Ecosyste.ms: Awesome

An open API service indexing awesome lists of open source software.

Awesome Lists | Featured Topics | Projects

https://github.com/jmfieldman/Mortar

A compact but full-featured Auto Layout DSL for Swift
https://github.com/jmfieldman/Mortar

Last synced: about 1 month ago
JSON representation

A compact but full-featured Auto Layout DSL for Swift

Awesome Lists containing this project

README 

        
![Mortar](/Development/Art/Banner.png)



![Swift 5.0](https://img.shields.io/badge/Swift_5.0-latest-orange.svg?style=flat)

![Swift 4.2](https://img.shields.io/badge/Swift_4.2-%7E%3E%201.5-orange.svg?style=flat)

![Swift 4.0](https://img.shields.io/badge/Swift_4.0-%7E%3E%201.4-orange.svg?style=flat)



Mortar allows you to create Auto Layout constraints using concise, simple code statements.



Use this:



```swift

view1.m_right |=| view2.m_left - 12.0

```



Instead of:



```swift

addConstraint(NSLayoutConstraint(

    item:        view1,

    attribute:  .right,

    relatedBy:  .equal,

    toItem:      view2,

    attribute:  .left,

    multiplier:  1.0,

    constant:   -12.0

))

```



Other examples:



```swift

/* Set the size of three views at once */

[view1, view2, view3].m_size |=| (100, 200)



/* Pin a 200px high, same-width view at the bottom of a container */

[view.m_sides, view.m_bottom, view.m_height] |=| [container, container, 200]



/* VFL syntax */

view1 |>> viewA || viewB[==44] | 20 | viewC[~~2]

```



#### Updating from a version prior to v1.1? Read this!



> A change in the default Mortar constraint priority took effect in v1.1.  Please read the README_DEFAULTS.md file for more information.



# Why?



Yes, there are many Auto Layout DSLs to choose from.  Mortar was created to fill perceived weaknesses in other offerings:



* Mortar does not use blocks/closures like SnapKit or Cartography. These are distracting and ugly when coding constraints for controllers with many views.



* Mortar is an attempt to get away from chaining methods like SnapKit does.  Chained methods are good for providing semantic meaning to the line of code, but they are hard to parse quickly when you come back to your constraint section later.



* Mortar supports multi-constraint macro properties (like ```m_edges```, ```m_frame```, ```m_size```, etc). SwiftAutoLayout and other operator-based DSLs don't seem to have support for these in a concise form.  Properties are prefixed with m_ to reduce potential for conflict with other View extensions.



* Mortar supports implicit property matching (other frameworks require declared properties on both sides of the statement.)



* Mortar supports implicit tuple processing (you don't need to call out a tuple as a specific element like ```CGRect``` or ```CGSize```).



* Mortar supports multi-view alignment/constraints in a single line.



* Mortar supports a robust compile-time VFL syntax that uses views directly (instead of dictionary lookups).



* Mortar has additional goodies, like the ```|+|``` operator to visually construct view hierarchies rather than using tons of sequential calls to ```addSubview()```.



* Mortar lets you construct a view hierarchy and supply layout constraints in a

single imperative expression.



# Installing



### Swift Package Manager (Preferred)



Add Mortar to your Package.swift dependency list:



```swift

.package(url: "https://github.com/jmfieldman/Mortar.git", from: "2.0.0")

```



### Cocoapods



> Notice: Cocoapods support has been deprecated; the last supported Mortar version is 1.7.0



You can install Mortar by adding it to your [CocoaPods](http://cocoapods.org/) ```Podfile```:



```ruby

pod 'Mortar'

```



If you would like to use the Mortar VFL language:



```ruby

pod 'Mortar/MortarVFL'

```



Or you can use a variety of ways to include the ```Mortar.framework``` file from this project into your own.



# Swift Version Support



Version 2.0.0, using the Swift Package Manager, has been tested with Xcode 14.2 and Swift 5.7.



> This README reflects the updated syntax and constants used in the Swift 3 Mortar release.

> For the Swift 2.x documentation, refer to the ```README_SWIFT2.md``` file.



```ruby

pod 'Mortar', '~> 1.6'  # Swift 5.0

pod 'Mortar', '~> 1.5'  # Swift 4.2

pod 'Mortar', '~> 1.4'  # Swift 4.0

pod 'Mortar', '~> 1.3'  # Swift 3.1

```



#### Disabling MortarCreatable (Cocoapods, Legacy Swift)



The default implementation of Mortar declares a MortarCreatable protocol (create), which in legacy versions of

swift causes problems with classes that do not expose the default init() method.



