Add UnsetAttr

Fully address #20

attribute.go

@@ -6,11 +6,12 @@ import "golang.org/x/image/font"
 import "git.tebibyte.media/tomo/tomo/canvas"
 
 // AttrSet is a set of attributes wherein only one/zero of each attribute type
-// can exist. It is keyed by the AttrNumber of each attribute and must not be
+// can exist. It is keyed by the AttrKind of each attribute and must not be
 // modified directly.
-type AttrSet map[int] Attr
+type AttrSet map[AttrKind] Attr
 
-// AS builds an AttrSet out of a vararg list of Attr values.
+// AS builds an AttrSet out of a vararg list of Attr values. If multiple Attrs
+// of the same kind are specified, the last one will override the others.
 func AS (attrs ...Attr) AttrSet {
 	set := AttrSet { }
 	set.Add(attrs...)
@@ -20,7 +21,7 @@ func AS (attrs ...Attr) AttrSet {
 // Add adds attributes to the set.
 func (this AttrSet) Add (attrs ...Attr) {
 	for _, attr := range attrs {
-		this[attr.attr()] = attr
+		this[attr.Kind()] = attr
 	}
 }
 
@@ -29,7 +30,7 @@ func (this AttrSet) Add (attrs ...Attr) {
 func (this AttrSet) MergeUnder (other AttrSet) {
 	if other == nil { return }
 	for _, attr := range other {
-		if _, exists := this[attr.attr()]; !exists {
+		if _, exists := this[attr.Kind()]; !exists {
 			this.Add(attr)
 		}
 	}
@@ -49,9 +50,27 @@ type Attr interface {
 	// Equals returns true if both attributes can reasonably be declared
 	// equal.
 	Equals (Attr) bool
-	attr () int
+	Kind   () AttrKind
+	attr   ()
 }
 
+type AttrKind int; const (
+	AttrKindColor AttrKind = iota
+	AttrKindTexture
+	AttrKindTextureMode
+	AttrKindBorder
+	AttrKindMinimumSize
+	AttrKindPadding
+	AttrKindGap
+	AttrKindTextColor
+	AttrKindDotColor
+	AttrKindFace
+	AttrKindWrap
+	AttrKindAlign
+	AttrKindOverflow
+	AttrKindLayout
+)
+
 // AttrColor sets the background color of a box.
 type AttrColor struct { color.Color }
 // AttrTexture sets the texture of a box to a named texture.
@@ -256,24 +275,32 @@ func (this AttrLayout) Equals (other Attr) bool {
 	return false
 }
 
-// AttrNumber returns the number of an attribute. Each attribute type has a
-// unique number. The exact values of these numbers are not part of the API and
-// may change.
-func AttrNumber (attr Attr) int {
-	return attr.attr()
-}
+func (AttrColor)       Kind () AttrKind { return AttrKindColor }
+func (AttrTexture)     Kind () AttrKind { return AttrKindTexture }
+func (AttrTextureMode) Kind () AttrKind { return AttrKindTextureMode }
+func (AttrBorder)      Kind () AttrKind { return AttrKindBorder }
+func (AttrMinimumSize) Kind () AttrKind { return AttrKindMinimumSize }
+func (AttrPadding)     Kind () AttrKind { return AttrKindPadding }
+func (AttrGap)         Kind () AttrKind { return AttrKindGap }
+func (AttrTextColor)   Kind () AttrKind { return AttrKindTextColor }
+func (AttrDotColor)    Kind () AttrKind { return AttrKindDotColor }
+func (AttrFace)        Kind () AttrKind { return AttrKindFace }
+func (AttrWrap)        Kind () AttrKind { return AttrKindWrap }
+func (AttrAlign)       Kind () AttrKind { return AttrKindAlign }
+func (AttrOverflow)    Kind () AttrKind { return AttrKindOverflow }
+func (AttrLayout)      Kind () AttrKind { return AttrKindLayout }
 
-func (AttrColor)       attr () int { return  0 }
-func (AttrTexture)     attr () int { return  1 }
-func (AttrTextureMode) attr () int { return  2 }
-func (AttrBorder)      attr () int { return  3 }
-func (AttrMinimumSize) attr () int { return  4 }
-func (AttrPadding)     attr () int { return  5 }
-func (AttrGap)         attr () int { return  6 }
-func (AttrTextColor)   attr () int { return  7 }
-func (AttrDotColor)    attr () int { return  8 }
-func (AttrFace)        attr () int { return  9 }
-func (AttrWrap)        attr () int { return 10 }
-func (AttrAlign)       attr () int { return 11 }
-func (AttrOverflow)    attr () int { return 12 }
-func (AttrLayout)      attr () int { return 13 }
+func (AttrColor)       attr () { }
+func (AttrTexture)     attr () { }
+func (AttrTextureMode) attr () { }
+func (AttrBorder)      attr () { }
+func (AttrMinimumSize) attr () { }
+func (AttrPadding)     attr () { }
+func (AttrGap)         attr () { }
+func (AttrTextColor)   attr () { }
+func (AttrDotColor)    attr () { }
+func (AttrFace)        attr () { }
+func (AttrWrap)        attr () { }
+func (AttrAlign)       attr () { }
+func (AttrOverflow)    attr () { }
+func (AttrLayout)      attr () { }

object.go

@@ -57,8 +57,12 @@ type Box interface {
 	// SetTag adds or removes a named tag.
 	SetTag (string, bool)
 
-	// SetAttr overrides a style attribute.
-	SetAttr(Attr)
+	// SetAttr sets a style attribute, overriding the currently applied
+	// style.
+	SetAttr (Attr)
+	// UnsetAttr reverts a style attribute to whatever is specified by the
+	// currently applied style.
+	UnsetAttr (AttrKind)
 
 	// SetDNDData sets the data that will be picked up if this Box is
 	// dragged. If this is nil (which is the default), this Box will not be