GitHub - psharanda/FixFlex: Declarative Auto Layout code that is easy to write, read, and modify

FixFlex is a simple yet powerful Auto Layout library built on top of the NSLayoutAnchor API, a swifty and type-safe reimagination of Visual Format Language

Features

Declarative Auto Layout code that is easy to write, read, and modify
Simple API with 2 functions and 4 specifiers, covering 99% of layout use cases
Single-file implementation with a tiny surface area
Compatible with any other Auto Layout code
Basically generates a bunch of activated NSLayoutConstraint and UILayoutGuide
Keeps your view hierarchy flat, no need for extra containers
Lightweight alternative to UIStackView
Super straightforward mental model
Typesafe alternative to VFL
Dynamic Type and Right-To-Left friendly
Automatically sets translatesAutoresizingMaskIntoConstraints to false
Fully documented with 100% test coverage
Supports iOS 12.0+ / macOS 10.13+ / tvOS 12.0+

Usage

Imagine we want to create a layout like this:

Let's scan the layout horizontally and translate it into FixFlex code:

Most of the views and spacings have a fixed width (Fix), while the title and subtitle widths are flexible, designed to occupy the remaining space (Flex):

parent.fx.hstack(Fix(15),
                 Fix(iconView, 44),
                 Fix(15),
                 Flex([titleLabel, subtitleLabel]),
                 Fix(15),
                 Fix(chevron, 20),
                 Fix(15))

Vertically, we have three distinct groups of views. Starting with the icon:

We do a spacing at the top using Fix. The bottom spacing should be at least 15pt, for the case when the labels' height is less than the icon's height:

parent.fx.vstack(Fix(15),
                 Fix(iconView, 44),
                 Flex(min: 15))

Next, we perform a vertical scan of the title and subtitle:

parent.fx.vstack(Fix(15),
                 Flex(titleLabel),
                 Flex(subtitleLabel),
                 Fix(15))

Finally, we scan the chevron vertically:

To center the chevron, we ensure the top spacing is equal to the bottom spacing using Fill:

parent.fx.vstack(Fill(),
                 Fix(chevron, 30),
                 Fill())

That's it! The best part is how easy it is to modify FixFlex layout code, inserting extra padding or views effortlessly, without the need to rewire constraints.

API

hstack/vstack

FixFlex provides two functions for laying out views horizontally (hstack) and vertically (vstack), accessible through the view.fx.* namespace.

You can specify startAnchor/endAnchor to layout items between arbitrary anchors instead of the view's edges. startOffset/endOffset are used to add spacing or offsets from the startAnchor and endAnchor respectively. Pass nil to leave a side unpinned (useful for allowing overflow).

By default, hstack works in natural positioning mode and operates using leadingAnchor/trailingAnchor. This setup ensures that the layout is mirrored for Right-to-Left languages. However, this behavior can be overridden by enabling the useAbsolutePositioning flag. When this flag is set to true, hstack shifts to using leftAnchor/rightAnchor for layout positioning.

func hstack(
        startAnchor: NSLayoutXAxisAnchor? = nil, // if nil, we use leadingAnchor or leftAnchor
        startOffset: CGFloat? = 0, // if nil, we do not pin to startAnchor
        endAnchor: NSLayoutXAxisAnchor? = nil, // if nil, we use trailingAnchor or rightAnchor
        endOffset: CGFloat? = 0, // if nil, we do not pin to endAnchor
        useAbsolutePositioning: Bool = false, // if true, we use leftAnchor/rightAnchor based positioning (force Left-To-Right)
        _ intents: SizingIntent...
    ) -> StackingResult

func vstack(
        startAnchor: NSLayoutYAxisAnchor? = nil, // if nil, we use topAnchor
        startOffset: CGFloat? = 0, // if nil, we do not pin to startAnchor
        endAnchor: NSLayoutYAxisAnchor? = nil, // if nil, we use bottomAnchor
        endOffset: CGFloat? = 0, // if nil, we do not pin to endAnchor
        _ intents: SizingIntent...
    ) -> StackingResult

StackingResult gives access to everything created during stacking so you can adjust or deactivate pieces later:

constraints: all constraints activated for the stack
layoutGuides: guides created for spacers/fills

