ui/app: create windows directly

Replace CreateWindow with NewWindow that immediately creates a Window
ready to use.

Drop the Windows channel of windows created by the system. For iOS
and Android where the system creates the windows, let them rendezvous
with the window created in the first NewWindow call.

Android is further changed so that destroying and re-creating the
Java Activity simply reconnects with the original Window.

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

ui/app/app.go

@@ -3,6 +3,7 @@
 package app
 
 import (
+	"errors"
 	"image"
 	"math"
 	"os"
@@ -47,6 +48,17 @@ type CommandEvent struct {
 type Stage uint8
 type CommandType uint8
 
+type windowRendezvous struct {
+	in   chan windowAndOptions
+	out  chan windowAndOptions
+	errs chan error
+}
+
+type windowAndOptions struct {
+	window *Window
+	opts   *WindowOptions
+}
+
 const (
 	StageDead Stage = iota
 	StagePaused
@@ -79,30 +91,6 @@ const (
 // Set with the go linker flag -X.
 var extraArgs string
 
-var windows = make(chan *Window)
-
-// CreateWindow creates a new window for a set of window
-// options. The options are hints; the platform is free to
-// ignore or adjust them.
-// CreateWindow is not supported on iOS and Android.
-func CreateWindow(opts *WindowOptions) error {
-	if opts == nil {
-		opts = &WindowOptions{
-			Width:  ui.Dp(800),
-			Height: ui.Dp(600),
-			Title:  "Gio program",
-		}
-	}
-	if opts.Width.V <= 0 || opts.Height.V <= 0 {
-		panic("window width and height must be larger than 0")
-	}
-	return createWindow(opts)
-}
-
-func Windows() <-chan *Window {
-	return windows
-}
-
 func (l Stage) String() string {
 	switch l {
 	case StageDead:
@@ -164,3 +152,29 @@ func (c *Config) Px(v ui.Value) int {
 	}
 	return int(math.Round(float64(r)))
 }
+
+func newWindowRendezvous() *windowRendezvous {
+	wr := &windowRendezvous{
+		in:   make(chan windowAndOptions),
+		out:  make(chan windowAndOptions),
+		errs: make(chan error),
+	}
+	go func() {
+		var main windowAndOptions
+		var out chan windowAndOptions
+		for {
+			select {
+			case w := <-wr.in:
+				var err error
+				if main.window != nil {
+					err = errors.New("multiple windows are not supported")
+				}
+				wr.errs <- err
+				main = w
+				out = wr.out
+			case out <- main:
+			}
+		}
+	}()
+	return wr
+}