io/pointer: add InputOp.Types to specify which types of events to receive

Signed-off-by: Gordon Klaus <gordon.klaus@gmail.com>

io/pointer/doc.go

@@ -8,6 +8,23 @@ object such as a finger.
 The InputOp operation is used to declare a handler ready for pointer
 events. Use an event.Queue to receive events.
 
+Types
+
+Only events that match a specified list of types are delivered to a handler.
+
+For example, to receive Press, Move, and Release events (but not Enter,
+Leave, or Scroll):
+
+	var ops op.Ops
+	var h *Handler = ...
+
+	pointer.InputOp{
+		Tag:   h,
+		Types: pointer.Press | pointer.Move | pointer.Release,
+	}.Add(ops)
+
+Cancel events are always delivered.
+
 Areas
 
 The area operations are used for specifying the area where
@@ -15,9 +32,6 @@ subsequent InputOp are active.
 
 For example, to set up a rectangular hit area:
 
-	var ops op.Ops
-	var h *Handler = ...
-
 	r := image.Rectangle{...}
 	pointer.Rect(r).Add(ops)
 	pointer.InputOp{Tag: h}.Add(ops)

io/pointer/pointer.go

@@ -56,6 +56,8 @@ type InputOp struct {
 	// Grab, if set, request that the handler get
 	// Grabbed priority.
 	Grab bool
+	// Types is a bitwise-or of event types to receive.
+	Types Type
 }
 
 // PassOp sets the pass-through mode.
@@ -83,7 +85,7 @@ type areaKind uint8
 const (
 	// A Cancel event is generated when the current gesture is
 	// interrupted by other handlers or the system.
-	Cancel Type = iota
+	Cancel Type = (1 << iota) >> 1
 	// Press of a pointer.
 	Press
 	// Release of a pointer.
@@ -157,6 +159,7 @@ func (h InputOp) Add(o *op.Ops) {
 	if h.Grab {
 		data[1] = 1
 	}
+	data[2] = byte(h.Types)
 }
 
 func (op PassOp) Add(o *op.Ops) {