A rendering engine library for Go supporting both SDL2 and WebAssembly (HTML Canvas) targets.
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package canvas
import (
"syscall/js"
"git.kirsle.net/apps/doodle/lib/events"
)
// EventClass to categorize JavaScript events.
type EventClass int
// EventClass values.
const (
MouseEvent EventClass = iota
ClickEvent
KeyEvent
ResizeEvent
)
// Event object queues up asynchronous JavaScript events to be processed linearly.
type Event struct {
Name string // mouseup, keydown, etc.
Class EventClass
// Mouse events.
X int
Y int
LeftClick bool
RightClick bool
// Key events.
KeyName string
State bool
Repeat bool
}
// AddEventListeners sets up bindings to collect events from the browser.
func (e *Engine) AddEventListeners() {
// Mouse movement.
e.canvas.Value.Call(
"addEventListener",
"mousemove",
js.FuncOf(func(this js.Value, args []js.Value) interface{} {
var (
x = args[0].Get("pageX").Int()
y = args[0].Get("pageY").Int()
)
e.queue <- Event{
Name: "mousemove",
Class: MouseEvent,
X: x,
Y: y,
}
return nil
}),
)
// Mouse clicks.
for _, ev := range []string{"mouseup", "mousedown"} {
ev := ev
e.canvas.Value.Call(
"addEventListener",
ev,
js.FuncOf(func(this js.Value, args []js.Value) interface{} {
var (
x = args[0].Get("pageX").Int()
y = args[0].Get("pageY").Int()
which = args[0].Get("which").Int()
)
// Is a mouse button pressed down?
checkDown := func(number int) bool {
if which == number {
return ev == "mousedown"
}
return false
}
e.queue <- Event{
Name: ev,
Class: ClickEvent,
X: x,
Y: y,
LeftClick: checkDown(1),
RightClick: checkDown(3),
}
return false
}),
)
}
// Supress context menu.
e.canvas.Value.Call(
"addEventListener",
"contextmenu",
js.FuncOf(func(this js.Value, args []js.Value) interface{} {
args[0].Call("preventDefault")
return false
}),
)
// Keyboard keys
for _, ev := range []string{"keydown", "keyup"} {
ev := ev
js.Global().Get("document").Call(
"addEventListener",
ev,
js.FuncOf(func(this js.Value, args []js.Value) interface{} {
var (
event = args[0]
key = event.Get("key").String()
repeat = event.Get("repeat").Bool()
pressed = ev == "keydown"
)
if key == "F3" {
args[0].Call("preventDefault")
}
e.queue <- Event{
Name: ev,
Class: KeyEvent,
KeyName: key,
Repeat: repeat,
State: pressed,
}
return nil
}),
)
}
}
// PollEvent returns the next event in the queue, or null.
func (e *Engine) PollEvent() *Event {
select {
case event := <-e.queue:
return &event
default:
return nil
}
return nil
}
// Poll for events.
func (e *Engine) Poll() (*events.State, error) {
s := e.events
for event := e.PollEvent(); event != nil; event = e.PollEvent() {
switch event.Class {
case MouseEvent:
s.CursorX.Push(int32(event.X))
s.CursorY.Push(int32(event.Y))
case ClickEvent:
s.CursorX.Push(int32(event.X))
s.CursorY.Push(int32(event.Y))
s.Button1.Push(event.LeftClick)
s.Button2.Push(event.RightClick)
case KeyEvent:
switch event.KeyName {
case "Escape":
if event.Repeat {
continue
}
if event.State {
s.EscapeKey.Push(true)
}
case "Enter":
if event.Repeat {
continue
}
if event.State {
s.EnterKey.Push(true)
}
case "F3":
if event.State {
s.KeyName.Push("F3")
}
case "ArrowUp":
s.Up.Push(event.State)
case "ArrowLeft":
s.Left.Push(event.State)
case "ArrowRight":
s.Right.Push(event.State)
case "ArrowDown":
s.Down.Push(event.State)
case "Shift":
s.ShiftActive.Push(event.State)
continue
case "Alt":
case "Control":
continue
case "Backspace":
if event.State {
s.KeyName.Push(`\b`)
}
default:
if event.State {
s.KeyName.Push(event.KeyName)
} else {
s.KeyName.Push("")
}
}
}
}
return e.events, nil
}