op

Overview ¶

Package op implements operations for updating a user interface.

Gio programs use operations, or ops, for describing their user interfaces. There are operations for drawing, defining input handlers, changing window properties as well as operations for controlling the execution of other operations.

Ops represents a list of operations. The most important use for an Ops list is to describe a complete user interface update to a ui/app.Window's Update method.

Drawing a colored square:

import "gioui.org/unit"
import "gioui.org/app"
import "gioui.org/op/paint"

var w app.Window
var e system.FrameEvent
ops := new(op.Ops)
...
ops.Reset()
paint.ColorOp{Color: ...}.Add(ops)
paint.PaintOp{Rect: ...}.Add(ops)
e.Frame(ops)

State ¶

An Ops list can be viewed as a very simple virtual machine: it has state such as transformation and color and execution flow can be controlled with macros.

Some state, such as the current color, is modified directly by operations with Add methods. Other state, such as transformation and clip shape, are represented by stacks.

This example sets the simple color state and pushes an offset to the transformation stack.

ops := new(op.Ops)
// Set the color.
paint.ColorOp{...}.Add(ops)
// Apply an offset to subsequent operations.
stack := op.Offset(...).Push(ops)
...
// Undo the offset transformation.
stack.Pop()

The MacroOp records a list of operations to be executed later:

ops := new(op.Ops)
macro := op.Record(ops)
// Record operations by adding them.
...
// End recording.
call := macro.Stop()

// replay the recorded operations:
call.Add(ops)