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· 5 min read
Feodor Fitsner

Flet 0.1.42 has been released with navigation and routing!

Navigation and routing is an essential feature of Single Page Applications (SPA) which allows organizing application user interface into virtual pages (views) and "navigate" between them while application URL reflects the current state of the app.

For mobile apps navigation and routing serves as a deep linking to specific application parts.

Well, it took more efforts than expected to add navigation and routing into Flet as the implementation is based on Navigator 2.0 Flutter API and required to replace Flet's "Page" abstraction with "Page and Views". Flutter's newer navigation and routing API has substantial improvements such as:

  1. Programmatic control over history stack.
  2. An easy way to intercept a call to "Back" button in AppBar.
  3. Robust synchronization with browser history.

Explore source code of the example above.

Page route

Page route is a portion of application URL after # symbol:

Default application route, if not set in application URL by the user, is /. All routes start with /, for example /store, /authors/1/books/2.

Application route can be obtained by reading page.route property, for example:

import flet as ft

def main(page: ft.Page):
page.add(ft.Text(f"Initial route: {page.route}"))

ft.app(target=main, view=ft.WEB_BROWSER)

Grab application URL, open a new browser tab, paste the URL, modify its part after # to /test and hit enter. You should see "Initial route: /test".

Every time the route in the URL is changed (by editing the URL or navigating browser history with Back/Forward buttons) Flet calls page.on_route_change event handler:

import flet as ft

def main(page: ft.Page):
page.add(ft.Text(f"Initial route: {page.route}"))

def route_change(route):
page.add(ft.Text(f"New route: {route}"))

page.on_route_change = route_change
page.update()

ft.app(target=main, view=ft.WEB_BROWSER)

Now try updating URL hash a few times and then use Back/Forward buttons! You should see a new message added to a page each time the route changes:

Route can be changed programmatically, by updating page.route property:

import flet as ft

def main(page: ft.Page):
page.add(ft.Text(f"Initial route: {page.route}"))

def route_change(route):
page.add(ft.Text(f"New route: {route}"))

def go_store(e):
page.route = "/store"
page.update()

page.on_route_change = route_change
page.add(ft.ElevatedButton("Go to Store", on_click=go_store))

ft.app(target=main, view=ft.WEB_BROWSER)

Click "Go to Store" button and you'll see application URL is changed and a new item is pushed in a browser history. You can use browser "Back" button to navigate to a previous route.

Page views

Flet's Page now is not just a single page, but a container for View layered on top of each other like a sandwich:

A collection of views represents navigator history. Page has page.views property to access views collection.

The last view in the list is the one currently displayed on a page. Views list must have at least one element (root view).

To simulate a transition between pages change page.route and add a new View in the end of page.view list.

Pop the last view from the collection and change route to a "previous" one in page.on_view_pop event handler to go back.

Building views on route change

To build a reliable navigation there must be a single place in the program which builds a list of views depending on the current route. Other words, navigation history stack (represented by the list of views) must be a function of a route.

This place is page.on_route_change event handler.

Let's put everything together into a complete example which allows navigating between two pages:

import flet as ft

def main(page: ft.Page):
page.title = "Routes Example"

def route_change(route):
page.views.clear()
page.views.append(
View(
"/",
[
ft.AppBar(title=ft.Text("Flet app"), bgcolor=ft.colors.SURFACE_VARIANT),
ft.ElevatedButton("Visit Store", on_click=lambda _: page.go("/store")),
],
)
)
if page.route == "/store":
page.views.append(
View(
"/store",
[
ft.AppBar(title=ft.Text("Store"), bgcolor=ft.colors.SURFACE_VARIANT),
ft.ElevatedButton("Go Home", on_click=lambda _: page.go("/")),
],
)
)
page.update()

def view_pop(view):
page.views.pop()
top_view = page.views[-1]
page.go(top_view.route)

page.on_route_change = route_change
page.on_view_pop = view_pop
page.go(page.route)


ft.app(target=main, view=ft.WEB_BROWSER)

Try navigating between pages using "Visit Store" and "Go Home" buttons, Back/Forward browser buttons, manually changing route in the URL - it works no matter what! :)

note

To "navigate" between pages we used page.go(route) - a helper method that updates page.route, calls page.on_route_change event handler to update views and finally calls page.update().

Notice the usage of page.on_view_pop event handler. It fires when the user clicks automatic "Back" button in AppBar control. In the handler we remove the last element from views collection and navigate to view's root "under" it.

Route templates

Flet offers TemplateRoute - an utility class based on repath library which allows matching ExpressJS-like routes and parsing their parameters, for example /account/:account_id/orders/:order_id.

TemplateRoute plays great with route change event:

troute = TemplateRoute(page.route)

if troute.match("/books/:id"):
print("Book view ID:", troute.id)
elif troute.match("/account/:account_id/orders/:order_id"):
print("Account:", troute.account_id, "Order:", troute.order_id)
else:
print("Unknown route")

You can read more about template syntax supported by repath library here.

