# That's It!

Minimal, cross-environment user interface framework.

## Definitions

### User interface

A **user interface** is a program which, repeatedly:
* **renders** information to be displayed to the user;
* **handles** input from the user;
thus interactively performing tasks until an exit condition is met.

### Engine

An **engine** is the underlying platform responsible for:
* Displaying your program's `render`ed output to the user
* Reading user input to be `handle`d by your program.

For example, the `tui` engine is based on `crossterm`,
a library for rendering text user interfaces.

### Widget

A **widget** is any struct that implements the `Input<E, I>` and `Output<E, O>` 
traits for a given engine `E`. This enables it to act as a component of the
user interface. Widgets may contain arbitrary state -- including other widgets.
We provide a set of basic widgets that allow you to define standard hierarchical
UI layouts. It's the implementor's responsibility to define
`render` and `handle` behaviors for custom widgets.

### Input

**To respond to user input**, implement the trait `Input`.
It has a single method, `handle`, which takes an input
event, and returns an engine-specific value.

In the case of the `tui` engine, the returned value is a
`bool` corresponding to whether the input event was captured
by the current widget. Returning `true` from `render` terminates
the handling of the current event; returning `false` "bubbles" it
to the parent widget.

### Output

**To display data to the user**, implement the trait `Output`.
It has a single method, `render`, which takes a mutable instance
of the engine, and returns and engine-specific value.

In the case of the `tui` engine, the returned value is `[u16;2]`,
corresponding to the size requested by the widget. This allows
the layout components to implement dynamic, responsive layouts in the
terminal.