draconisplusplus/subprojects/FTXUI-5.0.0/doc/mainpage.md

\mainpage

# Introduction {#introduction}

Welcome to the FTXUI documentation!

This is a brief tutorial. You are also encouraged to self-learn by reading the
[examples](./examples.html).

@tableofcontents 

**Short example**

To build a single frame, you need create an `ftxui::Element`, and display it on
a `ftxui::Screen`.

**main.cpp**
```cpp
#include <ftxui/dom/elements.hpp>
#include <ftxui/screen/screen.hpp>
#include <iostream>

int main(void) {
  using namespace ftxui;

  // Define the document
  Element document =
    hbox({
      text("left")   | border,
      text("middle") | border | flex,
      text("right")  | border,
    });

  auto screen = Screen::Create(
    Dimension::Full(),       // Width
    Dimension::Fit(document) // Height
  );
  Render(screen, document);
  screen.Print();

  return EXIT_SUCCESS;
}
```


**output**
```bash
┌────┐┌────────────────────────────────────┐┌─────┐
│left││middle                              ││right│
└────┘└────────────────────────────────────┘└─────┘
```

# Build {#build}

## Using CMake {#build-cmake}

This is an example configuration for your **CMakeLists.txt**

CMakeLists.txt
```cmake
cmake_minimum_required (VERSION 3.11)

# --- Fetch FTXUI --------------------------------------------------------------
include(FetchContent)

set(FETCHCONTENT_UPDATES_DISCONNECTED TRUE)
FetchContent_Declare(ftxui
  GIT_REPOSITORY https://github.com/ArthurSonzogni/ftxui
  # Important: Specify a GIT_TAG XXXXX here.
)

FetchContent_GetProperties(ftxui)
if(NOT ftxui_POPULATED)
  FetchContent_Populate(ftxui)
  add_subdirectory(${ftxui_SOURCE_DIR} ${ftxui_BINARY_DIR} EXCLUDE_FROM_ALL)
endif()

# ------------------------------------------------------------------------------

project(ftxui-starter
  LANGUAGES CXX
  VERSION 1.0.0
)

add_executable(ftxui-starter src/main.cpp)
target_include_directories(ftxui-starter PRIVATE src)

target_link_libraries(ftxui-starter
  PRIVATE ftxui::screen
  PRIVATE ftxui::dom
  PRIVATE ftxui::component # Not needed for this example.
)

```

Subsequently, you build the project in the standard fashion as follows:
```bash
mkdir build && cd build
cmake ..
make
./main
```

# List of modules. {#modules}

The project is comprised of 3 modules:

1. **ftxui/screen** defines a `ftxui::Screen`, a grid of `ftxui::Pixel`.

2. **ftxui/dom** is the main module. It defines a hierarchical set of
   `ftxui::Element`.  An element draws something on the `ftxui::Screen`. It is
   responsive to the size of its container.

3. **ftxui/component** The module is required if your program needs to respond
   to user input. It defines a set of `ftxui::Component`. These components can
   be utilized to navigate using the arrow keys *and/or* cursor. There are 
   several builtin widgets like checkbox/inputbox/etc to interact with. You can
   combine them, or even define your own custom components.

# screen {#module-screen}

This is the visual element of the program. It defines a `ftxui::Screen`, which
is a grid of `ftxui::Pixel`. A Pixel represents a Unicode character and its
associated style (bold, colors, etc.). The screen can be printed as a string
using `ftxui::Screen::ToString()`. The following example highlights this
process:

```cpp
  #include <ftxui/screen/screen.hpp>
  #include <iostream>

  int main(void) {
    using namespace ftxui;
    auto screen = Screen::Create(Dimension::Fixed(32), Dimension::Fixed(10));

    auto& pixel = screen.PixelAt(9,9);
    pixel.character = U'A';
    pixel.bold = true;
    pixel.foreground_color = Color::Blue;

    std::cout << screen.ToString();
    return EXIT_SUCCESS;
  }
```

# dom {#module-dom}

This module defines a hierarchical set of `ftxui::Element`. An element manages
the layout and can be responsive to the terminal dimension changes. Note the
following example where this module is used to create a simple layout with a
number of operators:

**Example:**
```cpp
// Define the document
Element document = vbox({
  text("The window") | bold | color(Color::Blue),
  gauge(0.5)
  text("The footer")
});

// Add a border, by calling the `ftxui::border` decorator function.
document = border(document);

// Add another border, using the pipe operator.
document = document | border.

// Add another border, using the |= operator.
document |= border

```

**List of elements**

The list of all elements are included and can be accessed by including the
corresponding header file:
```cpp
#include <ftxui/dom/elements.hpp>
```

\include ftxui/dom/elements.hpp

## text ## {#dom-text}

The most simple widget. It displays a text.
```cpp
text("I am a piece of text");
```
```bash
I am a piece of text.
```

## vtext {#dom-vtext}

Identical to `ftxui::text`, but displayed vertically.

