Tutorial for github.com/kjk/flex Go package (implementation of CSS flexbox algorithm)
Aug 4 2017
Package github.com/kjk/flex implements CSS flexbox layout algorithm in Go.
It’s a pure Go port of Facebook’s Yoga C library.

High-level API overview

Despite implementing CSS flexbox spec, it isn’t tied to CSS/HTML in any way. Yoga, for example, can be integrated with iOS app and used to layout UIView hierarchy.
The library works on abstract tree of nodes. In HTML a node would correspond to a block element like a div. When used in Cocoa app, a node could represent UIView or NSView.
When used on windows, it could represent a HWND-based control.
The high-level use is:

An exmple

Let’s assume that we want to re-create the following HTML layout:
<div id="percentage_multiple_nested_with_padding_margin_and_percentage_values" style="width: 200px; height: 200px; flex-direction: column;">
  <div style="flex-grow: 1; flex-basis: 10%; min-width: 60%; margin: 5px; padding: 3px;">
    <div style="width: 50%; margin: 5px; padding: 3%;">
      <div style="width: 45%; margin: 5%; padding: 3px;"></div>
  <div style="flex-grow: 4; flex-basis: 15%; min-width: 20%;"></div>
The equivalent Go code is:
config := flex.NewConfig()

root := flex.NewNodeWithConfig(config)

rootChild0 := flex.NewNodeWithConfig(config)
rootChild0.StyleSetMargin(EdgeLeft, 5)
rootChild0.StyleSetMargin(EdgeTop, 5)
rootChild0.StyleSetMargin(EdgeRight, 5)
rootChild0.StyleSetMargin(EdgeBottom, 5)
rootChild0.StyleSetPadding(EdgeLeft, 3)
rootChild0.StyleSetPadding(EdgeTop, 3)
rootChild0.StyleSetPadding(EdgeRight, 3)
rootChild0.StyleSetPadding(EdgeBottom, 3)
root.InsertChild(rootChild0, 0)

rootChild0Child0 := flex.NewNodeWithConfig(config)
rootChild0Child0.StyleSetMargin(EdgeLeft, 5)
rootChild0Child0.StyleSetMargin(EdgeTop, 5)
rootChild0Child0.StyleSetMargin(EdgeRight, 5)
rootChild0Child0.StyleSetMargin(EdgeBottom, 5)
rootChild0Child0.StyleSetPaddingPercent(EdgeLeft, 3)
rootChild0Child0.StyleSetPaddingPercent(EdgeTop, 3)
rootChild0Child0.StyleSetPaddingPercent(EdgeRight, 3)
rootChild0Child0.StyleSetPaddingPercent(EdgeBottom, 3)
rootChild0.InsertChild(rootChild0Child0, 0)

rootChild0Child0Child0 := flex.NewNodeWithConfig(config)
rootChild0Child0Child0.StyleSetMarginPercent(EdgeLeft, 5)
rootChild0Child0Child0.StyleSetMarginPercent(EdgeTop, 5)
rootChild0Child0Child0.StyleSetMarginPercent(EdgeRight, 5)
rootChild0Child0Child0.StyleSetMarginPercent(EdgeBottom, 5)
rootChild0Child0Child0.StyleSetPadding(EdgeLeft, 3)
rootChild0Child0Child0.StyleSetPadding(EdgeTop, 3)
rootChild0Child0Child0.StyleSetPadding(EdgeRight, 3)
rootChild0Child0Child0.StyleSetPadding(EdgeBottom, 3)
rootChild0Child0.InsertChild(rootChild0Child0Child0, 0)

rootChild1 := flex.NewNodeWithConfig(config)
root.InsertChild(rootChild1, 1)
flex.CalculateLayout(root, flex.Undefined, flex.Undefined, DirectionLTR)
After CalculateLayout we can see the position of each node e.g.:
fmt.Printf("root left: %f\n", root.LayoutGetLeft()) // 0
fmt.Printf"root top: %f\n", root.LayoutGetTop()) // 0
fmt.Printf("root width: %f\n", root.LayoutGetWidth()) // 200
fmt.Printf("root height: %f\n", root.LayoutGetHeight()) // 200
To see example for every flexbox property, look into github.com/facebook/yoga/gentest/fixtures. Their names hint at which properties are being used.
Each file there has corresponding *_test.go file in github.com/kjk/flex directory which shows how to express it in Go.

Size of root’s parent

Notice that in this particular example we used flex.Undefined as both height and width of the parent container.
Imagine you’re using flex to implment layout for a dekstop application where each flex.Node represents a control inside the window.
Window is the parent of root node.
In response to user resizing the window, you want to pass width/height of the window to flex.CalculateLayout().
When you create the window initially, you might do the reverse: pass flex.Undefined as width/height of parent container and then use the size of root node as the size of the window, to size it to its content.

Measure function

Imagine that a node represents an OS button. The button has some intrisic size dictated by its text.
To represent that size flex allows setting a measuring function with node.SetMeasureFunc(measureFunc MeasureFunc). It’s definition is:
type MeasureFunc func(node *Node, width float32, widthMode MeasureMode, height float32, heightMode MeasureMode) Size
The functions takes a hint width/height which is the size of parent container and returns intrinsic size of node.
This is usefule e.g. when a node represents a paragraph of text. When you know width of the parent container, you can break it into multi-line text.
If measuring function needs some state, you can use node.Context to store it.

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