io: [API] introduce event filters; convert pointer input to use them

Instead of having to supply the predicates for event filtering at the
time of layout, the new Filter type allows widgets to filter at the time
of calling Source.Events. There is then only the need for a single input
op type, in package event.

Filters most importantly allow the use of one tag for several event types,
and we can define that a widget w has &w as its primary tag, by convention.
This allows the replacement of per-widget Focus methods with direct uses
of FocusCmd{&w}, and the later addition of Source.Focused(&w) queries.

Note that the TestCursor test needed restructuring to avoid its use of
InputOps.

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

io/transfer/transfer.go

@@ -2,10 +2,10 @@
 //
 // The transfer protocol is as follows:
 //
-//   - Data sources use [SourceFilter] to receive [InitiateEvent]s when a drag
-//     is initiated, and [RequestEvent]s for each initiation of a data transfer.
-//     Sources respond to requests with [OfferCommand].
-//   - Data targets use [TargetFilter] to receive [DataEvent]s for receiving data.
+//   - Data sources use [SourceFilter] to receive an [InitiateEvent] when a drag
+//     is initiated, and an [RequestEvent] for each initiation of a data transfer.
+//     Sources respond to requests with [OfferCmd].
+//   - Data targets use [TargetFilter] to receive an [DataEvent] for receiving data.
 //     The target must close the data event after use.
 //
 // When a user initiates a pointer-guided drag and drop transfer, the
@@ -20,9 +20,7 @@ package transfer
 import (
 	"io"
 
-	"gioui.org/internal/ops"
 	"gioui.org/io/event"
-	"gioui.org/op"
 )
 
 // OfferCmd is used by data sources as a response to a RequestEvent.
@@ -34,38 +32,27 @@ type OfferCmd struct {
 	// Data contains the offered data. It is closed when the
 	// transfer is complete or cancelled.
 	// Data must be kept valid until closed, and it may be used from
-	// a goroutine separate from the one processing the frame..
+	// a goroutine separate from the one processing the frame.
 	Data io.ReadCloser
 }
 
 func (OfferCmd) ImplementsCommand() {}
 
-// SourceOp registers a tag as a data source for a MIME type.
-// Use multiple SourceOps if a tag supports multiple types.
-type SourceOp struct {
-	Tag event.Tag
+// SourceFilter filters for any [RequestEvent] that match a MIME type
+// as well as [InitiateEvent] and [CancelEvent].
+// Use multiple filters to offer multiple types.
+type SourceFilter struct {
 	// Type is the MIME type supported by this source.
 	Type string
 }
 
-// TargetOp registers a tag as a data target.
-// Use multiple TargetOps if a tag supports multiple types.
-type TargetOp struct {
-	Tag event.Tag
+// TargetFilter filters for any [DataEvent] whose type matches a MIME type
+// as well as [CancelEvent]. Use multiple filters to accept multiple types.
+type TargetFilter struct {
 	// Type is the MIME type accepted by this target.
 	Type string
 }
 
-func (op SourceOp) Add(o *op.Ops) {
-	data := ops.Write2(&o.Internal, ops.TypeSourceLen, op.Tag, op.Type)
-	data[0] = byte(ops.TypeSource)
-}
-
-func (op TargetOp) Add(o *op.Ops) {
-	data := ops.Write2(&o.Internal, ops.TypeTargetLen, op.Tag, op.Type)
-	data[0] = byte(ops.TypeTarget)
-}
-
 // RequestEvent requests data from a data source. The source must
 // respond with an OfferCmd.
 type RequestEvent struct {
@@ -99,3 +86,6 @@ type DataEvent struct {
 }
 
 func (DataEvent) ImplementsEvent() {}
+
+func (SourceFilter) ImplementsFilter() {}
+func (TargetFilter) ImplementsFilter() {}