Documentation
¶
Overview ¶
Package layout implements layouts common to GUI programs.
Constraints and dimensions ¶
Constraints and dimensions form the interface between layouts and interface child elements. This package operates on Widgets, functions that compute Dimensions from a a set of constraints for acceptable widths and heights. Both the constraints and dimensions are maintained in an implicit Context to keep the Widget declaration short.
For example, to add space above a widget:
var gtx layout.Context
// Configure a top inset.
inset := layout.Inset{Top: 8, ...}
// Use the inset to lay out a widget.
inset.Layout(gtx, func() {
// Lay out widget and determine its size given the constraints
// in gtx.Constraints.
...
return layout.Dimensions{...}
})
Note that the example does not generate any garbage even though the Inset is transient. Layouts that don't accept user input are designed to not escape to the heap during their use.
Layout operations are recursive: a child in a layout operation can itself be another layout. That way, complex user interfaces can be created from a few generic layouts.
This example both aligns and insets a child:
inset := layout.Inset{...}
inset.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
align := layout.Alignment(...)
return align.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
return widget.Layout(gtx, ...)
})
})
More complex layouts such as Stack and Flex lay out multiple children, and stateful layouts such as List accept user input.
Index ¶
Examples ¶
Constants ¶
This section is empty.
Variables ¶
This section is empty.
Functions ¶
Types ¶
type Axis ¶
type Axis uint8
Axis is the Horizontal or Vertical direction.
func (Axis) Convert ¶
Convert a point in (x, y) coordinates to (main, cross) coordinates, or vice versa. Specifically, Convert((x, y)) returns (x, y) unchanged for the horizontal axis, or (y, x) for the vertical axis.
type Constraints ¶
Constraints represent the minimum and maximum size of a widget.
A widget does not have to treat its constraints as "hard". For example, if it's passed a constraint with a minimum size that's smaller than its actual minimum size, it should return its minimum size dimensions instead. Parent widgets should deal appropriately with child widgets that return dimensions that do not fit their constraints (for example, by clipping).
func Exact ¶
func Exact(size image.Point) Constraints
Exact returns the Constraints with the minimum and maximum size set to size.
func (Constraints) AddMin ¶
func (c Constraints) AddMin(delta image.Point) Constraints
AddMin returns a copy of Constraints with the Min constraint enlarged by up to delta while still fitting within the Max constraint. The Max is unchanged, and the Min constraint will not go negative.
func (Constraints) Constrain ¶
func (c Constraints) Constrain(size image.Point) image.Point
Constrain a size so each dimension is in the range [min;max].
func (Constraints) SubMax ¶
func (c Constraints) SubMax(delta image.Point) Constraints
SubMax returns a copy of Constraints with the Max constraint shrunk by up to delta while not going negative. The values of delta are expected to be positive. The Min constraint is adjusted to fit within the new Max constraint.
type Context ¶
type Context struct {
// Constraints track the constraints for the active widget or
// layout.
Constraints Constraints
Metric unit.Metric
// By convention, a nil Queue is a signal to widgets to draw themselves
// in a disabled state.
Queue event.Queue
// Now is the animation time.
Now time.Time
// Locale provides information on the system's language preferences.
// BUG(whereswaldon): this field is not currently populated automatically.
// Interested users must look up and populate these values manually.
Locale system.Locale
*op.Ops
}
Context carries the state needed by almost all layouts and widgets. A zero value Context never returns events, map units to pixels with a scale of 1.0, and returns the zero time from Now.
func NewContext ¶
func NewContext(ops *op.Ops, e system.FrameEvent) Context
NewContext is a shorthand for
Context{
Ops: ops,
Now: e.Now,
Queue: e.Queue,
Config: e.Config,
Constraints: Exact(e.Size),
}
NewContext calls ops.Reset and adjusts ops for e.Insets.
func (Context) Disabled ¶
Disabled returns a copy of this context with a nil Queue, blocking events to widgets using it.
By convention, a nil Queue is a signal to widgets to draw themselves in a disabled state.
type Dimensions ¶
Dimensions are the resolved size and baseline for a widget.
Baseline is the distance from the bottom of a widget to the baseline of any text it contains (or 0). The purpose is to be able to align text that span multiple widgets.
type Direction ¶
type Direction uint8
Direction is the alignment of widgets relative to a containing space.
Example ¶
package main
import (
"fmt"
"image"
"gioui.org/layout"
"gioui.org/op"
)
func main() {
gtx := layout.Context{
Ops: new(op.Ops),
// Rigid constraints with both minimum and maximum set.