Code:
```cpp
vtext("HELLO");
```

Terminal output:
```bash
H
E
L
L
O
```

## paragraph {#dom-paragraph}

Similar to `ftxui::text`, but the individual word are wrapped along multiple
lines, depending on the width of its container.

Sample Code:
```cpp
paragraph("A very long text")
```

![ezgif com-gif-maker (4)](https://user-images.githubusercontent.com/4759106/147251370-983a06e7-6f41-4113-92b8-942f43d34d06.gif)

For a more detailed example refer to [detailed example](https://arthursonzogni.github.io/FTXUI/examples_2dom_2paragraph_8cpp-example.html). Paragraph also includes a number of other variants as shown below:
```cpp
Element paragraph(std::string text);
Element paragraphAlignLeft(std::string text);
Element paragraphAlignRight(std::string text);
Element paragraphAlignCenter(std::string text);
Element paragraphAlignJustify(std::string text);
```


## border {#dom-border}

Adds a border around an element.

Code:
```cpp
border(text("The element"))
```

Terminal output:
```bash
┌───────────┐
│The element│
└───────────┘
```

You can achieve the same behavior by using the pipe operator.

Code:
```cpp
text("The element") | border
```

Border also comes in a variety of styles as shown below:
```cpp
Element border(Element);
Element borderLight(Element);
Element borderHeavy(Element);
Element borderDouble(Element);
Element borderRounded(Element);
Element borderEmpty(Element);
Decorator borderStyled(BorderStyle);
Decorator borderWith(Pixel);
```


## window ## {#dom-window}

A `ftxui::window` is a `ftxui::border`, but with an additional header. To add a
window around an element, wrap it and specify a string as the header.
Code:
```cpp
window("The window", text("The element"))
```

Terminal output:
```bash
┌The window─┐
│The element│
└───────────┘
```

## separator {#dom-separator}

Displays a vertical/horizontal line to visually split the content of a
container in two.

Code:
```cpp
border(
  hbox({
    text("Left"), 
    separator(),
    text("Right")
  })
)
```

Terminal output:
```bash
┌────┬─────┐
│left│right│
└────┴─────┘
```


Separators come in a variety of flavors as shown below:
```cpp
Element separator(void);
Element separatorLight();
Element separatorHeavy();
Element separatorDouble();
Element separatorEmpty();
Element separatorStyled(BorderStyle);
Element separator(Pixel);
Element separatorCharacter(std::string);
Element separatorHSelector(float left,
                           float right,
                           Color background,
                           Color foreground);
Element separatorVSelector(float up,
                           float down,
                           Color background,
                           Color foreground);
```

## gauge {#dom-gauge}

It constitutes a gauge. It can be used to represent a progress bar.

Code:
```cpp
border(gauge(0.5))
```

Teminal output:
```bash
┌────────────────────────────────────────────────────────────────────────────┐
│██████████████████████████████████████                                      │
└────────────────────────────────────────────────────────────────────────────┘
```

Gauges can be displayed in many orientations as shown below:
```cpp
Element gauge(float ratio);
Element gaugeLeft(float ratio);
Element gaugeRight(float ratio);
Element gaugeUp(float ratio);
Element gaugeDown(float ratio);
Element gaugeDirection(float ratio, GaugeDirection);
```

## graph {#dom-graph}

@htmlonly
<script id="asciicast-223726" src="https://asciinema.org/a/223726.js" async></script>
@endhtmlonly

See:
```cpp
Element graph(GraphFunction);
```

## Colors {#dom-colors}

Most terminal consoles can display colored text and colored backgrounds. FTXUI
supports every color palette:
```cpp
Decorator color(Color);
Decorator bgcolor(Color);
```


