Setup

We’ll start off by creating a brand new Flutter project

flutter create flutter_todos

We can then replace the contents of pubspec.yaml with:

and finally install all of our dependencies

flutter packages get

Note: We’re overriding some dependencies because we’re going to be reusing them from Brian Egan’s Flutter Architecture Samples.

Todos Repository

In this tutorial we’re not going to go into the implementation details of the TodosRepository because it was implemented by Brian Egan and is shared among all of the Todo Architecture Samples.

At a high level, the TodosRepository will expose a method to loadTodos and to saveTodos . That's pretty much all we need to know so for the rest of the tutorial we'll focus on the Bloc and Presentation layers.

Todos Bloc

Our TodosBloc will be responsible for converting TodosEvents into TodosStates and will manage the list of todos in our application.

Model

The first thing we need to do is define our Todo model. Each todo will need to have an id, a task, an optional note, and an optional completed flag.

Let’s create a models directory and create todo.dart .

Note: We’re using the Equatable package so that we can compare instances of Todos without having to manually override == and hashCode .

Next up, we need to create the TodosState which our presentation layer will receive.

States

Let’s create blocs/todos/todos_state.dart and define the different states we'll need to handle.

The three states we will implement are:

TodosLoading - the state while our application is fetching todos from the repository.

- the state while our application is fetching todos from the repository. TodosLoaded - the state of our application after the todos have successfully been loaded.

- the state of our application after the todos have successfully been loaded. TodosNotLoaded - the state of our application if the todos were not successfully loaded.

Note: We are annotating our base TodosState with the immutable decorator so that we can indicate that all TodosStates cannot be changed.

Next, let’s implement the events we will need to handle.

Events

The events we will need to handle in our TodosBloc are:

LoadTodos - tells the bloc that it needs to load the todos from the TodosRepository .

- tells the bloc that it needs to load the todos from the . AddTodo - tells the bloc that it needs to add an new todo to the list of todos.

- tells the bloc that it needs to add an new todo to the list of todos. UpdateTodo - tells the bloc that it needs to update an existing todo.

- tells the bloc that it needs to update an existing todo. DeleteTodo - tells the bloc that it needs to remove an existing todo.

- tells the bloc that it needs to remove an existing todo. ClearCompleted - tells the bloc that it needs to remove all completed todos.

- tells the bloc that it needs to remove all completed todos. ToggleAll - tells the bloc that it needs to toggle the completed state of all todos.

Create blocs/todos/todos_event.dart and let's implement the events we described above.

Now that we have our TodosStates and TodosEvents implemented we can implement our TodosBloc .

Bloc

Let’s create blocs/todos/todos_bloc.dart and get started! We just need to implement initialState and mapEventToState .

Tip: Check out the Bloc VSCode Extension which provides tools for effectively creating blocs for both Flutter and AngularDart apps.

When we yield a state in the private mapEventToState handlers, we are always yielding a new state instead of mutating the currentState . This is because every time we yield, bloc will compare the currentState to the nextState and will only trigger a state change ( transition ) if the two states are not equal. If we just mutate and yield the same instance of state, then currentState == nextState would evaluate to true and no state change would occur.

Our TodosBloc will have a dependency on the TodosRepository so that it can load and save todos. It will have an initial state of TodosLoading and defines the private handlers for each of the events. Whenever the TodosBloc changes the list of todos it calls the saveTodos method in the TodosRepository in order to keep everything persisted locally.

Barrel File

Now that we’re done with our TodosBloc we can create a barrel file to export all of our bloc files and make it convenient to import them later on.

Create blocs/todos/todos.dart and export the bloc, events, and states:

Filtered Todos Bloc

The FilteredTodosBloc will be responsible for reacting to state changes in the TodosBloc we just created and will maintain the state of filtered todos in our application.

Model

Before we start defining and implementing the TodosStates , we will need to implement a VisibilityFilter model that will determine which todos our FilteredTodosState will contain. In this case, we will have three filters:

all - show all Todos (default)

- show all Todos (default) active - only show Todos which have not been completed

- only show Todos which have not been completed completed only show Todos which have been completed

We can create models/visibility_filter.dart and define our filter as an enum:

States

Just like we did with the TodosBloc , we'll need to define the different states for our FilteredTodosBloc .