If you targetting a very old version of Swift, you can use:



```ruby

pod 'Mortar/Core_NoCreatable'

pod 'Mortar/MortarVFL_NoCreatable'

```



# Usage



Mortar does not require closures of any kind.  The Mortar operators (```|=|```, ```|>|``` and ```|<|```) instantiate and return constraints that are activated by default.



Mortar will set ```translatesAutoresizingMaskIntoConstraints``` to ```false``` for every view declared on the left side of an operator.



### Equal, Less or Greater?



There are three mortar operators:



```swift

view1.m_width |=| 40                // Equal

view1.m_width |>| view2.m_width     // Greater than or equal

view1.m_size  |<| (100, 100)        // Less than or equal

```



### Attributes



Mortar supports all of the standard layout attributes:



* ```m_left```

* ```m_right```

* ```m_top```

* ```m_bottom```

* ```m_leading```

* ```m_trailing```

* ```m_width```

* ```m_height```

* ```m_centerX```

* ```m_centerY```

* ```m_baseline```



And iOS/tvOS spceific attributes:



* ```m_firstBaseline```

* ```m_leftMargin```

* ```m_rightMargin```

* ```m_topMargin```

* ```m_bottomMargin```

* ```m_leadingMargin```

* ```m_trailingMargin```

* ```m_centerXWithinMargin```

* ```m_centerYWithinMargin```



It also supports composite attributes:



* ```m_sides -- (left, right)```

* ```m_caps -- (top, bottom)```

* ```m_size -- (width, height)```

* ```m_center -- (centerX, centerY)```

* ```m_cornerTL -- (top, left)```

* ```m_cornerTR -- (top, right)```

* ```m_cornerBL -- (bottom, left)```

* ```m_cornerBR -- (bottom, right)```

* ```m_edges -- (top, left, bottom, right)```

* ```m_frame -- (top, left, width, height)```



### Implicit Attributes



_Mortar will do its best to infer implicit attributes!_



The ```m_edges``` attribute is implied when no attributes are declared on either side:



```swift

view1.m_edges |=| view2.m_edges     // These two lines

view1         |=| view2             // are equivalent.

```



If an attribute is declared on one side, it is implied on the other:



```swift

view1.m_top   |>| view2             // These two lines

view1         |>| view2.m_top       // are equivalent.

```



You are required to put attributes on both sides if they are not the same:



```swift

view1.m_top   |<| view2.m_bottom

```



### Using Layout Guides



On iOS you can access the layout guides of a ```UIViewController```.  An example from inside ```viewDidLoad()``` that puts a view just below the safe top:



```swift

// Super useful when trying to position views inside a navigation/tab controller!

view1.m_top   |<| self.m_safeTop

```



There is also a new ```UIViewController``` property ```m_safeRegion``` to help align views to the safe region of a controller's view.  *To use this property you must have the MortarVFL extension installed.*



```swift

// Center a view inside the safe region of a UIViewController that is a child of

// a navigation controller or tab controller

textField.m_center |=| self.m_safeRegion

```



Using ```m_safeRegion``` will create a "ghost" view as a subview of the controller's root view.  This ghost view is hidden and non-interactive, and used only for positioning.  Its class name is ```_MortarVFLGhostView``` in case you see it inside the view debugger.



### Multipliers and Constants



Auto Layout constraints can have multipliers and constants applied to them.  This is done with normal arithmetic operators.  Attributes must be explicitly declared on the _right_ side When arthmetic operators are used.



```swift

view1.m_size  |=| view2.m_size  * 2         // Multiplier

view1.m_left  |>| view2.m_right + 20        // Constant

view1         |<| view2.m_top   * 1.4 + 20  // Both -- m_top is implied on the left

```



You can also set attributes directly to constants:



```swift

view1.m_width |=| 100                // Single-dimension constants

view1.m_size  |=| 150.0              // Set multiple dimensions to the same constant

view1.m_size  |>| (200, 50)          // Set multiple dimensions to a tuple value

view1.m_frame |<| (0, 0, 50, 100)    // Four-dimension tuples supported

