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#353 in Asynchronous

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flo_binding, a data-driven binding library

flo_binding provides a way to create 'data bindings', which are a way to store and share state between different parts of an application. Applications designed around these data structures are sometimes referred to as 'reactive' applications.

A basic binding can be created using the bind() function:

    let binding = bind(1);

This can be updated by calling set() and retrieved using get():

    let value = binding.get(); // == 1
    let value = binding.get(); // == 2

Cloning a binding will share the state:

    let another_binding = binding.clone();
    let value = another_binding.get();  // == 3
    let value = binding.get();          // Also == 3

There are two main ways to be notified when a binding has changed. The when_changed() function provides a way to call a function whenever a binding has changed since it was last read, and the follow() function will return a Stream of the most recent value attached to the binding. The streaming approach is the most flexible.

    let binding             = bind(1);
    let mut event_lifetime  = binding.when_changed(notify(|| { println!("Binding changed"); }));
    let mut binding_stream  = follow(binding.clone());

    let value = binding_stream.next().await;        // == 1
    binding.set(2);                                 // Prints 'Binding changed'
    binding.set(3);                                 // Changed flag is set, so prints nothing
    let value = binding.get();                      // == 3

    binding.set(4);                                 // Prints 'Binding changed' again
    let value = binding_stream.next().await;        // == 4 (stream does not return intermediate values)
    let value = binding_stream.next().await;        // Blocks until something elsewhere updates the binding

    binding.set(5);                                 // Prints nothing as the 'when_changed' event has been deregistered

The inverse of follow() is bind_stream(), which creates a bindings that is kept up to date with whatever the last value received from a stream is:

    let binding             = bind(1);
    let binding_from_stream = bind_stream(follow(binding.clone()), 1, |_old_value, new_value| new_value);

    let value = binding.get();                  // == 1
    let value = binding_from_stream.get();      // == 1

    let value = binding_from_stream.get();      // == 2

A stream binding like this is read-only, but is a good way to convert any stream of state values into a value representing a static state. It implements all of the other binding operations.

Another important type of binding is the computed() binding, which makes it possible to create a binding that derives a value from other bindings. Computed bindings automatically monitor any bindings that were captured for changes, so they can be followed or when_changed as with any other binding:

    let binding             = bind(1);
    let binding_copy        = binding.clone();
    let one_more            = computed(move || binding_copy.get() + 1);

    let mut event_lifetime  = one_more.when_changed(notify(|| println!("Computed binding changed")));

    let value = one_more.get();     // == 2 (1 + 1)
    binding.set(2);                 // Prints 'Computed binding changed'
    binding.set(3);                 // Computed binding has not been read since the last notification so prints nothing
    let value = one_more.get();     // == 4 (3 + 1)

For collections of data, flo_binding uses the concept of a 'rope binding'. The general rope data type is provided by the flo_rope crate. These bindings send differences rather than their full state when streaming and are internally represented by a data structure that can perform deletions and insertions efficiently. Unlike the traditional concept of a rope, they aren't limited to editing strings, and can annotate their contents with attributes, which makes them suitable for representing sequences of any kind, or sequences of rich text annotated with style information.

    let mutable_rope        = RopeBindingMut::<usize, ()>::new();
    let rope_copy           = RopeBinding::from_stream(mutable_rope.follow_changes());
    let mut rope_stream     = rope_copy.follow_changes();

    mutable_rope.replace(0..0, vec![1, 2, 3, 4]);

    let next            = rope_stream.next().await;                         // == RopeAction::Replace(0..0, vec![1,2,3,4]))

    let rope_len        = rope_copy.len();                                  // == 4
    let rope_content    = rope_copy.read_cells(0..4).collect::<Vec<_>>();   // == vec![1, 2, 3, 4]

The flo_rope library provides some extra functionality - for example, a way to create the RopeActions for a rope by using a diff algorithm on a changing sequence instead of just reporting the changes as they arrive.

Companion libraries

Aside from flo_rope, the desync crate provides a novel approach to asynchronous code, including pipe operations that work very well with the streamed updates from the follow() function.

The flo_stream provides a pubsub system that provides more flexible ways for distributing state through streams.

flo_scene is a runtime for building complex systems out of entities that exchange messages with each other. It uses flo_binding as an ergonomic way to exchange state information.

FlowBetween is a vector and 2D animation editor that uses flo_binding to represent its UI.