cmd/viewer: add codegen tool for Views

Signed-off-by: Maisem Ali <maisem@tailscale.com>

types/views/views.go

@@ -14,6 +14,78 @@ import (
 	"tailscale.com/net/tsaddr"
 )
 
+func unmarshalJSON[T any](b []byte, x *[]T) error {
+	if *x != nil {
+		return errors.New("already initialized")
+	}
+	if len(b) == 0 {
+		return nil
+	}
+	return json.Unmarshal(b, x)
+}
+
+// StructView represents the corresponding StructView of a Viewable. The concrete types are
+// typically generated by tailscale.com/cmd/viewer.
+type StructView[T any] interface {
+	// Valid reports whether the underlying Viewable is nil.
+	Valid() bool
+	// AsStruct returns a deep-copy of the underlying value.
+	// It returns nil, if Valid() is false.
+	AsStruct() T
+}
+
+// ViewCloner is any type that has had View and Clone funcs generated using
+// tailscale.com/cmd/viewer.
+type ViewCloner[T any, V StructView[T]] interface {
+	// View returns a read-only view of Viewable.
+	// If Viewable is nil, View().Valid() reports false.
+	View() V
+	// Clone returns a deep-clone of Viewable.
+	// It returns nil, when Viewable is nil.
+	Clone() T
+}
+
+// SliceOfViews returns a ViewSlice for x.
+func SliceOfViews[T ViewCloner[T, V], V StructView[T]](x []T) SliceView[T, V] {
+	return SliceView[T, V]{x}
+}
+
+// SliceView is a read-only wrapper around a struct which should only be exposed
+// as a View.
+type SliceView[T ViewCloner[T, V], V StructView[T]] struct {
+	// It is named distinctively to make you think of how dangerous it is to escape
+	// to callers. You must not let callers be able to mutate it.
+	ж []T
+}
+
+// MarshalJSON implements json.Marshaler.
+func (v SliceView[T, V]) MarshalJSON() ([]byte, error) { return json.Marshal(v.ж) }
+
+// UnmarshalJSON implements json.Unmarshaler.
+func (v *SliceView[T, V]) UnmarshalJSON(b []byte) error { return unmarshalJSON(b, &v.ж) }
+
+// IsNil reports whether the underlying slice is nil.
+func (v SliceView[T, V]) IsNil() bool { return v.ж == nil }
+
+// Len returns the length of the slice.
+func (v SliceView[T, V]) Len() int { return len(v.ж) }
+
+// At returns a View of the element at index `i` of the slice.
+func (v SliceView[T, V]) At(i int) V { return v.ж[i].View() }
+
+// AppendTo appends the underlying slice values to dst.
+func (v SliceView[T, V]) AppendTo(dst []V) []V {
+	for _, x := range v.ж {
+		dst = append(dst, x.View())
+	}
+	return dst
+}
+
+// AsSlice returns a copy of underlying slice.
+func (v SliceView[T, V]) AsSlice() []V {
+	return v.AppendTo(nil)
+}
+
 // Slice is a read-only accessor for a slice.
 type Slice[T any] struct {
 	// It is named distinctively to make you think of how dangerous it is to escape
@@ -31,16 +103,7 @@ func (v Slice[T]) MarshalJSON() ([]byte, error) {
 
 // UnmarshalJSON implements json.Unmarshaler.
 func (v *Slice[T]) UnmarshalJSON(b []byte) error {
-	if v.ж != nil {
-		return errors.New("Slice is already initialized")
-	}
-	if len(b) == 0 {
-		return nil
-	}
-	if err := json.Unmarshal(b, &v.ж); err != nil {
-		return err
-	}
-	return nil
+	return unmarshalJSON(b, &v.ж)
 }
 
 // IsNil reports whether the underlying slice is nil.