In this case, we only have two states:

FilteredTodosLoading - the state while we are fetching todos

- the state while we are fetching todos FilteredTodosLoaded - the state when we are no longer fetching todos

Let’s create blocs/filtered_todos/filtered_todos_state.dart and implement the two states.

Note: The FilteredTodosLoaded state contains the list of filtered todos as well as the active visibility filter.

Events

We’re going to implement two events for our FilteredTodosBloc :

UpdateFilter - which notifies the bloc that the visibility filter has changed

- which notifies the bloc that the visibility filter has changed UpdateTodos - which notifies the bloc that the list of todos has changed

Create blocs/filtered_todos/filtered_todos_event.dart and let's implement the two events.

We’re ready to implement our FilteredTodosBloc next!

Bloc

Our FilteredTodosBloc will be similar to our TodosBloc ; however, instead of having a dependency on the TodosRepository , it will have a dependency on the TodosBloc itself. This will allow the FilteredTodosBloc to update its state in response to state changes in the TodosBloc .

Create blocs/filtered_todos/filtered_todos_bloc.dart and let's get started.

We create a StreamSubscription for the stream of TodosStates so that we can listen to the state changes in the TodosBloc . We override the bloc's dispose method and cancel the subscription so that we can clean up after the bloc is disposed.

Barrel File

Just like before, we can create a barrel file to make it more convenient to import the various filtered todos classes.

Create blocs/filtered_todos/filtered_todos.dart and export the three files:

Stats Bloc

The StatsBloc will be responsible for maintaining the statistics for number of active todos and number of completed todos. Similarly, to the FilteredTodosBloc , it will have a dependency on the TodosBloc itself so that it can react to changes in the TodosBloc state.

State

Our StatsBloc will have two states that it can be in:

StatsLoading - the state when the statistics have not yet been calculated.

- the state when the statistics have not yet been calculated. StatsLoaded - the state when the statistics have been calculated.

Create blocs/stats/stats_state.dart and let's implement our StatsState .

Next, let’s define and implement the StatsEvents .

Events

There will just be a single event our StatsBloc will respond to: UpdateStats . This event will be dispatched whenever the TodosBloc state changes so that our StatsBloc can recalculate the new statistics.

Create blocs/stats/states_event.dart and let's implement it.

Now we’re ready to implement our StatsBloc which will look very similar to the FilteredTodosBloc .

Bloc

Our StatsBloc will have a dependency on the TodosBloc itself which will allow it to update its state in response to state changes in the TodosBloc .

Create blocs/stats/stats_bloc.dart and let's get started.

That’s all there is to it! Our StatsBloc recalculates its state which contains the number of active todos and the number of completed todos on each state change of our TodosBloc .

Now that we’re done with the StatsBloc we just have one last bloc to implement: the TabBloc .

Tab Bloc

The TabBloc will be responsible for maintaining the state of the tabs in our application. It will be taking TabEvents as input and outputting AppTabs .

Model / State

We need to define an AppTab model which we will also use to represent the TabState . The AppTab will just be an enum which represents the active tab in our application. Since the app we're building will only have two tabs: todos and stats, we just need two values.

Create models/app_tab.dart :

Event

Our TabBloc will be responsible for handling a single TabEvent :

UpdateTab - which notifies the bloc that the active tab has updated

Create blocs/tab/tab_event.dart :

Bloc

Our TabBloc implementation will be super simple. As always, we just need to implement initialState and mapEventToState .

Create blocs/tab/tab_bloc.dart and let's quickly do the implementation.

I told you it’d be simple. All the TabBloc is doing is setting the initial state to the todos tab and handling the UpdateTab event by yielding a new AppTab instance.

Barrel File

Lastly, we’ll create another barrel file for our TabBloc exports. Create blocs/tab/tab.dart and export the two files:

Bloc Delegate

Before we move on to the presentation layer, we will implement our own BlocDelegate which will allow us to handle all state changes and errors in a single place. It's really useful for things like developer logs or analytics.