A SizingIntent is essentially an instruction for calculating the width or height of:

a spacer (for which a UILayoutGuide is created behind the scenes)
a view
an array of views (when they are aligned in parallel)

Concrete instances of SizingIntent can be created using specialized builder functions:

Fix

Used for specifying the exact size of a view/spacer.

func Fix(_ value: CGFloat) -> SizingIntent

func Fix(_ view: _View, _ value: CGFloat) -> SizingIntent

func Fix(_ views: [_View], _ value: CGFloat) -> SizingIntent

Flex

Useful for sizes that change dynamically. Optionally, it is possible to specify min/max constraints and in-place priority settings for hugging and compression resistance.

func Flex(min: CGFloat? = nil, max: CGFloat? = nil) -> SizingIntent

func Flex(_ view: _View, min: CGFloat? = nil, max: CGFloat? = nil, huggingPriority: _LayoutPriority? = nil, compressionResistancePriority: _LayoutPriority? = nil) -> SizingIntent

func Flex(_ views: [_View], min: CGFloat? = nil, max: CGFloat? = nil, huggingPriority: _LayoutPriority? = nil, compressionResistancePriority: _LayoutPriority? = nil) -> SizingIntent

Fill

Fill allows a view/spacer to proportionally occupy the available free space based on its weight. It's particularly useful for achieving equal spacing, centering elements, or for designing symmetrical layouts like tables or grids.

func Fill(weight: CGFloat = 1.0) -> SizingIntent

func Fill(_ view: _View, weight: CGFloat = 1.0) -> SizingIntent

func Fill(_ views: [_View], weight: CGFloat = 1.0) -> SizingIntent

Match

This is used to match the size of a view or spacer to a specified NSLayoutDimension. It is particularly useful for aligning the sizes of different views or spacers, or for making their sizes proportional to each other.

public func Match(dimension: NSLayoutDimension, multiplier: CGFloat = 1, offset: CGFloat = 0) -> SizingIntent

public func Match(_ view: _View, dimension: NSLayoutDimension, multiplier: CGFloat = 1, offset: CGFloat = 0) -> SizingIntent

public func Match(_ views: [_View], dimension: NSLayoutDimension, multiplier: CGFloat = 1, offset: CGFloat = 0) -> SizingIntent

SizingIntent Modifiers

You can customize generated pieces via callbacks on SizingIntent:

onCreateStartConstraint(_:) lets you tweak each constraint that pins an item to the previous anchor (or stack start)
onCreateEndConstraint(_:) lets you tweak the constraint that pins the final item to the end anchor
onCreateDimensionConstraint(_:) lets you tweak each size constraint (e.g., priority or identifier)
onCreateLayoutGuide(_:) gives you the implicit UILayoutGuide for configuration

Each SizingIntent can also specify a spacingBefore(_:) to add (or subtract, via negative values) the gap before that item—handy for overlaps or custom per-item spacing.

Behavior notes

hstack defaults to leading/trailing so layouts mirror in RTL; set useAbsolutePositioning: true to force left/right.
startOffset/endOffset = nil leaves the leading/trailing item unpinned and should only be used when other constraints define its position.
Flex on views keeps existing hugging/compression unless you pass overrides; the spacer-only overload applies .required by default.

How it works

FixFlex is not a black box and doesn't use any magic. It is simply a declarative and convenient way to create constraints and layout guides. Let's take a look at how FixFlex is translated into standard Auto Layout calls when we want to center vertically two labels:

parent.fx.hstack(Flex([topLabel, bottomLabel]))

parent.fx.vstack(Fill(),
                 Flex(topLabel),
                 Fix(5),
                 Flex(bottomLabel),
                 Fill())

Under the hood, FixFlex creates constraints and layout guides which are equivalent to the following:

topLabel.translatesAutoresizingMaskIntoConstraints = false
bottomLabel.translatesAutoresizingMaskIntoConstraints = false

let layoutGuideTop = UILayoutGuide()
let layoutGuideMiddle = UILayoutGuide()
let layoutGuideBottom = UILayoutGuide()

parent.addLayoutGuide(layoutGuideTop)
parent.addLayoutGuide(layoutGuideMiddle)
parent.addLayoutGuide(layoutGuideBottom)

