Spot

Spot is a simple, cross-platform, reactive GUI toolkit for Go using native widgets where available. It is designed to be easy to use and to provide a consistent API across different platforms.

Example

package main

import (
	"fmt"

	"github.com/roblillack/spot"
	"github.com/roblillack/spot/ui"
)

func main() {
	ui.Init()

	spot.MountFn(func(ctx *spot.RenderContext) spot.Component {
		counter, setCounter := spot.UseState[int](ctx, 0)

		buttonTitle := "Click me!"
		if counter > 0 {
			buttonTitle = fmt.Sprintf("Clicked %d times!", counter)
		}

		return &ui.Window{
			Title:  "Hello World!",
			Width:  200,
			Height: 125,
			Children: []spot.Component{
				&ui.Button{
					X: 25, Y: 50, Width: 150, Height: 25,
					Title: buttonTitle,
					OnClick: func() {
						setCounter(counter + 1)
					},
				},
			},
		}
	})

	ui.Run()
}

Features

• Simple: You can add Spot as a simple dependency to your project and start building your UI right away. No need to use additional tools or code generation steps. Just write Go code and get a native GUI application as a self-contained binary.
• Cross-platform: Spot uses native widgets where available and automatically selects the best backend for the platform you are running on at compile time. Currently, two backend implementations are provided: one based on FLTK using go-fltk and one based on Cocoa using (a modified version of) gocoa.
• Reactive: Spot automatically updates the UI when the state of the application changes. You just provide side-effect free rendering functions and manage the state of your application using the UseState hook.
• Broad widget support: Spot provides a wide range of UI controls out of the box, including buttons, labels, text inputs, sliders, dropdowns, and more. See the full list: List of supported UI controls.

FAQs

What does "reactive" mean?

In the context of Spot, reactive means that the UI is automatically updated when the state of the application changes. This is achieved by re-building an immutable component tree upon state changes which can quickly be compared to the previous state in order to determine what UI controls need to be updated. In the web world, this idea is often called a "virtual DOM" and Spot actually started as an experiment to bring this concept to Go by implementing a React-like GUI library for the desktop.

By using a reactive model, the developer does not need to worry about updating the UI manually. Instead, the developer can focus on the application logic and let Spot take care of updating the UI.

What are the "native widgets" that Spot uses?

Currently, Spot uses a Cocoa backend on macOS and a FLTK-based one on all other platforms. Optionally, FLTK can be used on the Mac, too. Better support for Windows is planned for the future.

Can I implement my own hooks?

Yes, just like in React, you can implement your own hooks. Just create a function which takes a *spot.RenderContext as first argument and use this to "hook" into the Spot lifecycle by calling spot.UseState, spot.UseEffect, etc. Convention here is to prefix the function with Use….

How do I write custom components?

There are a few different ways to separate your UI into components in Spot; for some ideas, check out the custom-components example. The main way to write custom components is to create a struct that implements the spot.Component interface. This interface has a single method, Render(ctx *spot.RenderContext) spot.Component, which is called to render the component. Components created like this can be used in the same way as the built-in ones.

Look at the BlinkingButton component in the example to see how this is done.

Can I use Spot with a completely different widget library than the provided one?

Yes, you can. You just need to create some structs that implement the spot.Component interface and which take care of managing the native widgets.

Can I use spot/ui, but with a different backend than Cocoa or FLTK?

Currently, these are the only backends that are supported. But feel free to create a PR if you want to add support for another backend. *hint hint*

What's the difference between spot/ui and spot?

spot is the core package that provides the reactive model and the rendering functionality. It is backend-agnostic and can be used with any set of controls which implement the spot.Control interface.

spot/ui is a package that provides a set of pre-built cross-platform GUI controls that which can be used with spot.

What's the difference between a “component” and a “control”?

In Spot, a component is a logical unit of the application that contains business logic and state. Any component is made out of other componens and can ultimately be rendered down to a single or multiple "controls".

A control is special kind component is mounted to the UI tree and represents a visual element on the screen. Usually a control is backed by a native implementation of the GUI backend, like a button, a label, or a text input.

What do the terms ”make”, “render”, “build”, “mount”, and “update” mean in the context of Spot?

• Make: The process of creating a new component instance. This is done by creating a reference to an instance of a struct that implements the spot.Component interface or by calling spot.Make with a render function.

• Render: The process of applying a component's state to its building blocks and hereby returning another component instance. This is done by calling the Render method on a component instance.

• Build: The process of creating a new UI tree from a component instance. This is done by recursively rendering a component to create a tree of controls. This can be done by calling spot.Build with a component instance or spot.BuildFn with a render function.

• Mount: The process of creating real UI controls from a (virtual) tree of controls. This is done by calling Mount on a tree node or spot.Mount with a component instance or spot.MountFn with a render function.

• Update: The process of updating a tree of (mounted) controls. This is done by calling Update on a tree node.

Features, Spot does not have right now

• Automatic layouting
• Multiple windows
• Modal dialogs
• Resizable windows
• Menu bars
• Custom widgets
• Access to native widgets
• Drag and drop
• Internationalization

List of supported UI controls

Explanation of the status column:
❓ Not implemented / 🚧 Work in progress / ⚠️ Partially implemented / ✅ Done

Name	Description	Native controls used	Status
Button	Simple button to initiate an action	Fl_Button NSButton	✅
Checkbox	Control offering the user a choice between two mutually exclusive options	Fl_Check_Button NSButton (NSButtonTypeSwitch)	✅
ComboBox	A combined dropdown menu with text input	ComboBox NSComboBox	Not started
Dial	Circular status control	Fl_Dial NSProgressIndicator (with NSCircular style)	⚠️
Dropdown	Drop-down menu to select a single item out of multiple options	Fl_Choice NSComboBox	✅
Image	Control to display bitmap images on	Fl_Box custom NSButton	✅
Label	Simple, non-editable text label	Fl_Box NSTextField	✅
ListBox	Scrollable control which allows the user to select a single or multible items from a given list	Fl_Select_Browser/Fl_Multi_Browser NSTableView	✅
ProgressBar	Progress bar control to visualize the progression of a long-running operation	Fl_Progress NSProgressIndicator	✅
Slider	Horizontal slider input control	Fl_Slider NSSlider	✅
Spinner	Number input control with up/down buttons	Fl_Spinner NSTextField+NSStepper	✅
TextField	Control for single-line text input	Fl_Input NSTextField	✅
TextEditor	General-purpose text box to edit multi-line text content	Fl_Text_Editor NSTextView	✅
Window	Control representing a (top-level) window on the screen	Fl_Window NSWindow	✅

Potential future backends to look at

• Native Windows controls: https://github.com/rodrigocfd/windigo