Create blocs/simple_bloc_delegate.dart and let's get started.

All we’re doing in this case is printing all state changes ( transitions ) and errors to the console just so that we can see what's going on when we're running our app locally. You can hook up your BlocDelegate to Google Analytics, Sentry, Crashlytics, etc...

Blocs Barrel File

Now that we have all of our blocs implemented we can create a barrel file. Create blocs/blocs.dart and export all of our blocs so that we can conveniently import any bloc code with a single import.

Up next, we’ll focus on implementing the major screens in our Todos application.

Screens

Home Screen

Our HomeScreen will be responsible for creating the Scaffold of our application. It will maintain the AppBar , BottomNavigationBar , as well as the Stats / FilteredTodos widgets (depending on the active tab).

Let’s create a new directory called screens where we will put all of our new screen widgets and then create screens/home_screen.dart .

Our HomeScreen will be a StatefulWidget because it will need to create and dispose the TabBloc , FilteredTodosBloc , and StatsBloc .

The HomeScreen creates the TabBloc , FilteredTodosBloc , and StatsBloc as part of its state. It uses BlocProvider.of<TodosBloc>(context) in order to access the TodosBloc which will be made available from our root TodosApp widget (we'll get to it later in this tutorial).

Since the HomeScreen needs to respond to changes in the TodosBloc state, we use BlocBuilder in order to build the correct widget based on the current TodosState .

The HomeScreen also makes the TabBloc , FilteredTodosBloc , and StatsBloc available to the widgets in its subtree by using the BlocProviderTree widget from flutter_bloc.

BlocProviderTree(

blocProviders: [

BlocProvider<TabBloc>(bloc: _tabBloc),

BlocProvider<FilteredTodosBloc>(bloc: _filteredTodosBloc),

BlocProvider<StatsBloc>(bloc: _statsBloc),

],

child: Scaffold(...),

);

is equivalent to writing

BlocProvider<TabBloc>(

bloc: _tabBloc,

child: BlocProvider<FilteredTodosBloc>(

bloc: _filteredTodosBloc,

child: BlocProvider<StatsBloc>(

bloc: _statsBloc,

child: Scaffold(...),

),

),

);

You can see how using BlocProviderTree helps reduce the levels of nesting and makes the code easier to read and maintain.

Next, we’ll implement the DetailsScreen .

Details Screen

The DetailsScreen displays the full details of the selected todo and allows the user to either edit or delete the todo.

Create screens/details_screen.dart and let's build it.

Note: The DetailsScreen requires a todo id so that it can pull the todo details from the TodosBloc and so that it can update whenever a todo's details have been changed (a todo's id cannot be changed).

The main things to note are that there is an IconButton which dispatches a DeleteTodo event as well as a checkbox which dispatches an UpdateTodo event.

There is also another FloatingActionButton which navigates the user to the AddEditScreen with isEditing set to true . We'll take a look at the AddEditScreen next.

Add/Edit Screen

The AddEditScreen widget allows the user to either create a new todo or update an existing todo based on the isEditing flag that is passed via the constructor.

Create screens/add_edit_screen.dart and let's have a look at the implementation.

There’s nothing bloc-specific in this widget. It’s simply presenting a form and:

if isEditing is true the form is populated it with the existing todo details.

is true the form is populated it with the existing todo details. otherwise the inputs are empty so that the user can create a new todo.

It uses an onSave callback function to notify its parent of the updated or newly created todo.

That’s it for the screens in our application so before we forget, let’s create a barrel file to export them.

Screens Barrel File

Create screens/screens.dart and export all three.

Widgets

FilterButton

The FilterButton widget will be responsible for providing the user with a list of filter options and will notify the FilteredTodosBloc when a new filter is selected.

Let’s create a new directory called widgets and put our FilterButton implementation in widgets/filter_button.dart .

The FilterButton needs to respond to state changes in the FilteredTodosBloc so it uses BlocProvider to access the FilteredTodosBloc from the BuildContext . It then uses BlocBuilder to re-render whenever the FilteredTodosBloc changes state.

