-last mixin
->
super class
.
Is there any way for me to control this behavior, i.e. call the super call method even when a mixin
has a method with the same name?
You might be askin why I would want to do this and not just rename the methods.
Well, in Flutter all State
's have a dispose
method and if I have a mixin
that has dispose
method as well, it will break the State
's functionality because the mixin
's dispose
method takes priority as illustrated above.
super.output
will call the mixin method as well, which is why that does not work. You can try adding the following constructor to Baz
:
Baz() {
super.output();
}
Even if this worked, it would not help as the dispose
method in the Flutter case is called from the outside.
The answer is, the mixin method completely overwrites the original method.
We make use of the with keyword followed by one or more mixins names. Mixins can be used in two ways, the first case is when we want to make use of class code in such a way that the class doesn't have any constructor and the object of the class is extended.
Mixins are a language concept that allows a programmer to inject some code into a class. Mixin programming is a style of software development, in which units of functionality are created in a class and then mixed in with other classes. A mixin class acts as the parent class, containing the desired functionality.
A mixin is a class whose methods and properties can be used by other classes – without subclassing. It's a reusable chunk of code that can be “plugged in” to any class that needs this functionality. Mixins are supported by Dart, and Flutter uses them in many places.
In mixins - the order in which the mixins are declared is very important.
When you apply a mixin to a class,
Mixins in Dart work by creating a new class that layers the implementation of the mixin on top of a superclass to create a new class — it is not “on the side” but “on top” of the superclass, so there is no ambiguity in how to resolve lookups source
class A {
String getMessage() => 'A';
}
class B {
String getMessage() => 'B';
}
class P {
String getMessage() => 'P';
}
class AB extends P with A, B {}
class BA extends P with B, A {}
void main() {
String result = '';
AB ab = AB();
result += ab.getMessage();
BA ba = BA();
result += ba.getMessage();
print(result);
}
Both, AB and BA classes extend the P class with A and B mixins but in a different order. All three A, B and P classes have a method called getMessage.
First, we call the getMessage method of the AB class, then the getMessage method of the BA class.
And the output will be BA
.
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