add layout constraints and abs/rel units

3 files changed, 75 insertions, 0 deletions

diff --git a/constraint.go b/constraint.go
new file mode 100644
index 0000000..c08e222
--- /dev/null
+++ b/constraint.go
@@ -0,0 +1,34 @@
+package gui
+
+// Constraint imposes a restriction on the size of a widget or layout.
+type Constraint struct {
+	// Dim is the dimension to constrain: width/height.
+	Dim
+
+	// Relation declares whether the constraint is an upper, lower, or exact bound.
+	Relation
+
+	// Length is the target or threshold value.
+	Length
+}
+
+// Dim is a dimension of a widget or layout that can be constrained.
+type Dim int
+
+const (
+	_ Dim = iota
+	Width
+	Height
+)
+
+// Relation is an (in)equality.
+type Relation int
+
+const (
+	_    Relation = iota
+	Eq            // ==
+	Gteq          // >=
+	Gt            // >
+	Lteq          // <=
+	Lt            // <
+)
diff --git a/len.go b/len.go
new file mode 100644
index 0000000..895688d
--- /dev/null
+++ b/len.go
@@ -0,0 +1,25 @@
+package gui
+
+// TODO: add font-size-relative units once a text rendering package is added.
+
+// Length allows distance or size to be expressed in absolute or relative units.
+type Length interface {
+	// Px resolves the Length to pixels for the given parent Env's size in pixels.
+	Px(parent int) int
+}
+
+// Px is a Length expressed in pixels.
+type Px int
+
+// Px implements the Length interface.
+func (p Px) Px(parent int) int { return int(p) }
+
+// Relative is a Length relative to the width or height of the parent Env.
+//
+// Relative(0.10) <=> 10%.
+type Relative float64
+
+// Px implements the Length interface.
+func (r Relative) Px(parent int) int {
+	return int(float64(r) * float64(parent))
+}
diff --git a/len_test.go b/len_test.go
new file mode 100644
index 0000000..91b0886
--- /dev/null
+++ b/len_test.go
@@ -0,0 +1,16 @@
+package gui_test
+
+import (
+	"fmt"
+	"image"
+
+	"github.com/faiface/gui"
+)
+
+func ExampleRelative() {
+	var l Length = Relative(0.10)   // 10%
+	r := image.Rect(0, 0, 100, 100) // 100x100 rectangle
+	fmt.Println(l.Px(r))
+	// Output:
+	// 10
+}