The rest of the implementation is pure Flutter and there isn’t much going on so we can move on to the ExtraActions widget.

Extra Actions

Similarly to the FilterButton , the ExtraActions widget is responsible for providing the user with a list of extra options: Toggling Todos and Clearing Completed Todos.

Since this widget doesn’t care about the filters it will interact with the TodosBloc instead of the FilteredTodosBloc .

Let’s create widgets/extra_actions.dart and implement it.

Just like with the FilterButton , we use BlocProvider to access the TodosBloc from the BuildContext and BlocBuilder to respond to state changes in the TodosBloc .

Based on the action selected, the widget dispatches an event to the TodosBloc to either ToggleAll todos' completion states or ClearCompleted todos.

Next we’ll take a look at the TabSelector widget.

Tab Selector

The TabSelector widget is responsible for displaying the tabs in the BottomNavigationBar and handling user input.

Let’s create widgets/tab_selector.dart and implement it.

You can see that there is no dependency on blocs in this widget; it just calls onTabSelected when a tab is selected and also takes an activeTab as input so it knows which tab is currently selected.

Next, we’ll take a look at the FilteredTodos widget.

Filtered Todos

The FilteredTodos widget is responsible for showing a list of todos based on the current active filter.

Create widgets/filtered_todos.dart and let's implement it.

Just like the previous widgets we’ve written, the FilteredTodos widget uses BlocProvider to access blocs (in this case both the FilteredTodosBloc and the TodosBloc are needed).

The FilteredTodosBloc is needed to help us render the correct todos based on the current filter

is needed to help us render the correct todos based on the current filter The TodosBloc is needed to allow us to add/delete todos in response to user interactions such as swiping on an individual todo.

Todo Item

TodoItem is a stateless widget which is responsible for rendering a single todo and handling user interactions (taps/swipes).

Create widgets/todo_item.dart and let's build it.

Again, notice that the TodoItem has no bloc-specific code in it. It simply renders based on the todo we pass via the constructor and calls the injected callback functions whenever the user interacts with the todo.

Next up, we’ll create the DeleteTodoSnackBar .

Delete Todo SnackBar

The DeleteTodoSnackBar is responsible for indicating to the user that a todo was deleted and allows the user to undo his/her action.

Create widgets/delete_todo_snack_bar.dart and let's implement it.

By now, you’re probably noticing a pattern: this widget also has no bloc-specific code. It simply takes in a todo in order to render the task and calls a callback function called onUndo if a user presses the undo button.

We’re almost done; just two more widgets to go!

Loading Indicator

The LoadingIndicator widget is a stateless widget that is responsible for indicating to the user that something is in progress.

Create widgets/loading_indicator.dart and let's write it.

Not much to discuss here; we’re just using a CircularProgressIndicator wrapped in a Center widget (again no bloc-specific code).

Lastly, we need to build our Stats widget.

Stats

The Stats widget is responsible for showing the user how many todos are active (in progress vs completed.

Let’s create widgets/stats.dart and take a look at the implementation.

We’re accessing the StatsBloc using BlocProvider and using BlocBuilder to rebuild in response to state changes in the StatsBloc state.

Putting it all together

Let’s create main.dart and our TodosApp widget. We need to create a main function and run our TodosApp .

Note: We are setting our BlocSupervisor’s delegate to the SimpleBlocDelegate we created earlier so that we can hook into all transitions and errors.

Next, let’s implement our TodosApp widget.

Our TodosApp is a stateless widget which creates a TodosBloc and makes it available through the entire application by using the BlocProvider widget from flutter_bloc.

The TodosApp has two routes:

Home - which renders a HomeScreen

- which renders a AddTodo - which renders a AddEditScreen with isEditing set to false .

The entire main.dart should look like this:

That’s all there is to it! We’ve now implemented a todos app in flutter using the bloc and flutter_bloc packages and we’ve successfully separated our presentation layer from our business logic.

The full source for this example can be found here.

If you enjoyed this exercise as much as I did you can support me by ⭐️the repository, or 👏 for this story.