Subscription (shorten to Sub) is a variant of Cmd.
Beyond hello how to#
We’ll see together in a later blog post how to unit test all parts of a Fabulous app UI included! Subscriptions
Beyond hello update#
This will keep the init and update functions pure which will let you unit test them easily. Tips: Try to put all side effects in external functions and call them through Cmd, even when they are synchronous or could be inlined in the update function (like logging, sending metrics to AppCenter, etc.). You can find a few other examples in FabulousContacts like using Xamarin.Essentials to start a phone call, author an email, etc. The exact same process is used for any asynchronous calls. The comma between the model and the Cmd means we're constructing a Tuple (commonly noted as `A * B`) We’ll need to update those functions to return the correct type let init () = This time, init and update need to return a tuple of both a Model and a Cmd (the side effect). Let’s change it to mkProgram: let program = XamarinFormsProgram.mkProgram init update view This declaration wants init and update to only return a Model. If you remember from the last blog post, we saw the following declaration to bring together all our functions and start the app: let program = XamarinFormsProgram.mkSimple init update view Fabulous will treat this message like any other message coming from the view. Fabulous will run those for you outside of the main MVU loop, so it can continue to process messages.Īt any point in time, you’ll have the possibility to dispatch a Msg to update your app state and your UI. CommandsĬommand (shorten to Cmd) provides you with a way to tell Fabulous you’d like to run some side effects. This is done in 2 ways: commands and subscriptions. Here, Fabulous allows you to run side effects in a controlled manner while keeping the init and update functions pure and reactive. Everything else is considered a side effect, even just updating a property of its own class or logging something in the debug console because the function would leave the app in a different state than it originally was. Functional programming has a broader definition for side effect:Ī function is considered pure of side effect when it always gives the same output for the same input without changing anything in the application. In object-oriented programming, side effects are considered bad because they change things outside of their scope and lead to unpredictable behaviors. So how does one make async calls with Fabulous? All these things are necessary when writing real world applications. Querying an API, accessing the storage of the device, interacting with a local database, etc. You don’t want your app to stop processing messages while calling an API over an EDGE connection. To ensure reactivity when a new message arrives, the init, update and view functions are synchronous. If several messages are received at once, they will be queued and processed one by one. By design, Fabulous guarantees that you won’t have any concurrency issue, even if multiple things go on simultaneously thanks to this Msg mechanism. Fabulous then proceeds to call the view function with the new state.įor more information on MVU in Fabulous, please read Fabulous: Functional App Development.
When the user interacts with the UI, it dispatches messages (aka Msg) back to Fabulous to let it know that the state needs to change.Īt this point, Fabulous call the update function with the current state and the received message to let you define a brand new state. The state is created by the init function, and Fabulous passes it to the view function to know what to display on the UI. This state is immutable, no one can change it. MVU uses a single state object (aka the Model) for all the data needed by your app. If you haven’t read it yet, I strongly encourage you to do so first as this blog post will build on it.Ī quick reminder on the MVU architecture as used in Fabulous: You can find him on Twitter Fabulous: Functional App Development, we saw how to leverage functional programming and the Model-View-Update (MVU) architecture to build mobile and desktop apps with Fabulous. Timothé is the maintainer of Fabulous: Functional App Development and a Microsoft MVP. This is a guest blog by Timothé Larivière.
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