Color [gallery](https://arthursonzogni.github.io/FTXUI/examples_2dom_2color_gallery_8cpp-example.html):
![image](https://user-images.githubusercontent.com/4759106/147248595-04c7245a-5b85-4544-809d-a5984fc6f9e7.png)

### Palette16 #{#dom-colors-palette-16}

On most terminals the following colors are supported:
- Default

- Black
- GrayDark
- GrayLight

- White

- Blue
- BlueLight

- Cyan
- CyanLight

- Green
- GreenLight

- Magenta
- MagentaLight

- Red
- RedLight

- Yellow
- YellowLight

Example use of the above colors using the pipe operator:
```cpp
text("Blue foreground") | color(Color::Blue);
text("Blue background") | bgcolor(Color::Blue);
text("Black on white") | color(Color::Black) | bgcolor(Color::White);
```

### Palette256 #{#dom-colors-palette-256}

On terminal supporting 256 colors. 
@htmlonly
<script id="asciicast-OAUc3n6QrkmrLt7XEEb8AzbLt" src="https://asciinema.org/a/OAUc3n6QrkmrLt7XEEb8AzbLt.js" async></script>
@endhtmlonly

```cpp
text("HotPink") | color(Color::HotPink);
```

### TrueColor #{#dom-colors-true-color}

On terminal supporting trueColor, you can directly use the 24bit RGB color
space:

Use the constructors below to specify the **RGB** or **HSV** values for your
color:

There are two constructors:
```cpp
ftxui::Color::RGB(uint8_t red, uint8_t green, uint8_t blue);
ftxui::Color::HSV(uint8_t hue, uint8_t saturation, uint8_t value);
```

@htmlonly
<script id="asciicast-dk5r8IcCH0aFIIgWG0keSEHMG" src="https://asciinema.org/a/dk5r8IcCH0aFIIgWG0keSEHMG.js" async></script>
<script id="asciicast-xwzzghmqcqzIuyLwCpQFEqbEu" src="https://asciinema.org/a/xwzzghmqcqzIuyLwCpQFEqbEu.js" async></script>
@endhtmlonly

## LinearGradient #{#dom-linear-gradient}

FTXUI supports linear gradient. Either on the foreground or the background.

```cpp
Decorator color(const LinearGradient&);
Decorator bgcolor(const LinearGradient&);
```

A `ftxui::LinearGradient` is defined by an angle in degree, and a list of color
stops.
```cpp
auto gradient = LinearGradient()
  .Angle(45)
  .AddStop(0.0, Color::Red)
  .AddStop(0.5, Color::Green)
  .AddStop(1.0, Color::Blue);
```

You can also use simplified constructors:
```cpp
LinearGradient(Color::Red, Color::Blue);
```
```cpp
LinearGradient(45, Color::Red, Color::Blue);
```

See [demo](https://arthursonzogni.github.io/FTXUI/examples/?file=component/linear_gradient_gallery).


## Style {#dom-style}
In addition to colored text and colored backgrounds. Many terminals support text
effects such as: `bold`, `dim`, `underlined`, `inverted`, `blink`.

```cpp
Element bold(Element);
Element dim(Element);
Element inverted(Element);
Element underlined(Element);
Element underlinedDouble(Element);
Element strikethrough(Element);
Element blink(Element);
Decorator color(Color);
Decorator bgcolor(Color);
Decorator colorgrad(LinearGradient);
Decorator bgcolorgrad(LinearGradient);
```

[Example](https://arthursonzogni.github.io/FTXUI/examples_2dom_2style_gallery_8cpp-example.html)

![image](https://user-images.githubusercontent.com/4759106/147244118-380bf834-9e33-40df-9ff0-07c10f2598ef.png)

To use these effects, simply wrap your elements with your desired effect:
```cpp
underlined(bold(text("This text is bold and underlined")))
```

Alternatively, use the pipe operator to chain it on your element:
```cpp
text("This text is bold") | bold | underlined
```

## Layout {#dom-layout}

Enables elements to be arranged in the following ways:
  - **Horizontally** with `ftxui::hbox`
  - **Vertically** with `ftxui::vbox`
  - **Inside a grid** with `ftxui::gridbox`
  - **Wrapped along one direction** using the `ftxui::flexbox`.
  
[Example](https://arthursonzogni.github.io/FTXUI/examples_2dom_2vbox_hbox_8cpp-example.html) using `ftxui::hbox`, `ftxui::vbox` and `ftxui::filler`.

