ui: make OpPush and OpPop explicit

We're about to allow OpBlock for invoking ops from multiple (cached)
Ops containers. To allow for drawing state changes to stick after
invoking such a cached block, we can't let OpBlock perform an implicit
save and restore of drawing state.

Instead, introduce OpPush and OpPop for explicit drawing state stack
management.

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

ui/ops.go

@@ -24,8 +24,6 @@ type OpsReader struct {
 	pc    pc
 	stack []block
 	ops   opsData
-
-	pseudoOp [1]byte
 }
 
 type block struct {
@@ -73,6 +71,10 @@ var refLengths = [...]int{
 	ops.TypePopRefs,
 }
 
+type OpPush struct{}
+
+type OpPop struct{}
+
 type OpBlock struct {
 	ops *Ops
 	pc  pc
@@ -82,6 +84,14 @@ type opBlockDef struct {
 	endpc pc
 }
 
+func (p OpPush) Add(o *Ops) {
+	o.Write([]byte{byte(ops.TypePush)}, nil)
+}
+
+func (p OpPop) Add(o *Ops) {
+	o.Write([]byte{byte(ops.TypePop)}, nil)
+}
+
 // Begin a block of ops.
 func (o *Ops) Begin() {
 	o.stack = append(o.stack, o.ops.pc())
@@ -183,8 +193,7 @@ func (r *OpsReader) Decode() ([]byte, []interface{}, bool) {
 				r.ops = b.ops
 				r.pc = b.retPC
 				r.stack = r.stack[:len(r.stack)-1]
-				r.pseudoOp[0] = byte(ops.TypePop)
-				return r.pseudoOp[:], nil, true
+				continue
 			}
 		}
 		if r.pc.data == len(r.ops.data) {
@@ -217,8 +226,7 @@ func (r *OpsReader) Decode() ([]byte, []interface{}, bool) {
 			r.pc = op.pc
 			r.pc.data += ops.TypeBlockDefLen
 			r.pc.refs += ops.TypeBlockDefRefs
-			r.pseudoOp[0] = byte(ops.TypePush)
-			return r.pseudoOp[:], nil, true
+			continue
 		case ops.TypeBlockDef:
 			var op opBlockDef
 			op.decode(data)