```



Arithmetic can be done using tuples for multi-dimension attributes:



```swift

view1.m_size  |=| view2.m_size + (50, 30)

view1.m_size  |>| view2.m_size * (2, 3) + (10, 10)

```



The special inset operator ```~``` operates on multi-dimension attributes:



```swift

view1         |=| view2.m_edges ~ (20, 20, 20, 20)  // view1 is inset by 20 points on each side

```



### Attributes in Tuples



You are allowed to put attributes inside of tuples:



```swift

view1.m_size  |=| (view2.m_width, 100)

```



### Multiple Simultaneous Constraints



Multiple constraints can be created using arrays:



```swift

view1 |=| [view2.m_top, view3.m_bottom, view4.m_size]



/* Is equivalent to: */

view1 |=| view2.m_top

view1 |=| view3.m_bottom

view1 |=| view4.m_size

```



```swift

[view1, view2, view3].m_size |=| (100, 200)



/* Is equivalent to: */

[view1.m_size, view2.m_size, view3.m_size] |=| (100, 200)



/* Is equivalent to: */

view1.m_size |=| (100, 200)

view2.m_size |=| (100, 200)

view3.m_size |=| (100, 200)

```



This might be a convenient way to align an array of views, for example:



```swift

[view1, view2, view3].m_centerY |=| otherView

[view1, view2, view3].m_height  |=| otherView

```



If you put arrays on both sides of the constraint, it will only constrain elements at the same index.  That is:



```swift

[view1.m_left, view2, view3] |=| [view4.m_right, view5, view6]



/* Is equivalent to: */

view1.m_left |=| view4.m_right

view2        |=| view5

view3        |=| view6

```



You can use this to create complex constraints on one line.  For example, to create a 200-point high view that sits at the bottom of a container view:



```swift

[view.m_sides, view.m_bottom, view.m_height] |=| [container, container, 200]

```



### Priority



You can assign priority to constraints using the ```!``` operator.  Valid priorities are:



* ```.low```, ```.medium```, ```.high```, ```.required```

* Any ```UILayoutPriority``` value



```swift

v0 |=| self.container.m_height

v1 |=| self.container.m_height ! .low

v2 |=| self.container.m_height ! .medium

v3 |=| self.container.m_height ! .high

v4 |=| self.container.m_height ! .required

v5 |=| self.container.m_height ! 300

```



You can also put priorities inside tuples or arrays:



```swift

view1        |=| [view2.m_caps   ! .high, view2.m_sides      ! .low]  // Inside array

view1.m_size |=| (view2.m_height ! .high, view2.m_width + 20 ! .low)  // Inside tuple

```



### Default Priority



> Defaults have changed in Mortar v1.1; See README_DEFAULTS.md if you are updating.



By default, constraints are given priority of ```.required``` which is equal to 1000 (out of 1000) and is the same default used by Apple's constraint methods. Sometimes you

may want large batches of constraints to have a different priority, and it is messy to include something like

```! .medium``` after every constraint.



You can change the global base default value by using ```set```:



```swift

MortarDefault.priority.set(base: .medium)

```



You can use this in the ```AppDelegate``` to change the app-wide default constraint priority.



Because this can only be changed on the main thread, it is safe to call just before your

layout code.  Keep in mind it will affect all future Mortar contraints!  If you are adjusting

the default for a single layout section, it is usually wiser to use the stack mechanism

to change the default priority used in a frame of code:



```swift

MortarDefault.priority.push(.low)



v1 |=| v2 // Given priority .low automatically

...



MortarDefault.priority.pop()

```



You may only call the push/pop methods on the main thread, and Mortar will raise an exception if you do not

properly balance your pushes and pops.



### Change Priority



You can change the priority of a ```MortarConstraint``` or ```MortarGroup``` by calling the ```changePriority``` method.  This takes either a ```MortarLayoutPriority``` enum, or a ```UILayoutPriority``` value:



```swift

let c = view1 |=| view2 ! .low     // Creates 4 low-priority constraints (1 per edge)

c.changePriority(to: .high)        // Sets all 4 constraints to high priority

```



Remember that you can't switch to or from ```Required``` from any other priority level (this is an Auto Layout limitation.)