![image](https://user-images.githubusercontent.com/4759106/147242524-7103b5d9-1a92-4e2d-ac70-b3d6740061e3.png)
  
  
[Example](https://arthursonzogni.github.io/FTXUI/examples_2dom_2gridbox_8cpp-example.htmlp)
using `ftxui::gridbox`:

![image](https://user-images.githubusercontent.com/4759106/147242972-0db1f2e9-0790-496f-86e6-ed2c604f7a73.png)

[Example](https://github.com/ArthurSonzogni/FTXUI/blob/master/examples/dom/hflow.cpp)
using flexbox:

![image](https://user-images.githubusercontent.com/4759106/147243064-780ac7cc-605b-475f-94b8-cf7c4aed03a5.png)

Checkout this
[example](https://arthursonzogni.github.io/FTXUI/examples_2dom_2hflow_8cpp-example.html)
and the associated
[demo](https://arthursonzogni.github.io/FTXUI/examples/?file=component/flexbox).

Element can also become flexible using the the `ftxui::flex` decorator.

Code:
```cpp
  hbox({
    text("left") | border ,
    text("middle") | border | flex,
    text("right") | border,
  });
```
Terminal output:
```bash
┌────┐┌─────────────────────────────────────────────────────┐┌─────┐
│left││middle                                               ││right│
└────┘└─────────────────────────────────────────────────────┘└─────┘
```

Code:
```cpp
  hbox({
    text("left") | border ,
    text("middle") | border | flex,
    text("right") | border | flex,
  });
```

Terminal output:
```bash
┌────┐┌───────────────────────────────┐┌───────────────────────────────┐
│left││middle                         ││right                          │
└────┘└───────────────────────────────┘└───────────────────────────────┘
```

## Table {#dom-table}

Enables easy formatting of data into a neat table like visual form.

[Code example](https://arthursonzogni.github.io/FTXUI/examples_2dom_2table_8cpp-example.html):
  
![image](https://user-images.githubusercontent.com/4759106/147250766-77d8ec9e-cf2b-486d-9866-1fd9f1bd2e6b.png)

## Canvas {#dom-canvas}

See the API [<ftxui/dom/canvas.hpp>](./canvas_8hpp_source.html)

```cpp
  auto c = Canvas(100, 100);
  c.DrawPointLine(10, 10, 80, 10, Color::Red);
  auto element = canvas(c);
```

Drawing can be performed on a `ftxui::Canvas`, using braille, block, or simple
characters:
  
Simple [example](https://github.com/ArthurSonzogni/FTXUI/blob/master/examples/dom/canvas.cpp):
  
![image](https://user-images.githubusercontent.com/4759106/147245843-76cc62fb-ccb4-421b-aacf-939f9afb42fe.png)

Complex [example](https://github.com/ArthurSonzogni/FTXUI/blob/master/examples/component/canvas_animated.cpp):
  
![ezgif com-gif-maker (3)](https://user-images.githubusercontent.com/4759106/147250538-783a8246-98e0-4a25-b032-3bd3710549d1.gif)
# component {#module-component}

The `ftxui::component` module defines the logic that produces interactive
components that respond to user events (keyboard, mouse, etc.).

A `ftxui::ScreenInteractive` defines a main loop that renders a component.

A `ftxui::Component` is a shared pointer to a `ftxui::ComponentBase`. The latter defines:
  - `ftxui::ComponentBase::Render()`: How to render the interface.
  - `ftxui::ComponentBase::OnEvent()`: How to react to events.
  - `ftxui::ComponentBase::Add()`: Construct a parent/child relationship
    between two components. The tree of component is used to define how to
    navigate using the keyboard.

`ftxui::Element` are used to render a single frame.

`ftxui::Component` are used to render dynamic user interface, producing multiple
frame, and updating its state on events.

[Gallery](https://arthursonzogni.github.io/FTXUI/examples_2component_2gallery_8cpp-example.html) of multiple components. ([demo](https://arthursonzogni.github.io/FTXUI/examples/?file=component/gallery))

![image](https://user-images.githubusercontent.com/4759106/147247330-b60beb9f-e665-48b4-81c0-4b01ee95bc66.png)

All predefined components are available in
["ftxui/dom/component.hpp"](./component_8hpp.html)

\include ftxui/component/component.hpp

## Input {#component-input}

[Example](https://arthursonzogni.github.io/FTXUI/examples_2component_2input_8cpp-example.html):

![image](https://user-images.githubusercontent.com/4759106/147247671-f1d6f606-1845-4e94-a4a0-d4273e9ae6bd.png)

Produced by: `ftxui::Input()` from "ftxui/component/component.hpp"

@htmlonly
<script id="asciicast-223719" src="https://asciinema.org/a/223719.js" async></script>
@endhtmlonly

## Menu {#component-menu}

Defines a menu object. It contains a list of entries, one of them is selected.

