RoutedApp

Introduction

Say you want to design an app for a government agency with multiple, let's say 20+ pages (MPA = Multi Page Application). Since Flet is technically SPA (Single Page Application) only, you'll use views and some routing to simulate the MPA behaviour.

Not every person in the agency should be able to access every page/view. Also, they shouldn't be able to see anything but the login page until they're logged in. The roles defined in the OAuth token the app receives upon login will determine what pages/views a user has access to.

You'll probably want to design your app in a way that bundles every page/view into its own module. If you used an architecture design pattern (which you definitely should at this scale), obtaining the view requires building its model and presenter or controller as well and thus you need some function or method to obtain the view.

The way flet routing works ATM, a view will have to be recreated after a route change, so you'll want to match each route of your app to the function or method that creates the appropriate view for it. You'll also want the function/method to return a different view or raise an exception if the user is not authorized to access it. This can create a lot of boilerplate code if you don't have the help of a library.

Usage

ViewBuilder class

In the module of your page/view, create a file called (something like) build.py. In it, create a class called (something like) {page_name}ViewBuilder. This class should inherit from the ViewBuilder class of this library and at minimum define a method with the signature

def build_view(self, route_parameters: dict[str, str]) -> flet.View

This library also contains convenience ViewBuilder subclasses that provide a shortcut for common architecture design patterns. The MvpViewBuilder for example only requires you to define three class variables:

from fletched.mvp import MvpViewBuilder

from my_package.views.counter import CounterDataSource, CounterPresenter, CounterView


class CounterViewBuilder(MvpViewBuilder):
    data_source_class = CounterDataSource
    presenter_class = CounterPresenter
    view_class = CounterView

Route assignment

from fletched.routed_app import route
from fletched.mvp import MvpViewBuilder

from my_package.views.counter import CounterDataSource, CounterPresenter, CounterView

@route("/counter")
class CounterViewBuilder(MvpViewBuilder):
    data_source_class = CounterDataSource
    presenter_class = CounterPresenter
    view_class = CounterView

You can use template routes too! They just have to follow the repath spec, which both flet and fletched use under the hood.

from fletched.routed_app import route
from fletched.mvp import MvpViewBuilder

from my_package.views.counter import CounterDataSource, CounterPresenter, CounterView

@route("/counter/:user/count/:id")
class CounterViewBuilder(MvpViewBuilder):
    data_source_class = CounterDataSource
    presenter_class = CounterPresenter
    view_class = CounterView

The variables user and id will automatically be extracted by fletched and put into the route_params dictionary the build_view() method of the ViewBuilder accepts as a parameter. When no template route is specified or used (parameters can be optional), that dictionary is empty. Otherwise, it will contain a mapping of variable names to variable values.

In the MvpViewBuilder, the route_params mapping will automatically be passed to the DataSource.

In case the route we assigned in the code block above gets called like this: yourappdomain/counter/23/count/4, the route_params dictionary will look like this: {"user"="23", "id"="4"}. Note that both name and value will be strings, so you'll have to convert the value in case it's supposed to be of a different data type.

The repath library allows you to specify a lot of constraints with the help of regular expressions, e.g. that a parameter is supposed to consist of 1-3 digits. If the route the user input does not match the route template of a given ViewBuilder, a RoutedApp will return a simple PageNotFoundView.

Due to the limits of regular expressions, you can not always assume that an input route that was successfully matched to a route template satisfies all the constraints the parameters of that route template should have. The best example for that would be a user ID. We might be able to specify that it must be a number between 1 and 999, but we can't ensure this way that the ID actually exists. That is why it is up to you to handle the error cases that can happen this way.

In general, you probably want to do this in the build_view() method since it is easily possible to return a different view from there. If you use the MvpViewBuilder, this is done by overriding the route_params_valid property of the MvpDataSource, which is defined but not implemented by default.

The build_view() method of the ViewBuilder will automatically create a new DataSource (and thus a new model/view state) when the route parameters change and return the aforementioned PageNotFoundView if the DataSource has a non-empty route_params mapping and the route_params_valid property returns False.

Route protection

from fletched.routed_app import login_required, route
from fletched.mvp import MvpViewBuilder

from my_package.views.counter import CounterDataSource, CounterPresenter, CounterView

@login_required
@route("/counter")
class CounterViewBuilder(MvpViewBuilder):
    data_source_class = CounterDataSource
    presenter_class = CounterPresenter
    view_class = CounterView

from fletched.routed_app import group_required, route
from fletched.mvp import MvpViewBuilder

from my_package.views.counter import CounterDataSource, CounterPresenter, CounterView

@group_required("demo")
@route("/counter")
class CounterViewBuilder(MvpViewBuilder):
    data_source_class = CounterDataSource
    presenter_class = CounterPresenter
    view_class = CounterView

You can also easily write your own auth decorator, all it has to do is define a function that returns a bool and set the auth_func attribute of the ViewBuilder class it wraps to that function.

Aggregating ViewBuilder classes

Somewhere in your project, you will have to import all ViewBuilder classes and aggregate them in a list. The recommended approach is to do this in the __init__.py of the module that contains all your view modules.

It is also possible to create multiple lists of different ViewBuilders in different places in your project and to then add these lists to the app one after another.

RoutedApp usage

In your main() function, create an instance of RoutedApp and add the previously imported list of ViewBuilder classes to the instance.

import flet as ft

from fletched.routed_app import RoutedApp

from mypackage import views

def main(page: ft.Page):
    app = RoutedApp(page)
    app.add_view_builders(views.view_builders)

ft.app(target=main)

App state

You can share data between different pages/views by storing it in the state dictionary of the app instance and retrieving it from there.

state is a defaultdict; if a key does not exist, it will return the string Literal "not set".

Each ViewBuilder will be passed the app instance when it is added to that very instance.

If you know exactly which variables you will need to pass at runtime and you want to have autocomplete in your editor, you can create custom app and state classes in an app.py file like this:

from fletched.routed_app import CustomAppState, RoutedApp


class AppState(CustomAppState):
    test: int = 0
    demo: str = ""


class App(RoutedApp):
    state: AppState = AppState()

Please remember to use those throughout your project when typehinting, otherwise you won't reap the autocomplete benefits.

CustomAppState is an empty dataclass which saves you the trouble of having to import dataclasses and decorate your class and ensures better type safety for the library.

You will also need to pass the custom_state=True flag when creating the app instance, so the constructor of RoutedApp knows not to set the state class variable to an empty defaultdict.