ui: move macro recording from Ops to MacroOp

Move the Record and Stop methods from Ops to MacroOp itself.

Before this change, Ops.Stop stopped the recording of the most
recent macro, which could be a different macro than intended.
After this change, there is no such confusion.

As a bonus, the Ops API becomes less cluttered.

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

ui/ops.go

@@ -2,16 +2,12 @@ package ui
 
 import (
 	"encoding/binary"
-	"errors"
-	"fmt"
 
 	"gioui.org/ui/internal/ops"
 )
 
 // Ops holds a list of serialized Ops.
 type Ops struct {
-	// Stack of macro start indices.
-	stack   []pc
 	version int
 	// Serialized ops.
 	data []byte
@@ -61,9 +57,10 @@ type PushOp struct{}
 type PopOp struct{}
 
 type MacroOp struct {
-	ops     *Ops
-	version int
-	pc      pc
+	recording bool
+	ops       *Ops
+	version   int
+	pc        pc
 }
 
 type opMacroDef struct {
@@ -82,14 +79,6 @@ func (p PopOp) Add(o *Ops) {
 	o.Write([]byte{byte(ops.TypePop)})
 }
 
-// Record starts recording a macro. Multiple simultaneous
-// recordings are supported. Stop ends the most recent.
-func (o *Ops) Record() {
-	o.stack = append(o.stack, o.pc())
-	// Make room for a macro definition. Filled out in Stop.
-	o.Write(make([]byte, ops.TypeMacroDefLen))
-}
-
 func (op *opAux) decode(data []byte) {
 	if ops.OpType(data[0]) != ops.TypeAux {
 		panic("invalid op")
@@ -115,28 +104,9 @@ func (op *opMacroDef) decode(data []byte) {
 	}
 }
 
-// Stop the most recent recording and return the macro for later
-// use.
-func (o *Ops) Stop() MacroOp {
-	if len(o.stack) == 0 {
-		panic(errors.New("not recording a macro"))
-	}
-	start := o.stack[len(o.stack)-1]
-	o.stack = o.stack[:len(o.stack)-1]
-	pc := o.pc()
-	// Write the macro header reserved in Begin.
-	data := o.data[start.data : start.data+ops.TypeMacroDefLen]
-	data[0] = byte(ops.TypeMacroDef)
-	bo := binary.LittleEndian
-	bo.PutUint32(data[1:], uint32(pc.data))
-	bo.PutUint32(data[5:], uint32(pc.refs))
-	return MacroOp{ops: o, pc: start, version: o.version}
-}
-
 // Reset the Ops, preparing it for re-use.
 func (o *Ops) Reset() {
 	o.inAux = false
-	o.stack = o.stack[:0]
 	// Leave references to the GC.
 	for i := range o.refs {
 		o.refs[i] = nil
@@ -191,7 +161,35 @@ func (d *Ops) pc() pc {
 	return pc{data: len(d.data), refs: len(d.refs)}
 }
 
-func (b *MacroOp) decode(data []byte, refs []interface{}) {
+// Record a macro of operations.
+func (m *MacroOp) Record(o *Ops) {
+	if m.recording {
+		panic("already recording")
+	}
+	m.recording = true
+	m.ops = o
+	m.pc = o.pc()
+	// Make room for a macro definition. Filled out in Stop.
+	m.ops.Write(make([]byte, ops.TypeMacroDefLen))
+}
+
+// Stop recording the macro.
+func (m *MacroOp) Stop() {
+	if !m.recording {
+		panic("not recording")
+	}
+	m.recording = false
+	pc := m.ops.pc()
+	// Fill out the macro definition reserved in Record.
+	data := m.ops.data[m.pc.data : m.pc.data+ops.TypeMacroDefLen]
+	data[0] = byte(ops.TypeMacroDef)
+	bo := binary.LittleEndian
+	bo.PutUint32(data[1:], uint32(pc.data))
+	bo.PutUint32(data[5:], uint32(pc.refs))
+	m.version = m.ops.version
+}
+
+func (m *MacroOp) decode(data []byte, refs []interface{}) {
 	if ops.OpType(data[0]) != ops.TypeMacro {
 		panic("invalid op")
 	}
@@ -199,7 +197,7 @@ func (b *MacroOp) decode(data []byte, refs []interface{}) {
 	dataIdx := int(bo.Uint32(data[1:]))
 	refsIdx := int(bo.Uint32(data[5:]))
 	version := int(bo.Uint32(data[9:]))
-	*b = MacroOp{
+	*m = MacroOp{
 		ops: refs[0].(*Ops),
 		pc: pc{
 			data: dataIdx,
@@ -209,17 +207,20 @@ func (b *MacroOp) decode(data []byte, refs []interface{}) {
 	}
 }
 
-func (b MacroOp) Add(o *Ops) {
-	if b.ops == nil {
+func (m MacroOp) Add(o *Ops) {
+	if m.recording {
+		panic("a recording is in progress")
+	}
+	if m.ops == nil {
 		return
 	}
 	data := make([]byte, ops.TypeMacroLen)
 	data[0] = byte(ops.TypeMacro)
 	bo := binary.LittleEndian
-	bo.PutUint32(data[1:], uint32(b.pc.data))
-	bo.PutUint32(data[5:], uint32(b.pc.refs))
-	bo.PutUint32(data[9:], uint32(b.version))
-	o.Write(data, b.ops)
+	bo.PutUint32(data[1:], uint32(m.pc.data))
+	bo.PutUint32(data[5:], uint32(m.pc.refs))
+	bo.PutUint32(data[9:], uint32(m.version))
+	o.Write(data, m.ops)
 }
 
 // Reset start reading from the op list.
@@ -230,9 +231,6 @@ func (r *OpsReader) Reset(ops *Ops) {
 	if ops == nil {
 		return
 	}
-	if n := len(ops.stack); n > 0 {
-		panic(fmt.Errorf("%d Begin(s) not matched with End", n))
-	}
 	r.ops = ops
 }