That's all for today!

Give Flet a try and let us know what you think!

· 2 min read
Feodor Fitsner

We have just released Flet 0.1.41 with drag-and-drop support and other neat features such as absolute positioning of controls inside stack and clickable container!

Drag and Drop

Making drag-and-drop in Flet is a real joy - thanks to a smart drag-and-drop implementation in Flutter! You just have "draggable" control which could be dragged to a "drag target" which calls on_accept event handler when draggable is dropped.

Take a look at Drag-and-Drop example.

Explore Draggable and DragTarget controls, their properties and events.

Absolute positioning inside Stack

All visible controls now have left top, right and bottom properties to let them be absolutely positioned inside Stack, for example:

import flet as ft

def main(page: ft.Page):

page.horizontal_alignment = ft.CrossAxisAlignment.CENTER
page.vertical_alignment = ft.MainAxisAlignment.CENTER

page.add(
ft.Container(
ft.Stack(
[
ft.Text("1", color=ft.colors.WHITE),
ft.Text("2", color=ft.colors.WHITE, right=0),
ft.Text("3", color=ft.colors.WHITE, right=0, bottom=0),
ft.Text("4", color=ft.colors.WHITE, left=0, bottom=0),
ft.Text("5", color=ft.colors.WHITE, left=40, top=35),
]
),
border_radius=8,
padding=5,
width=100,
height=100,
bgcolor=ft.colors.BROWN_700,
)
)

ft.app(target=main)

Clickable container

Container control has got on_click event which allows you to make a button from any control and with a beautiful material ripple effect when ink is set to True!

See source code for the example above.

Give Flet a try and let us know what you think!

· 2 min read
Feodor Fitsner

You can now use your own fonts in a Flet app!

The following font formats are supported:

  • .ttc
  • .ttf
  • .otf

Use page.fonts property to import fonts.

Set page.fonts property to a dictionary where key is the font family name to refer that font and the value is the URL of the font file to import:

def main(page: ft.Page):
page.fonts = {
"Kanit": "https://raw.githubusercontent.com/google/fonts/master/ofl/kanit/Kanit-Bold.ttf",
"Aleo Bold Italic": "https://raw.githubusercontent.com/google/fonts/master/ofl/aleo/Aleo-BoldItalic.ttf"
}
page.update()

# ...

Font can be imported from external resource by providing an absolute URL or from application assets by providing relative URL and assets_dir.

Specify assets_dir in flet.app() call to set the location of assets that should be available to the application. assets_dir could be a relative to your main.py directory or an absolute path. For example, consider the following program structure:

/assets
/fonts
/OpenSans-Regular.ttf
main.py

Code sample

The following program loads "Kanit" font from GitHub and "Open Sans" from the assets. "Kanit" is set as a default app font and "Open Sans" is used for a specific Text control:

import flet as ft

def main(page: ft.Page):
page.title = "Custom fonts"

page.fonts = {
"Kanit": "https://raw.githubusercontent.com/google/fonts/master/ofl/kanit/Kanit-Bold.ttf",
"Open Sans": "fonts/OpenSans-Regular.ttf",
}

page.theme = Theme(font_family="Kanit")

page.add(
ft.Text("This is rendered with Kanit font"),
ft.Text("This is Open Sans font example", font_family="Open Sans"),
)

ft.app(target=main, assets_dir="assets")

Static vs Variable fonts

At the moment only static fonts are supported, i.e. fonts containing only one spacific width/weight/style combination, for example "Open Sans Regular" or "Roboto Bold Italic".

Variable fonts support is still work in progress.

However, if you need to use a variable font in your app you can create static "instantiations" at specific weights using fonttools, then use those:

fonttools varLib.mutator ./YourVariableFont-VF.ttf wght=140 wdth=85

To explore available font features (e.g. possible options for wght) use Wakamai Fondue online tool.

Give Flet a try and let us know what you think!

· 2 min read
Feodor Fitsner

Today we announce the first release of Flet!

Flet is a framework for building real-time web, desktop and mobile applications in Python.

No more complex architecture with JavaScript frontend, REST API backend, database, cache, etc. With Flet you just write a monolith stateful app in Python only and get multi-user, realtime Single-Page Application (SPA) or a mobile app.

To start developing with Flet, you just need your favorite IDE or text editor. No SDKs, no thousands of dependencies, no complex tooling - Flet has built-in web server with assets hosting and desktop clients.

Flet UI is built with Flutter, so your app looks professional and can be delivered to any platform. Flet simplifies Flutter model by combining smaller "widgets" into ready-to-use "controls" with imperative programming model. You get all the power of Flutter without having to learn Dart!

Flet app is deployed as a regular web app and can be instanly accessed with a browser or installed as a PWA on a mobile device. Web app also exposes an API that can be used by a Flet client (planned for future releases) running on iOS and Android and providing native mobile experience.

Some examples:

Give Flet a try and let us know what you think!