NSLayoutConstraint.activate([
    // hstack
    topLabel.leadingAnchor.constraint(equalTo: parent.leadingAnchor),
    topLabel.trailingAnchor.constraint(equalTo: parent.trailingAnchor),
    bottomLabel.leadingAnchor.constraint(equalTo: parent.leadingAnchor),
    bottomLabel.trailingAnchor.constraint(equalTo: parent.trailingAnchor),
    //vstack
    layoutGuideTop.topAnchor.constraint(equalTo: parent.topAnchor),
    layoutGuideTop.bottomAnchor.constraint(equalTo: topLabel.topAnchor),
    topLabel.bottomAnchor.constraint(equalTo: layoutGuideMiddle.topAnchor),
    layoutGuideMiddle.heightAnchor.constraint(equalToConstant: 5),
    layoutGuideMiddle.bottomAnchor.constraint(equalTo: bottomLabel.topAnchor),
    bottomLabel.bottomAnchor.constraint(equalTo: layoutGuideBottom.topAnchor),
    layoutGuideBottom.bottomAnchor.constraint(equalTo: parent.bottomAnchor),
    layoutGuideTop.heightAnchor.constraint(equalTo: layoutGuideBottom.heightAnchor),
])

Huh, that's a lot of code to write, and imagine needing to modify it — inserting an extra view or changing the order. Once you try FixFlex, you won't want to go back!

Examples

Fill Parent With Inset

parent.fx.hstack(
    Fix(15),
    Flex(child),
    Fix(15)
)

parent.fx.vstack(
    Fix(15),
    Flex(child),
    Fix(15)
)

Pin To Parent Trailing Bottom

parent.fx.hstack(
    Flex(),
    Fix(child, 100),
    Fix(15)
)

parent.fx.vstack(
    Flex(),
    Fix(child, 50),
    Fix(15)
)

Center In Parent

parent.fx.hstack(
    Fill(),
    Fix(child, 100),
    Fill()
)

parent.fx.vstack(
    Fill(),
    Fix(child, 50),
    Fill()
)

Center Label In Parent

parent.fx.hstack(
    Fill(),
    Flex(label),
    Fill()
)

parent.fx.vstack(
    Fill(),
    Flex(label),
    Fill()
)

Vertically Center Two Labels

parent.fx.hstack(Flex([topLabel, bottomLabel]))

parent.fx.vstack(
    Fill(),
    Flex(topLabel),
    Fix(5),
    Flex(bottomLabel),
    Fill()
)

Cell With Icon Title Subtitle And Chevron

parent.fx.hstack(
    Fix(15),
    Fix(iconView, 44),
    Fix(15),
    Flex([titleLabel, subtitleLabel]),
    Fix(15),
    Fix(chevron, 20),
    Fix(15)
)

parent.fx.vstack(
    Fix(15),
    Fix(iconView, 44),
    Flex(min: 15)
)

parent.fx.vstack(
    Fix(15),
    Flex(titleLabel),
    Flex(subtitleLabel),
    Fix(15)
)

parent.fx.vstack(
    Fill(),
    Fix(chevron, 30),
    Fill()
)

Card With Icon Title And Subtitle

parent.fx.hstack(
    Fix(5),
    Flex([iconView, titleLabel, subtitleLabel]),
    Fix(5)
)

parent.fx.vstack(
    Fix(5),
    Fix(iconView, 50),
    Fix(10),
    Flex(titleLabel),
    Flex(subtitleLabel),
    Fix(5)
)

Labels Row With Not Enough Space For Both

parent.fx.vstack(
    Flex([leftLabel, rightLabel])
)

parent.fx.hstack(
    Flex(leftLabel, compressionResistancePriority: .required),
    Fix(5),
    Flex(rightLabel)
)

Labels Split

parent.fx.vstack(
    Fix(5),
    Flex([label1, label2, label3]),
    Fix(5)
)

parent.fx.hstack(
    Fix(5),
    Fill(label1, weight: 2),
    Fix(5),
    Fill(label2),
    Fix(5),
    Fill(label3),
    Fix(5)
)