[Example](https://arthursonzogni.github.io/FTXUI/examples_2component_2menu_8cpp-example.html):

![image](https://user-images.githubusercontent.com/4759106/147247822-0035fd6f-bb13-4b3a-b057-77eb9291582f.png)


Produced by: `ftxui::Menu()` from "ftxui/component/component.hpp"

@htmlonly
<script id="asciicast-223720" src="https://asciinema.org/a/223720.js" async></script>
@endhtmlonly

## Toggle {#component-toggle}

A special kind of menu. The entries are displayed horizontally.

[Example](https://arthursonzogni.github.io/FTXUI/examples_2component_2toggle_8cpp-example.html):

![image](https://user-images.githubusercontent.com/4759106/147249383-e2201cf1-b7b8-4a5a-916f-d761e3e7ae40.png)

Produced by: `ftxui::Toggle()` from "ftxui/component/component.hpp"

@htmlonly
<script id="asciicast-223722" src="https://asciinema.org/a/223722.js" async></script>
@endhtmlonly

## CheckBox {#component-checkbox}

This component defines a checkbox. It is a single entry that can be turned
on/off.

[Example](https://arthursonzogni.github.io/FTXUI/examples_2component_2checkbox_8cpp-example.html):

![image](https://user-images.githubusercontent.com/4759106/147246646-b86926a9-1ef9-4efb-af98-48a9b62acd81.png)

Produced by: `ftxui::Checkbox()` from "ftxui/component/component.hpp"

@htmlonly
<script id="asciicast-223724" src="https://asciinema.org/a/223724.js" async></script>
@endhtmlonly

## RadioBox {#component-radiobox}

A radiobutton component. This is a list of entries, where one can be turned on.

[Example](https://arthursonzogni.github.io/FTXUI/examples_2component_2radiobox_8cpp-example.html):
  
![image](https://user-images.githubusercontent.com/4759106/147246401-809d14a5-6621-4e36-8dd9-a2d75ef2a94e.png)

Produced by: `ftxui::Radiobox()` from "ftxui/component/component.hpp"

@htmlonly
<script id="asciicast-223725" src="https://asciinema.org/a/223725.js" async></script>
@endhtmlonly

## Dropdown {#component-dropdown}

A drop down menu is a component that when checked display a list of element for
the user to select one.

[Example](https://arthursonzogni.github.io/FTXUI/examples_2component_2dropdown_8cpp-example.html):

![youtube-video-gif (3)](https://user-images.githubusercontent.com/4759106/147246982-1e821751-531c-4e1f-bc37-2fa290e143cd.gif)

Produced by: `ftxui::Dropdown()` from "ftxui/component/component.hpp"

## Slider {#component-slider}

Represents a slider object that consists of a range with binned intermediate
intervals. It can be created by `ftxui::Slider()`.

[Example](https://arthursonzogni.github.io/FTXUI/examples_2component_2slider_8cpp-example.html):

![image](https://user-images.githubusercontent.com/4759106/147249265-7e2cad75-082c-436e-affe-44a550c480ab.png)

Produced by: `ftxui::Slider()` from "ftxui/component/component.hpp"

## Renderer {#component-renderer}

Produced by: `ftxui::Renderer()` from \ref ftxui/component/component.hpp. This
component decorate another one by using a different function to render an
interface.

Example:
```cpp
auto inner = [...]

auto renderer = Renderer(inner, [&] {
  return inner->Render() | border
});
```

`ftxui::Renderer` also supports the component decorator pattern:
```cpp
auto component = [...]
component = component
  | Renderer([](Element e) { return e | border))
  | Renderer(bold)
```

As a short hand, you can also compose a component with an element decorator:
```cpp
auto component = [...]
component = component | border | bold;
```

## CatchEvent {#component-catchevent}

Produced by: `ftxui::CatchEvent()` from \ref ftxui/component/component.hpp.
This component decorate others, catching events before the underlying component.