Constraints: layout.Exact(image.Point{X: 100, Y: 100}),
}
dims := layout.Center.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
// Lay out a 50x50 sized widget.
dims := layoutWidget(gtx, 50, 50)
fmt.Println(dims.Size)
return dims
})
fmt.Println(dims.Size)
}
func layoutWidget(ctx layout.Context, width, height int) layout.Dimensions {
return layout.Dimensions{
Size: image.Point{
X: width,
Y: height,
},
}
}
Output: (50,50) (100,100)
func (Direction) Layout ¶
func (d Direction) Layout(gtx Context, w Widget) Dimensions
Layout a widget according to the direction. The widget is called with the context constraints minimum cleared.
type Flex ¶
type Flex struct {
// Axis is the main axis, either Horizontal or Vertical.
Axis Axis
// Spacing controls the distribution of space left after
// layout.
Spacing Spacing
// Alignment is the alignment in the cross axis.
Alignment Alignment
// WeightSum is the sum of weights used for the weighted
// size of Flexed children. If WeightSum is zero, the sum
// of all Flexed weights is used.
WeightSum float32
}
Flex lays out child elements along an axis, according to alignment and weights.
Example ¶
package main
import (
"fmt"
"image"
"gioui.org/layout"
"gioui.org/op"
)
func main() {
gtx := layout.Context{
Ops: new(op.Ops),
// Rigid constraints with both minimum and maximum set.
Constraints: layout.Exact(image.Point{X: 100, Y: 100}),
}
layout.Flex{WeightSum: 2}.Layout(gtx,
// Rigid 10x10 widget.
layout.Rigid(func(gtx layout.Context) layout.Dimensions {
fmt.Printf("Rigid: %v\n", gtx.Constraints)
return layoutWidget(gtx, 10, 10)
}),
// Child with 50% space allowance.
layout.Flexed(1, func(gtx layout.Context) layout.Dimensions {
fmt.Printf("50%%: %v\n", gtx.Constraints)
return layoutWidget(gtx, 10, 10)
}),
)
}
func layoutWidget(ctx layout.Context, width, height int) layout.Dimensions {
return layout.Dimensions{
Size: image.Point{
X: width,
Y: height,
},
}
}
Output: Rigid: {(0,100) (100,100)} 50%: {(45,100) (45,100)}
type FlexChild ¶
type FlexChild struct {
// contains filtered or unexported fields
}
FlexChild is the descriptor for a Flex child.
type Inset ¶
Inset adds space around a widget by decreasing its maximum constraints. The minimum constraints will be adjusted to ensure they do not exceed the maximum.
Example ¶
package main
import (
"fmt"
"image"
"gioui.org/layout"
"gioui.org/op"
)
func main() {
gtx := layout.Context{
Ops: new(op.Ops),
// Loose constraints with no minimal size.
Constraints: layout.Constraints{
Max: image.Point{X: 100, Y: 100},
},
}
// Inset all edges by 10.
inset := layout.UniformInset(10)
dims := inset.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
// Lay out a 50x50 sized widget.
dims := layoutWidget(gtx, 50, 50)
fmt.Println(dims.Size)
return dims
})
fmt.Println(dims.Size)
}
func layoutWidget(ctx layout.Context, width, height int) layout.Dimensions {
return layout.Dimensions{
Size: image.Point{
X: width,
Y: height,
},
}
}
Output: (50,50) (70,70)
func UniformInset ¶
UniformInset returns an Inset with a single inset applied to all edges.
type List ¶
type List struct {
Axis Axis
// ScrollToEnd instructs the list to stay scrolled to the far end position
// once reached. A List with ScrollToEnd == true and Position.BeforeEnd ==
// false draws its content with the last item at the bottom of the list
// area.
ScrollToEnd bool
// Alignment is the cross axis alignment of list elements.
Alignment Alignment
// Position is updated during Layout. To save the list scroll position,
// just save Position after Layout finishes. To scroll the list
// programmatically, update Position (e.g. restore it from a saved value)
// before calling Layout.
Position Position
// contains filtered or unexported fields
}
List displays a subsection of a potentially infinitely large underlying list. List accepts user input to scroll the subsection.
Example ¶
package main
import (
"fmt"
"image"
"gioui.org/layout"
"gioui.org/op"
)
func main() {
gtx := layout.Context{
Ops: new(op.Ops),
// Rigid constraints with both minimum and maximum set.
