Added SetIcon method and new IconKind type

icon.go

@@ -0,0 +1,18 @@
+//go:build !windows
+// +build !windows
+
+package webview
+
+import (
+	"image"
+	"unsafe"
+)
+
+type icons struct{}
+
+func (*icons) setIcon(window unsafe.Pointer, icon image.Image, kind IconKind) {
+	return nil
+}
+
+func (*icons) free() {
+}

icon_windows.go

@@ -0,0 +1,93 @@
+package webview
+
+/*
+#cgo LDFLAGS: -lgdi32
+
+#include <windows.h>
+
+LONG_PTR hIconToLongPtr(HICON hIcon) {
+	return (LONG_PTR)hIcon;
+}
+*/
+import "C"
+
+import (
+	"image"
+	"sync"
+	"unsafe"
+)
+
+type icons struct {
+	mutex sync.Mutex
+	icons []C.HICON
+}
+
+func (c *icons) setIcon(window unsafe.Pointer, icon image.Image, kind IconKind) {
+	hIcon := prepareIcon(icon)
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+	if c.icons == nil {
+		c.icons = make([]C.HICON, maxIconKind)
+	}
+	previousHIcon := c.icons[kind]
+	c.icons[kind] = hIcon
+	C.SetClassLongPtr(C.HWND(window), prepareIconKind(kind), C.hIconToLongPtr(hIcon))
+	if previousHIcon != nil {
+		C.DestroyIcon(previousHIcon)
+	}
+}
+
+func (c *icons) free() {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+	for i, hIcon := range c.icons {
+		if hIcon != nil {
+			C.DestroyIcon(hIcon)
+			c.icons[i] = nil
+		}
+	}
+}
+
+func prepareIcon(icon image.Image) C.HICON {
+	width, height, mask, color := prepareIconData(icon)
+	iconInfo := C.ICONINFO{}
+	iconInfo.fIcon = C.TRUE
+	iconInfo.xHotspot = 0
+	iconInfo.yHotspot = 0
+	iconInfo.hbmMask = C.CreateBitmap(C.int(width), C.int(height), 1, 8, unsafe.Pointer(&mask[0]))
+	iconInfo.hbmColor = C.CreateBitmap(C.int(width), C.int(height), 1, 32, unsafe.Pointer(&color[0]))
+	hIcon := C.CreateIconIndirect(&iconInfo)
+	C.DeleteObject(C.HGDIOBJ(iconInfo.hbmMask))
+	C.DeleteObject(C.HGDIOBJ(iconInfo.hbmColor))
+	return hIcon
+}
+
+func prepareIconData(img image.Image) (width, height int, mask, color []byte) {
+	bounds := img.Bounds()
+	width, height = bounds.Dx(), bounds.Dy()
+	mask = make([]byte, width*height)
+	color = make([]byte, 4*width*height)
+	for y, maskIndex, colorIndex := 0, 0, 0; y < height; y++ {
+		for x := 0; x < width; x++ {
+			r, g, b, a := img.At(x, y).RGBA()
+			color[colorIndex] = byte(b >> 8)
+			colorIndex++
+			color[colorIndex] = byte(g >> 8)
+			colorIndex++
+			color[colorIndex] = byte(r >> 8)
+			colorIndex++
+			color[colorIndex] = byte(a >> 8)
+			colorIndex++
+			mask[maskIndex] = byte(a >> 8)
+			maskIndex++
+		}
+	}
+	return width, height, mask, color
+}
+
+func prepareIconKind(kind IconKind) C.int {
+	if kind == IconKindSmaller {
+		return C.GCLP_HICONSM
+	}
+	return C.GCLP_HICON
+}

webview.go

@@ -27,6 +27,7 @@ import "C"
 import (
 	"encoding/json"
 	"errors"
+	"image"
 	"reflect"
 	"runtime"
 	"sync"
@@ -38,6 +39,15 @@ func init() {
 	runtime.LockOSThread()
 }
 
+// IconKind is used to specify usage of icon.
+type IconKind int
+
+const (
+	IconKindDefault = IconKind(iota)
+	IconKindSmaller
+	maxIconKind
+)
+
 // Hints are used to configure window sizing and resizing
 type Hint int
 
@@ -78,6 +88,10 @@ type WebView interface {
 	// NSWindow pointer, when using Win32 backend the pointer is HWND pointer.
 	Window() unsafe.Pointer
 
+	// SetIcon updates the icon of the native window. Must be called from the UI
+	// thread.
+	SetIcon(icon image.Image, kind IconKind)
+
 	// SetTitle updates the title of the native window. Must be called from the UI
 	// thread.
 	SetTitle(title string)
@@ -122,6 +136,7 @@ type WebView interface {
 
 type webview struct {
 	w C.webview_t
+	i icons
 }
 
 var (
@@ -156,6 +171,7 @@ func NewWindow(debug bool, window unsafe.Pointer) WebView {
 
 func (w *webview) Destroy() {
 	C.webview_destroy(w.w)
+	w.i.free()
 }
 
 func (w *webview) Run() {
@@ -182,6 +198,10 @@ func (w *webview) SetHtml(html string) {
 	C.webview_set_html(w.w, s)
 }
 
+func (w *webview) SetIcon(icon image.Image, kind IconKind) {
+	w.i.setIcon(w.Window(), icon, kind)
+}
+
 func (w *webview) SetTitle(title string) {
 	s := C.CString(title)
 	defer C.free(unsafe.Pointer(s))