### Create Deactivated Constraints



You can use the ```~~``` operator as a shorthand for constraint activation and deactivation.  This makes the most sense as part of constraint declarations when you want to create initially-deactivated constraints:



```swift

let constraint = view1 |=| view2 ~~ .deactivated



// Later on, it makes more semantic sense to call .activate():

constraint.activate()



// Even though this is functionally equivalent:

constraint ~~ .activated



// It works with groups too:

let group = [

    view1 |=| view2

    view3 |=| view4

] ~~ .deactivated



```



# Keeping Constraint References



The basic building block is the ```MortarConstraint```, which wraps several ```NSLayoutConstraint``` instances that are relevant to multi-affinity attributes like ```m_frame``` (4) or ```m_size``` (2).



You can capture a ```MortarConstraint``` for later reference:



```swift

let constraint = view1.m_top |=| view2.m_bottom

```



The raw ```NSLayoutConstraint``` elements can be accessed through the ```layoutConstraints``` accessor:



```swift

let mortarConstraint = view1.m_top |=| view2.m_bottom

for rawLayoutConstraint in mortarConstraint.layoutConstraints {

    ...

}

```



You can create an entire group of constraints:



```swift

let group = [

    view1.m_origin |=| view2,

    view1.m_size   |=| (100, 100)

]

```



Mortar includes a convenient typealias to refer to arrays of ```MortarConstraint``` objects:



```swift

public typealias MortarGroup = [MortarConstraint]

```



You can now activate/deactivate constraints:



```swift

let constraint = view1.m_top |=| view2.m_bottom



constraint.activate()

constraint.deactivate()



let group = [

    view1.m_origin |=| view2,

    view1.m_size   |=| (100, 100)

]



group.activate()

group.deactivate()

```



### Replacing Constraints and Groups of Constraints



Constraints and groups have a ```replace``` method that deactives the target and activates the parameter:



```swift

let constraint1 = view1.m_sides |=| view2

let constraint2 = view1.m_width |=| view2 ~~ .deactivated



constraint1.replace(with: constraint2)



let group1 = [

    view1.m_sides |<| view2,

    view1.m_caps  |>| view2,

]



let group2 = [

    view1.m_width |=| view2

] ~~ .deactivated



group1.replace(with: group2)

```



## Compression Resistance and Content Hugging



Mortar provides some shorthand properties to adjust a view's compression resistance and content hugging priorities:



```swift

// Set both horizontal and vertical compression resistance priority simultaneously:

view1.m_compResist = 1



// Set horizontal and vertical compression resistance independently:

view1.m_compResistH = 300

view1.m_compResistV = 800



// Set both horizontal and vertical content hugging priority simultaneously:

view1.m_hugging = 1



// Set horizontal and vertical content hugging independently:

view1.m_huggingH = 300

view1.m_huggingV = 800

```



You can get the horizontal and vertical values independently, but not together:



```swift

// These getters are fine:

let c1 = view1.m_compResistH

let c2 = view1.m_compResistV

let h1 = view1.m_huggingH

let h2 = view1.m_huggingV



// These getters raise exceptions:

let cr = view1.m_compResist

let hg = view1.m_hugging

```



# MortarVFL



Mortar supports a VFL language that is roughly equivalent to Apple's own [Auto Layout VFL langauge](https://developer.apple.com/library/content/documentation/UserExperience/Conceptual/AutolayoutPG/VisualFormatLanguage.html).  The primary advanges are:



* You can use it directly with existing Mortar attribute support

* Views are referenced directly (instead of using dictionaries) for compile-time checking

* Full weight-based support for relative sizing

* More concise: Operator-based instead of function/string-based



MortarVFL is contrained to its own extension because it makes heavy use of custom operators.  These operators may not be compatible with other libraries you are using, so we don't want Mortar core to conflict with those.



```ruby

pod 'Mortar/MortarVFL'

```



## MortarVFL Internal Composition



The heart of a MortarVFL statement is a list of VFL nodes that are positioned sequentially along either the horizontal or vertical axis.  A node list might look like:



```swift

viewA | viewB[==viewA] || viewC[==40] | 30 | viewD[~~1] | ~~1 | viewE[~~2]



// viewA has a size determinde by its intrinsic content size

// viewA is separated from viewB by 0 points (| operator)

// viewB has a size equal to viewA

// viewB is separated from viewC by the default padding (8 points; || operator)

// viewC has a fixed size of 40

// viewC is separated from viewD by a space of 30 points

// viewD has a weighted size of 1

// viewD is separated fom viewE by a weighted space of 1

// viewE has a weighted size of 2

```



VFL nodes:

* Represent either whitespace, one view, or multiple views

* Have either fixed spacing or weighted spacing



Nodes are separated by either a ```|``` or ```||``` operator.