Constraints: layout.Exact(image.Point{X: 100, Y: 100}),
}
// The list is 1e6 elements, but only 5 fit the constraints.
const listLen = 1e6
var list layout.List
list.Layout(gtx, listLen, func(gtx layout.Context, i int) layout.Dimensions {
return layoutWidget(gtx, 20, 20)
})
fmt.Println(list.Position.Count)
}
func layoutWidget(ctx layout.Context, width, height int) layout.Dimensions {
return layout.Dimensions{
Size: image.Point{
X: width,
Y: height,
},
}
}
Output: 5
func (*List) Layout ¶
func (l *List) Layout(gtx Context, len int, w ListElement) Dimensions
Layout a List of len items, where each item is implicitly defined by the callback w. Layout can handle very large lists because it only calls w to fill its viewport and the distance scrolled, if any.
type ListElement ¶
type ListElement func(gtx Context, index int) Dimensions
ListElement is a function that computes the dimensions of a list element.
type Position ¶
type Position struct {
// BeforeEnd tracks whether the List position is before the very end. We
// use "before end" instead of "at end" so that the zero value of a
// Position struct is useful.
//
// When laying out a list, if ScrollToEnd is true and BeforeEnd is false,
// then First and Offset are ignored, and the list is drawn with the last
// item at the bottom. If ScrollToEnd is false then BeforeEnd is ignored.
BeforeEnd bool
// First is the index of the first visible child.
First int
// Offset is the distance in pixels from the leading edge to the child at index
// First.
Offset int
// OffsetLast is the signed distance in pixels from the trailing edge to the
// bottom edge of the child at index First+Count.
OffsetLast int
// Count is the number of visible children.
Count int
// Length is the estimated total size of all children, measured in pixels.
Length int
}
Position is a List scroll offset represented as an offset from the top edge of a child element.
type Spacer ¶
Spacer adds space between widgets.
func (Spacer) Layout ¶
func (s Spacer) Layout(gtx Context) Dimensions
type Spacing ¶
type Spacing uint8
Spacing determine the spacing mode for a Flex.
const ( // SpaceEnd leaves space at the end. SpaceEnd Spacing = iota // SpaceStart leaves space at the start. SpaceStart // SpaceSides shares space between the start and end. SpaceSides // SpaceAround distributes space evenly between children, // with half as much space at the start and end. SpaceAround // SpaceBetween distributes space evenly between children, // leaving no space at the start and end. SpaceBetween // SpaceEvenly distributes space evenly between children and // at the start and end. SpaceEvenly )
type Stack ¶
type Stack struct {
// Alignment is the direction to align children
// smaller than the available space.
Alignment Direction
}
Stack lays out child elements on top of each other, according to an alignment direction.
Example ¶
package main
import (
"fmt"
"image"
"gioui.org/layout"
"gioui.org/op"
)
func main() {
gtx := layout.Context{
Ops: new(op.Ops),
Constraints: layout.Constraints{
Max: image.Point{X: 100, Y: 100},
},
}
layout.Stack{}.Layout(gtx,
// Force widget to the same size as the second.
layout.Expanded(func(gtx layout.Context) layout.Dimensions {
fmt.Printf("Expand: %v\n", gtx.Constraints)
return layoutWidget(gtx, 10, 10)
}),
// Rigid 50x50 widget.
layout.Stacked(func(gtx layout.Context) layout.Dimensions {
return layoutWidget(gtx, 50, 50)
}),
)
}
func layoutWidget(ctx layout.Context, width, height int) layout.Dimensions {
return layout.Dimensions{
Size: image.Point{
X: width,
Y: height,
},
}
}
Output: Expand: {(50,50) (100,100)}
func (Stack) Layout ¶
func (s Stack) Layout(gtx Context, children ...StackChild) Dimensions
Layout a stack of children. The position of the children are determined by the specified order, but Stacked children are laid out before Expanded children.
type StackChild ¶
type StackChild struct {
// contains filtered or unexported fields
}
StackChild represents a child for a Stack layout.
func Expanded ¶
func Expanded(w Widget) StackChild
Expanded returns a Stack child with the minimum constraints set to the largest Stacked child. The maximum constraints are set to the same as passed to Stack.Layout.
func Stacked ¶
func Stacked(w Widget) StackChild
Stacked returns a Stack child that is laid out with no minimum constraints and the maximum constraints passed to Stack.Layout.
type Widget ¶
type Widget func(gtx Context) Dimensions
Widget is a function scope for drawing, processing events and computing dimensions for a user interface element.
Notes ¶
Bugs ¶
this field is not currently populated automatically. Interested users must look up and populate these values manually.
Source Files