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package layout

import "gioui.org/layout"

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: unit.Dp(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.Align(...)
	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

Package Files

context.go doc.go flex.go layout.go list.go stack.go

func FPt Uses

func FPt(p image.Point) f32.Point

FPt converts an point to a f32.Point.

func FRect Uses

func FRect(r image.Rectangle) f32.Rectangle

FRect converts a rectangle to a f32.Rectangle.

type Alignment Uses

type Alignment uint8

Alignment is the mutual alignment of a list of widgets.

const (
    Start Alignment = iota
    End
    Middle
    Baseline
)

func (Alignment) String Uses

func (a Alignment) String() string

type Axis Uses

type Axis uint8

Axis is the Horizontal or Vertical direction.

const (
    Horizontal Axis = iota
    Vertical
)

func (Axis) String Uses

func (a Axis) String() string

type Constraints Uses

type Constraints struct {
    Min, Max image.Point
}

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 Uses

func Exact(size image.Point) Constraints

Exact returns the Constraints with the minimum and maximum size set to size.

func (Constraints) Constrain Uses

func (c Constraints) Constrain(size image.Point) image.Point

Constrain a size so each dimension is in the range [min;max].

type Context Uses

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

    *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 Uses

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.

func (Context) Disabled Uses

func (c Context) Disabled() Context

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.

func (Context) Events Uses

func (c Context) Events(k event.Tag) []event.Event

Events returns the events available for the key. If no queue is configured, Events returns nil.

func (Context) Px Uses

func (c Context) Px(v unit.Value) int

Px maps the value to pixels.

type Dimensions Uses

type Dimensions struct {
    Size     image.Point
    Baseline int
}

Dimensions are the resolved size and baseline for a widget.

type Direction Uses

type Direction uint8

Direction is the alignment of widgets relative to a containing space.

const (
    NW  Direction = iota
    N
    NE
    E
    SE
    S
    SW
    W
    Center
)

Code:

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)

Output:

(50,50)
(100,100)

func (Direction) Layout Uses

func (a Direction) Layout(gtx Context, w Widget) Dimensions

Layout a widget according to the direction.

func (Direction) String Uses

func (d Direction) String() string

type Flex Uses

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.

Code:

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)
    }),
)

Output:

Rigid: {(0,100) (100,100)}
50%: {(45,100) (45,100)}

func (Flex) Layout Uses

func (f Flex) Layout(gtx Context, children ...FlexChild) Dimensions

Layout a list of children. The position of the children are determined by the specified order, but Rigid children are laid out before Flexed children.

type FlexChild Uses

type FlexChild struct {
    // contains filtered or unexported fields
}

FlexChild is the descriptor for a Flex child.

func Flexed Uses

func Flexed(weight float32, widget Widget) FlexChild

Flexed returns a Flex child forced to take up w fraction of the of the space left over from Rigid children. The fraction is weight divided by the weight sum of all Flexed children.

func Rigid Uses

func Rigid(widget Widget) FlexChild

Rigid returns a Flex child with a maximal constraint of the remaining space.

type Inset Uses

type Inset struct {
    Top, Right, Bottom, Left unit.Value
}

Inset adds space around a widget.

Code:

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(unit.Dp(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)

Output:

(50,50)
(70,70)

func UniformInset Uses

func UniformInset(v unit.Value) Inset

UniformInset returns an Inset with a single inset applied to all edges.

func (Inset) Layout Uses

func (in Inset) Layout(gtx Context, w Widget) Dimensions

Layout a widget.

type List Uses

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
    // programatically, 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.

Code:

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
count := 0
list.Layout(gtx, listLen, func(gtx layout.Context, i int) layout.Dimensions {
    count++
    return layoutWidget(gtx, 20, 20)
})

fmt.Println(count)

Output:

5

func (*List) Dragging Uses

func (l *List) Dragging() bool

Dragging reports whether the List is being dragged.

func (*List) Layout Uses

func (l *List) Layout(gtx Context, len int, w ListElement) Dimensions

Layout the List.

type ListElement Uses

type ListElement func(gtx Context, index int) Dimensions

ListElement is a function that computes the dimensions of a list element.

type Position Uses

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 top edge to the child at index
    // First.
    Offset int
}

Position is a List scroll offset represented as an offset from the top edge of a child element.

type Spacing Uses

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
)

func (Spacing) String Uses

func (s Spacing) String() string

type Stack Uses

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.

Code:

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)
    }),
)

Output:

Expand: {(50,50) (100,100)}

func (Stack) Layout Uses

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 Uses

type StackChild struct {
    // contains filtered or unexported fields
}

StackChild represents a child for a Stack layout.

func Expanded Uses

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 Uses

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 Uses

type Widget func(gtx Context) Dimensions

Widget is a function scope for drawing, processing events and computing dimensions for a user interface element.

Package layout imports 10 packages (graph) and is imported by 37 packages. Updated 2020-07-05. Refresh now. Tools for package owners.