add window focus event to gui module

1 files changed, 70 insertions, 0 deletions

diff --git a/gui/event.go b/gui/event.go
new file mode 100644
index 0000000..533317e
--- /dev/null
+++ b/gui/event.go
@@ -0,0 +1,70 @@
+package gui
+
+import (
+	"fmt"
+	"image"
+)
+
+// Event is something that can happen in an environment.
+//
+// This package defines only one kind of event: Resize. Other packages implementing environments
+// may implement more kinds of events. For example, the win package implements all kinds of
+// events for mouse and keyboard.
+type Event interface {
+	String() string
+}
+
+// Resize is an event that happens when the environment changes the size of its drawing area.
+type Resize struct {
+	image.Rectangle
+}
+
+func (r Resize) String() string {
+	return fmt.Sprintf("resize/%d/%d/%d/%d", r.Min.X, r.Min.Y, r.Max.X, r.Max.Y)
+}
+
+// MakeEventsChan implements a channel of events with an unlimited capacity. It does so
+// by creating a goroutine that queues incoming events. Sending to this channel never blocks
+// and no events get lost.
+//
+// The unlimited capacity channel is very suitable for delivering events because the consumer
+// may be unavailable for some time (doing a heavy computation), but will get to the events
+// later.
+//
+// An unlimited capacity channel has its dangers in general, but is completely fine for
+// the purpose of delivering events. This is because the production of events is fairly
+// infrequent and should never out-run their consumption in the long term.
+func MakeEventsChan() (<-chan Event, chan<- Event) {
+	out, in := make(chan Event), make(chan Event)
+
+	go func() {
+		var queue []Event
+
+		for {
+			x, ok := <-in
+			if !ok {
+				close(out)
+				return
+			}
+			queue = append(queue, x)
+
+			for len(queue) > 0 {
+				select {
+				case out <- queue[0]:
+					queue = queue[1:]
+				case x, ok := <-in:
+					if !ok {
+						for _, x := range queue {
+							out <- x
+						}
+						close(out)
+						return
+					}
+					queue = append(queue, x)
+				}
+			}
+		}
+	}()
+
+	return out, in
+}