Flex Min Max

parent.fx.vstack(
    Fix(5),
    Flex(label1),
    Flex(label2),
    Flex(label3),
    Fix(5)
)

parent.fx.hstack(
    Fix(5),
    Flex(label1),
    Flex(),
    Fix(5)
)

parent.fx.hstack(
    Fix(5),
    Flex(label2, min: 175),
    Flex(),
    Fix(5)
)

parent.fx.hstack(
    Fix(5),
    Flex(label3, max: 100),
    Flex(),
    Fix(5)
)

Put Between Anchors

parent.fx.vstack(
    Flex([label, leadingView, trailingView])
)

parent.fx.hstack(
    Fill(),
    Flex(label),
    Fill()
)

parent.fx.hstack(
    startAnchor: label.leadingAnchor,
    endAnchor: label.trailingAnchor,
    Fix(leadingView, 20),
    Flex(),
    Fix(trailingView, 20)
)

Put Between Anchors Absolute

parent.fx.vstack(
    Flex([label, leadingView, trailingView])
)

parent.fx.hstack(
    Fill(),
    Flex(label),
    Fill()
)

parent.fx.hstack(
    startAnchor: label.leftAnchor,
    endAnchor: label.rightAnchor,
    useAbsolutePositioning: true,
    Fix(leadingView, 20),
    Flex(),
    Fix(trailingView, 20)
)

Optional Offsets Allow Overflow

container.fx.vstack(
    Fix(10),
    Flex(leadingOverflowLabel),
    Fix(10),
    Flex(trailingOverflowLabel),
    Fix(10)
)

container.fx.hstack(
    startOffset: nil,
    Flex(leadingOverflowLabel)
)

container.fx.hstack(
    endOffset: nil,
    Flex(trailingOverflowLabel)
)

Overlap With Spacing Before

parent.fx.hstack(
    Fill(),
    Fix(first, 60),
    Fix(second, 60).spacingBefore(-20),
    Fix(third, 60).spacingBefore(-20),
    Fill()
)

parent.fx.vstack(
    Fill(),
    Fix([first, second, third], 60),
    Fill()
)

Shadow Using Match

parent.fx.vstack(
    Fill(),
    Flex(label),
    Fill()
)

parent.fx.hstack(
    Fill(),
    Flex(label),
    Fill()
)

parent.fx.vstack(
    startAnchor: label.topAnchor,
    Fix(10),
    Match(matchView, dimension: label.heightAnchor),
    Flex()
)

parent.fx.hstack(
    startAnchor: label.leadingAnchor,
    Fix(10),
    Match(matchView, dimension: label.widthAnchor),
    Flex()
)

Integration

Swift Package Manager

Use Swift Package Manager and add dependency to Package.swift file.

  dependencies: [
    .package(url: "https://github.com/psharanda/FixFlex.git", .upToNextMajor(from: "1.3.0"))
  ]

Alternatively, in Xcode select File > Add Package Dependencies… and add FixFlex repository URL:

https://github.com/psharanda/FixFlex.git

Carthage

Add github "psharanda/FixFlex" to your Cartfile

CocoaPods

FixFlex is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod "FixFlex"

Contributing

We welcome contributions! If you find a bug, have a feature request, or want to contribute code, please open an issue or submit a pull request.

License

FixFlex is available under the MIT license. See the LICENSE file for more info.

Name		Name	Last commit message	Last commit date
Latest commit History 27 Commits
FixFlexSamples		FixFlexSamples
Readme		Readme
Sources		Sources
Tests/FixFlexTests		Tests/FixFlexTests
.gitignore		.gitignore
FixFlex.podspec		FixFlex.podspec
LICENSE		LICENSE
Package.swift		Package.swift
README.md		README.md
generate_readme.sh		generate_readme.sh

License

psharanda/FixFlex

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Features

Usage

API

hstack/vstack

Fix

Flex

Fill

Match

SizingIntent Modifiers

Behavior notes

How it works

Examples

Fill Parent With Inset

Pin To Parent Trailing Bottom

Center In Parent

Center Label In Parent

Vertically Center Two Labels

Cell With Icon Title Subtitle And Chevron

Card With Icon Title And Subtitle

Labels Row With Not Enough Space For Both

Labels Split

Flex Min Max

Put Between Anchors

Put Between Anchors Absolute

Optional Offsets Allow Overflow

Overlap With Spacing Before

Shadow Using Match

Integration

Swift Package Manager

Carthage

CocoaPods

Contributing

License

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