1 files changed, 134 insertions, 0 deletions
diff --git a/gui/mux.go b/gui/mux.go
new file mode 100644
index 0000000..4198d61
--- /dev/null
+++ b/gui/mux.go
@@ -0,0 +1,134 @@
+package gui
+
+import (
+	"image"
+	"image/draw"
+	"sync"
+)
+
+// Mux can be used to multiplex an Env, let's call it a root Env. Mux implements a way to
+// create multiple virtual Envs that all interact with the root Env. They receive the same
+// events and their draw functions get redirected to the root Env.
+type Mux struct {
+	mu         sync.Mutex
+	lastResize Event
+	eventsIns  []chan<- Event
+	draw       chan<- func(draw.Image) image.Rectangle
+}
+
+// NewMux creates a new Mux that multiplexes the given Env. It returns the Mux along with
+// a master Env. The master Env is just like any other Env created by the Mux, except that
+// closing the Draw() channel on the master Env closes the whole Mux and all other Envs
+// created by the Mux.
+func NewMux(env Env) (mux *Mux, master Env) {
+	drawChan := make(chan func(draw.Image) image.Rectangle)
+	mux = &Mux{draw: drawChan}
+	master = mux.makeEnv(true)
+
+	go func() {
+		for d := range drawChan {
+			env.Draw() <- d
+		}
+		close(env.Draw())
+	}()
+
+	go func() {
+		for e := range env.Events() {
+			mux.mu.Lock()
+			if resize, ok := e.(Resize); ok {
+				mux.lastResize = resize
+			}
+			for _, eventsIn := range mux.eventsIns {
+				eventsIn <- e
+			}
+			mux.mu.Unlock()
+		}
+		mux.mu.Lock()
+		for _, eventsIn := range mux.eventsIns {
+			close(eventsIn)
+		}
+		mux.mu.Unlock()
+	}()
+
+	return mux, master
+}
+
+// MakeEnv creates a new virtual Env that interacts with the root Env of the Mux. Closing
+// the Draw() channel of the Env will not close the Mux, or any other Env created by the Mux
+// but will delete the Env from the Mux.
+func (mux *Mux) MakeEnv() Env {
+	return mux.makeEnv(false)
+}
+
+type muxEnv struct {
+	events <-chan Event
+	draw   chan<- func(draw.Image) image.Rectangle
+}
+
+func (m *muxEnv) Events() <-chan Event                          { return m.events }
+func (m *muxEnv) Draw() chan<- func(draw.Image) image.Rectangle { return m.draw }
+
+func (mux *Mux) makeEnv(master bool) Env {
+	eventsOut, eventsIn := MakeEventsChan()
+	drawChan := make(chan func(draw.Image) image.Rectangle)
+	env := &muxEnv{eventsOut, drawChan}
+
+	mux.mu.Lock()
+	mux.eventsIns = append(mux.eventsIns, eventsIn)
+	// make sure to always send a resize event to a new Env if we got the size already
+	// that means it missed the resize event by the root Env
+	if mux.lastResize != nil {
+		eventsIn <- mux.lastResize
+	}
+	mux.mu.Unlock()
+
+	go func() {
+		func() {
+			// When the master Env gets its Draw() channel closed, it closes all the Events()
+			// channels of all the children Envs, and it also closes the internal draw channel
+			// of the Mux. Otherwise, closing the Draw() channel of the master Env wouldn't
+			// close the Env the Mux is muxing. However, some child Envs of the Mux may still
+			// send some drawing commmands before they realize that their Events() channel got
+			// closed.
+			//
+			// That is perfectly fine if their drawing commands simply get ignored. This down here
+			// is a little hacky, but (I hope) perfectly fine solution to the problem.
+			//
+			// When the internal draw channel of the Mux gets closed, the line marked with ! will
+			// cause panic. We recover this panic, then we receive, but ignore all furhter draw
+			// commands, correctly draining the Env until it closes itself.
+			defer func() {
+				if recover() != nil {
+					for range drawChan {
+					}
+				}
+			}()
+			for d := range drawChan {
+				mux.draw <- d // !
+			}
+		}()
+		if master {
+			mux.mu.Lock()
+			for _, eventsIn := range mux.eventsIns {
+				close(eventsIn)
+			}
+			mux.eventsIns = nil
+			close(mux.draw)
+			mux.mu.Unlock()
+		} else {
+			mux.mu.Lock()
+			i := -1
+			for i = range mux.eventsIns {
+				if mux.eventsIns[i] == eventsIn {
+					break
+				}
+			}
+			if i != -1 {
+				mux.eventsIns = append(mux.eventsIns[:i], mux.eventsIns[i+1:]...)
+			}
+			mux.mu.Unlock()
+		}
+	}()
+
+	return env
+}