The ```|``` operator introduces zero extra distance between nodes.  You can use this operator to connect nodes directly with zero spacing, or insert your own fixed/weighted numerical value between them (e.g. ```| 30 |``` or ```| ~~2 |```).  In these cases, the ```30``` and ```~~2``` are considered nodes that represent whitespace (no attached view).



The ```||``` operator separates nodes by the default padding (8 points).



Nodes that represent views respect their intrinsic content as much as possible given the realvent constraints and priorities. View nodes can also constain a subscript that gives them a size constraint.  You can use ```[==#]``` to give the view a fixed size, or ```[~~#]``` to give the view a weighted size.  You can also reference other views, e.g. ```[==viewA]``` to give the node's view the same constraint as the one it references.



MortarVFL will throw an error if you have cyclic view references, e.g. ```viewA[==viewB] | viewB[==viewA]```



### Arrays in a Node



As an advanced technique, you can use an array of views in a node.  It would look something like this:



```swift

viewA || [viewB, viewC, viewD][==40] || viewE

```



This positions the arrayed nodes in parallel with each other.  In the above example, all three of viewB, viewC and viewD will be sized 40 points and be adjacent to viewA and viewE.  This is very useful for complex grid-based layouts.



## Capture



MortarVFL statements must be captured on at least one end by a view attribute.  These captures look something like:



```swift

// viewB and viewC will take equal width between the

// right edge of viewA and the left edge of viewD

viewA.m_right |> viewB[~~1] | viewC[~~1] <| viewD.m_left



// viewB and viewC will be equal width between the

// left/right edges of viewA, inset by 8pt padding

// and separated by 40pts.

viewA ||>> viewB[~~1] | 40 | viewC[~~1]

```



MortarVFL support horizontal and vertical spacing in a similar manner.  The horizontal operators use the ```>``` character while the vertical operators use the ```^``` character.  Otherwise they act similarly.  For example, the vertical version of the above statement would be:



```swift

// viewB and viewC will take equal height between the

// bottom edge of viewA and the top edge of viewD

viewA.m_bottom |^ viewB[~~1] | viewC[~~1] ^| viewD.m_top

```



Mortar will make sure your operators are compatible with the attributes you've selected.  For example, using ```|>``` with ```m_top``` would be an axis mismatch and raise an exception.



### Implicit Capture Attributes



If you don't provide attributes on the capture terminals, Mortar will derive them based on the axis and position:



```swift

// These are equivalent:

viewA.m_left |> viewB | viewC <| viewD.m_right

viewA        |> viewB | viewC <| viewD



// These are equivalent:

viewA.m_top  |^ viewB | viewC ^| viewD.m_bottom

viewA        |^ viewB | viewC ^| viewD

```



***Important Observation:*** Implicit attributes might be opposite of what you expect.  This is because implicit attributes are normally used to capture views inside the bounds of parent views and so we use the outer edges, not the inner edges.



### Implicit Surround



If you want the MortarVFL nodes to be inside the bounds of a single view, you can use the surround operators instead of placing the same view at both terminals.



The surround operators use either ```>>``` or ```^^```:



```swift

// viewB and viewc will be equal width between the

// left/right edges of viewA, inset by 8pt padding

// and separated by 40pts.

viewA ||>> viewB[~~1] | 40 | viewC[~~1]



// viewB will be twice as tall as viewC; both will be between

// the top/bottom edges of viewA.

viewA |^^ viewB[~~2] | viewC[~~1]



// Using m_visibleRegion is helpful for layouts in child view controllers

// to get views laid out inside the visible region, not under nav/tab bars

self.m_visibleRegion ||^^ viewA | viewB | viewC

```



### Single-Ended Statements



Up until now, all of the examples have shown statements bordered by two attributes (left and right, top and bottom).



For statements surrounded on both sides, you ***cannot have all*** fixed spacing.  This means you will need at least one weighted or intrinsically sized node.  This allows Mortar to make your constraints flexible between the terminals.  You may see odd behavior if you only have intrinsically sized nodes, and their compression resistance and content hugging are artificially forced to .required.



For statements that have a single terminal, the opposite is true.  ***You cannot use any*** weight-based nodes, and they must all be fixed size or intrinsic content size.  This is because there is no second endpoing to use as an anchor for relative sizing.