Examples:
```cpp
auto screen = ScreenInteractive::TerminalOutput();
auto renderer = Renderer([] {
  return text("My interface");
});
auto component = CatchEvent(renderer, [&](Event event) {
  if (event == Event::Character('q')) {
    screen.ExitLoopClosure()();
    return true;
  }
  return false;
});
screen.Loop(component);
```

The `ftxui::CatchEvent` can also be used as a decorator:
```cpp
component = component
  | CatchEvent(handler_1)
  | CatchEvent(handler_2)
  | CatchEvent(handler_3)
  ;
```

## Collapsible {#component-collapsible}

Useful for visual elements whose visibility can be toggle on/off by the user.
Essentially, this the combination of the `ftxui::Checkbox()` and
`ftxui::Maybe()` components.

```cpp
auto collabsible = Collapsible("Show more", inner_element);
```

## Maybe {#component-maybe}

Produced by: `ftxui::Maybe()` from \ref ftxui/component/component.hpp.
This component can be utilized to show/hide any other component via a boolean or
a predicate.

Example with a boolean:
```cpp
bool show = true;
auto component = Renderer([]{ return "Hello World!"; });
auto maybe_component = Maybe(component, &show)
```

Example with a predicate:
```cpp
auto component = Renderer([]{ return "Hello World!"; });
auto maybe_component = Maybe(component, [&] { return time > 10; })
```

As usual, `ftxui::Maybe` can also be used as a decorator:
```cpp
component = component
  | Maybe(&a_boolean)
  | Maybe([&] { return time > 10; })
  ;
```

## Container {#component-container}

### Horizontal {#component-horizontal}

Produced by: `ftxui::Container::Horizontal()` from
"ftxui/component/component.hpp". It displays a list of components horizontally
and handle keyboard/mouse navigation.

### Vertical {#component-vertical}

Produced by: `ftxui::Container::Vertical()` from
"ftxui/component/component.hpp". It displays a list of components vertically 
and handles keyboard/mouse navigation.

### Tab {#component-tab}

Produced by: `ftxui::Container::Tab()` from
"ftxui/component/component.hpp". It take a list of component and display only
one of them. This is useful for implementing a tab bar.

[Vertical](https://arthursonzogni.github.io/FTXUI/examples_2component_2tab_vertical_8cpp-example.html):
  
![ezgif com-gif-maker (1)](https://user-images.githubusercontent.com/4759106/147250144-22ff044a-4773-4ff7-a49c-12ba4034acb4.gif)

[Horizontal](https://arthursonzogni.github.io/FTXUI/examples_2component_2tab_horizontal_8cpp-example.html):
  
  ![ezgif com-gif-maker (2)](https://user-images.githubusercontent.com/4759106/147250217-fe447e0f-7a99-4e08-948a-995087d9b40e.gif)


## ResizableSplit  {#component-resizable-split}

It defines a horizontal or vertical separation between two children components.
The position of the split is variable and controllable using the mouse. 
There are four possible splits:
- `ftxui::ResizableSplitLeft()`
- `ftxui::ResizableSplitRight()`
- `ftxui::ResizableSplitTop()`
- `ftxui::ResizableSplitBottom()`
from "ftxui/component/component.hpp"

[Example](https://arthursonzogni.github.io/FTXUI/examples_2component_2resizable_split_8cpp-example.html):

![ezgif com-gif-maker](https://user-images.githubusercontent.com/4759106/147248372-c55512fe-9b96-4b08-a1df-d05cf2cae431.gif)  

@htmlonly
<script id="asciicast-tprMH2EdkUoMb7D2YxgMGgpzx" src="https://asciinema.org/a/tprMH2EdkUoMb7D2YxgMGgpzx.js" async></script>
@endhtmlonly

## Force a frame redraw. {#component-force-redraw}

Typically, `ftxui::ScreenInteractive::Loop()` is responsible for drawing a new
frame whenever a new group of events (e.g keyboard, mouse, window resize, etc.)
has been processed. However, you might want to react to arbitrary events that
are unknown to FTXUI. To accomplish this, you must post events using
`ftxui::ScreenInteractive::PostEvent` (**this is thread safe**) via a thread.
You will have to post the event `ftxui::Event::Custom`.

Example:
```cpp
screen->PostEvent(Event::Custom);
```