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When developing an app with multiple screens, we tend to reuse the same piece of code over many classes: showing error messages, using the same page layout and wiring up some dependencies like for example a Bloc. All this could be solved if were using an abstract base class, however, what if we have a set of features/classes that we want to use on a particular screen but not on others? Since a class can't be a child of more than one class, should we create different base classes, as much as the number of combinations that we have? That's why we have mixin s.

Mixins and Base Classes: An introduction

Mixins let us add a set of “features” to a class without using parent-child hierarchy, allowing us to have in the same class one parent and multiple mixin components. As such, since it’s not a parent of our class, mixin s don't allow any declaration of constructors. You can read more about them in this article by Romain Rastel with the caveat that Dart 2 now has the mixin keyword, as seen in the documentation.

But how do mixins work? Let's take as an example an abstract class Person

We can use this class as a parent using the extend keyword, such as:

With this, we can initialize the class and call the parent’s method think() .

But what if we want to add new features to Mike ? What if Mike is a coder and needs a code function that could be used in other Person , but is not used by all Person s? mixin s save that problem.

First, we need to create a mixin class and expose the new methods we want to use.

With the keyword with , we can add this "feature" to our Mike class:

And, as with the parent, we can call all the functions that we created in Coder .

Now every class that uses the mixin Coder can effectively code. However, this poses a problem: this means that if we have a parent class Animal that has a child Squirrel , we can also have a Squirrel that can code() ! To prevent this, we can "lock" the usage of the mixin to a class and all classes that inherited from it with the keyword on :

This also gives us a powerful tool: we can now override methods that were set in the Person class to add or change functionality.

Calling the super.think() ensures that we are still calling the code that was defined in Person . The above code gives us the following output for the method think in. Mike :

By grasping the concepts of both base class es and mixin s we can now apply them to our Flutter apps.

Mixins and Base Classes: A practical Flutter Example

How can we apply this in our Flutter apps?

Take as an example the following two screens:

Our app has several screens with the layout shown above. Instead of copying and pasting the appbar and the background for each screen, we can solve our problem using mixin s.

In both cases we have a screen title defined, we are going to create a base class that has a method to provide our screen name called BasePage . We also are going to apply the mixin s only in StatefulWidgets since our classes will maintain and change their state. With this, we create two classes to be used in our pages: a BasePage and a BaseState<BasePage> that extend StatefulWidget and State<StatefulWidget> respectively.

Focusing on the second screen, we can now create its custom mixin BasicPageMixin , where we define the background and appbar of our page.

Since the method body() doesn't have a body, each class that uses this mixin must implement it, ensuring that we don't forget to add a body to our page.

In the screenshot above, we see a FloatingActionButton , but we may not need it for every screen, so how can we define it? By declaring a new method, fab() that by default returns a Container . If a class needs to add a FloatingActionButton , they can override this method.

With our mixin created, we can apply it to a new page.

And with this, we now only have to declare a body() and a possible fab() widget to be used in each screen, saving us a few dozens of lines of code.

Combining mixins

As a new feature, some of our screens will make API calls and if an error occurs, we need to display an error message in the form of a Snackbar. Additionally, we decide to use the BLoC architecture in which we need to inject a new bloc when each page is created. These two problems will need the following steps:

Change our BasePage by changing its constructor with the new bloc parameter.

by changing its constructor with the new parameter. Change BaseState by adding a new GlobalKey<ScaffoldState>

by adding a new Create a new mixin that let us display errors messages sent by the bloc in the page using a Snackbar

In our BaseBloc we are just exposing a Sink and a Stream in order to relay error messages.

Since we don’t want any other interactions with the bloc , our HomeBloc will just extend this class

We proceed by changing the constructor of our BasePage to include the bloc object. This will force us to also change all the classes that extend it to add the bloc to their constructors too. The bloc parameter is used as a Generic type so that each class that extends it can declare the correct type of bloc it is using. This makes sure that when we are calling it in the BaseState , we are going to get the correct type of bloc allowing us to access its methods.

As for the BaseState , we are going to declare a scaffoldKey to be used with the ScaffoldWidget so that we can show the Snackbar.

As seen before, one of the curious properties of mixins is that if they are “linked” to a class, they can override its methods. This is useful since in our StatefulWidget we can listen do the bloc 's streams in the initState method. As such, to show the error messages, we can create a mixin that overrides the initState method and provides methods to show the error Snackbar with the correct message.

Finally, we can add it to our HomePage class by adding it after the BasicPage mixin.

Conclusion

And there we go! ✌️ Now we can use both mixin s and abstract class es to reuse code throughout our application.

Maybe we don't need to do a base UI for our app, but we can use mixin s like the ErrorHandlingMixin to provide error feedback to the user, loading screens and showing a "App is Offline" screen.

However, creating both base classes and mixin s is a process that needs some deliberation, else we might face a "Deadly Diamond of Death", in which when calling one method that is declared in both base classes and mixin s, the compiler doesn't know which one to choose. You can read more about it in Shubham Soni 's article about mixin s in Flutter.