Single-terminal statements look the same as the others, but trailing operators use a bang: ```!```  Unfortunately this looks very much like the pipe operator, so don't be confused.  Specifically, when just attaching a statement to a trailing attribute, use ``` viewB[==44] || viewC[==88]



// viewC will be placed at the left edge of viewA and be 88pts wide.

// viewB will be placed 8pts (padding) left of viewC and will be 44pts wide.

viewB[==44] || viewC[==88] ```



## Examples



There are several examples of MortarVFL in the Examples/MortarVFL project.



# Visual View Hierarchy Creation



Mortar provides the ```|+|``` and ```|^|``` operators to quickly add a subview or array of subviews.  This can be used to create visual expressions of the view hierarchy.



Now this:



```swift

self.view.addSubview(backgroundView)

self.view.addSubview(myCoolPanel)

myCoolPanel.addSubview(nameLabel)

myCoolPanel.addSubview(nameField)

```



Turns into:



```swift

    self.view |+| [

        backgroundView,

        myCoolPanel |+| [

            nameLabel,

            nameField

        ]

    ]

```



Alternatively, if you want to see the upper subviews at the beginning of the array (so that visually, the views closer to the top of the file are closer to the user), use the ```|^|``` operator:



```swift

    self.view |^| [

        myCoolPanel |^| [      // myCoolPanel is added second and

            nameField,         // is therefore on top of backgroundView

            nameLabel

        ],

        backgroundView

    ]

```



### Initializing NSObject at Creation



Mortar extends `NSObject` with the `create` class function.  This class function performs a parameter-less

instantiation of the class, and passes the new instance into the provided closure.  This allows you to configure

an instance at creation-time, which is really nice for compartmentalizing view configuration.



As you can see in the below example, configuration of the view is separated from the code needed to attach it

to the view controller hierarchy and layout.



```swift

class MyController: UIViewController {



    // Instantiation/configuration

    let myLabel = UILabel.create {

        $0.text          = "Some Text"

        $0.textAlignment = .center

        $0.textColor     = .red

    }



    // UIViews can use the immediate init block

    let myButton = UIButton {

        $0.setTitle("Hello", for: .normal)

    }



    override func viewDidLoad() {

        super.viewDidLoad()



        // Hierarchy

        self.view |+| [

            myLabel,

            myButton

        ]



        // Layout

        myLabel.m_top     |=| self.view

        myLabel.m_centerX |=| self.view

        myButton.m_width  |=| myLabel

    }

}

```



### Combining Hierarchy and Layout



One of the historical problems with combining hierarchy and layout into a

single function (like SwiftUI) is that UIKit requires two views to be in

the same view hierarchy *before* a constrait is activated.



This prevented you from doing something like:



```swift

view1 |+| [

    UILabel.create {

        // Crash here, because the newly created UILabel is not

        // a subview of view1 until *after* the top-level |+| is

        // executed.

        $0.m_width |=| view1

    }

]

```



As of version 2.0.0, Mortar understands how to defer constraint activation

while the hierarchy is being created:



```swift

view1 |+| [

    UILabel.create {

        // This is OK in Mortar 2.0.0; the constraint will not

        // activate until after the outer-most |+| executes.

        $0.m_width |=| view1

    }

]

```



That is, any time the `|+|` or `|^|` operators are in progress, or you use the

new `.addSubviews` or `.addArrangedSubviews` result builders, Mortar knows *not*

to activate any constraint assignments until after the outer-most hierarchy

assignment is completed.



Mortar 2.0.0 now also offers result builders as the right hand side of the

add subview operators. The result builders take the left-side view in as a

block parameter, which lets children easily reference anonymous parents:



```swift

view |+| { view in

    UIStackView {

        $0.spacing = 1

        $0.axis = .vertical

        $0.m_width |=| view

    } |+| { stack in

        UILabel {

            $0.text = viewModel.helloText

            $0.m_height |=| stack

        }

        UILabel {

            $0.text = "World"

            $0.m_height |=| stack

            $0.reactive.text <~ viewModel.worldText

        }

        UIButton {

            $0.reactive.pressed = viewModel.pressed

        }

    }

]

```



Combining this with a reactive framework like ReactiveSwift can get you

nearly all the way to a single-expression view definition, even for large

custom layouts that can leverage stack views properly.