app/headless: implement headless windows

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/headless/headless.go

@@ -0,0 +1,128 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package headless
+
+import (
+	"fmt"
+	"image"
+	"runtime"
+
+	"gioui.org/app/internal/gl"
+	"gioui.org/app/internal/gpu"
+	"gioui.org/op"
+)
+
+// Window is a headless window.
+type Window struct {
+	size image.Point
+	ctx  context
+	fbo  *gl.SRGBFBO
+	gpu  *gpu.GPU
+}
+
+type context interface {
+	Functions() *gl.Functions
+	MakeCurrent() error
+	ReleaseCurrent()
+	Release()
+}
+
+// NewWindow creates a new headless window.
+func NewWindow(width, height int) (*Window, error) {
+	ctx, err := newContext()
+	if err != nil {
+		return nil, err
+	}
+	w := &Window{
+		size: image.Point{X: width, Y: height},
+		ctx:  ctx,
+	}
+	err = contextDo(ctx, func() error {
+		f := ctx.Functions()
+		fbo, err := gl.NewSRGBFBO(f)
+		if err != nil {
+			ctx.Release()
+			return err
+		}
+		if err := fbo.Refresh(width, height); err != nil {
+			fbo.Release()
+			ctx.Release()
+			return err
+		}
+		gpu, err := gpu.New(f)
+		if err != nil {
+			fbo.Release()
+			ctx.Release()
+			return err
+		}
+		w.fbo = fbo
+		w.gpu = gpu
+		return err
+	})
+	if err != nil {
+		return nil, err
+	}
+	return w, nil
+}
+
+// Release resources associated with the window.
+func (w *Window) Release() {
+	contextDo(w.ctx, func() error {
+		if w.gpu != nil {
+			w.gpu.Release()
+			w.gpu = nil
+		}
+		if w.fbo != nil {
+			w.fbo.Release()
+			w.fbo = nil
+		}
+		if w.ctx != nil {
+			w.ctx.Release()
+			w.ctx = nil
+		}
+		return nil
+	})
+}
+
+// Frame replace the window content and state with the
+// operation list.
+func (w *Window) Frame(frame *op.Ops) {
+	contextDo(w.ctx, func() error {
+		w.gpu.Collect(false, w.size, frame)
+		w.gpu.Frame(false, w.size)
+		w.gpu.EndFrame(false)
+		return nil
+	})
+}
+
+// Screenshot returns an image with the content of the window.
+func (w *Window) Screenshot() (*image.RGBA, error) {
+	img := image.NewRGBA(image.Rectangle{Max: w.size})
+	if len(img.Pix) != w.size.X*w.size.Y*4 {
+		panic("unexpected RGBA size")
+	}
+	contextDo(w.ctx, func() error {
+		f := w.ctx.Functions()
+		f.ReadPixels(0, 0, w.size.X, w.size.Y, gl.RGBA, gl.UNSIGNED_BYTE, img.Pix)
+		if glErr := f.GetError(); glErr != gl.NO_ERROR {
+			return fmt.Errorf("glReadPixels failed: %d", glErr)
+		}
+		return nil
+	})
+	return img, nil
+}
+
+func contextDo(ctx context, f func() error) error {
+	errCh := make(chan error)
+	go func() {
+		runtime.LockOSThread()
+		defer runtime.UnlockOSThread()
+		if err := ctx.MakeCurrent(); err != nil {
+			errCh <- err
+			return
+		}
+		defer ctx.ReleaseCurrent()
+		errCh <- f()
+	}()
+	return <-errCh
+}