package tomo

import "sync"
import "image"
import "git.tebibyte.media/tomo/tomo/data"
import "git.tebibyte.media/tomo/tomo/canvas"

// Backend is any Tomo implementation. Backends handle window creation, layout,
// rendering, and events so that there can be as many platform-specific
// optimizations as possible.
type Backend interface {
	// These methods create new Objects. The backend must reject any Object
	// that was not made by it.
	NewBox          () Box
	NewTextBox      () TextBox
	NewCanvasBox    () CanvasBox
	NewSurfaceBox   () (SurfaceBox, error)
	NewContainerBox () ContainerBox

	// NewWindow creates a normal Window and returns it.
	NewWindow (WindowKind, image.Rectangle) (Window, error)

	// NewTexture creates a new canvs.Texture from an image. The backend
	// must reject any texture that was not made by it.
	NewTexture (image.Image) canvas.TextureCloser

	// NewCanvas creates a new canvas.Canvas with the specified bounds. The
	// backend must reject any canvas that was not made by it.
	NewCanvas (image.Rectangle) canvas.CanvasCloser

	// ColorRGBA returns the RGBA of a color according to the current style,
	// as specified in color.Color.RGBA. It may be rendered invalid if the
	// visual style changes, but the Backend must send a StyleChange event
	// to all managed boxes when this happens.
	ColorRGBA (id Color) (r, g, b, a uint32)

	// IconTexture returns the canvas.Texture of an Icon. It may be closed
	// and therefore rendered invalid if the icon set changes, but the
	// Backend must send an IconSetChange event to all managed boxes when
	// this happens.
	IconTexture (Icon, IconSize) canvas.Texture

	// MimeIconTexture returns the canvas.Texture of an icon corresponding
	// to the specified MIME type. It may be closed and therefore rendered
	// invalid if the icon set changes, but the Backend must send an
	// IconSetChange event to all managed boxes when this happens.
	MimeIconTexture (data.Mime, IconSize) canvas.Texture

	// Run runs the event loop until Stop is called, or the Backend
	// experiences a fatal error.
	Run () error
	
	// Stop must unblock Run. This behavior may only be called from within
	// tomo.Stop.
	Stop ()

	// Do performs a callback function in the event loop thread as soon as
	// possible. This method must be safe to call concurrently.
	Do (func ())
}

var backendLock sync.Mutex
var backend Backend

func assertBackend () {
	if backend == nil { panic("nil backend") }
}

// SetBackend sets the Backend that functions in this package will call upon.
// This function will panic if there is already a Backend running.
func SetBackend (back Backend) {
	backendLock.Lock()
	defer backendLock.Unlock()

	if backend != nil {
		panic("SetBackend called while another backend was running")
	}
	backend = back
}