mirror of https://github.com/SnapKit/SnapKit
282 lines
9.5 KiB
Markdown
282 lines
9.5 KiB
Markdown
# Snappy
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Snappy is a light-weight layout framework which wraps AutoLayout with a nicer syntax. Snappy has its own layout DSL which provides a chainable way of describing your NSLayoutConstraints which results in layout code that is more concise and readable. Snappy supports both iOS and OS X.
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> Snappy uses some Swift only features like function overloading it cannot be used from Objective-C. Because of this we’ve chosen to swap prefixes from Masonry’s `mas_` to `snp_` so you can use both Masonry and Snappy in the same project.
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## Requirements
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## Installation
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_Due to the current lack of [proper infrastructure](http://cocoapods.org) for Swift dependency management, using Snappy in your project requires the following steps:_
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1. Add Snappy as a [submodule](http://git-scm.com/docs/git-submodule) by opening the Terminal, `cd`-ing into your top-level project directory, and entering the command `git submodule add https://github.com/Masonry/Snappy.git`
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2. Open the `Snappy` folder, and drag `Snappy.xcodeproj` into the file navigator of your Xcode project<sup>1</sup>.
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3. In Xcode, navigate to the target configuration window by clicking on the blue project icon, and selecting the application target under the "Targets" heading in the sidebar.
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4. In the tab bar at the top of that window, open the "General" panel.
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5. Click on the `+` button in the "Embedded Binaries" group of the panel and select and add `Snappy.framework`.
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<sup>1</sup><small>It's important you add Snappy as a subproject of your Xcode Project and not of a potential Xcode Workspace containing your project</small>
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## What's wrong with NSLayoutConstraints?
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Under the hood Auto Layout is a powerful and flexible way of organising and laying out your views. However creating constraints from code is verbose and not very descriptive.
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Imagine a simple example in which you want to have a view fill its superview but inset by 10 pixels on every side
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```swift
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let superview = self;
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let view1 = UIView()
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view1.setTranslatesAutoresizingMaskIntoConstraints(false)
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view1.backgroundColor = UIColor.greenColor()
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superview.addSubview(view1)
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let padding = UIEdgeInsetsMake(10, 10, 10, 10)
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superview.addConstraints([
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NSLayoutConstraint(
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item: view1,
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attribute: NSLayoutAttribute.Top,
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relatedBy: NSLayoutRelation.Equal,
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toItem: superview,
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attribute: NSLayoutAttribute.Top,
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multiplier: 1.0,
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constant: padding.top
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),
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NSLayoutConstraint(
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item: view1,
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attribute: NSLayoutAttribute.Left,
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relatedBy: NSLayoutRelation.Equal,
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toItem: superview,
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attribute: NSLayoutAttribute.Left,
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multiplier: 1.0,
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constant: padding.left
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),
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NSLayoutConstraint(
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item: view1,
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attribute: NSLayoutAttribute.Bottom,
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relatedBy: NSLayoutRelation.Equal,
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toItem: superview,
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attribute: NSLayoutAttribute.Bottom,
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multiplier: 1.0,
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constant: -padding.bottom
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),
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NSLayoutConstraint(
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item: view1,
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attribute: NSLayoutAttribute.Right,
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relatedBy: NSLayoutRelation.Equal,
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toItem: superview,
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attribute: NSLayoutAttribute.Right,
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multiplier: 1.0,
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constant: -padding.right
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)
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])
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```
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Even with such a simple example the code needed is quite verbose and quickly becomes unreadable when you have more than 2 or 3 views.
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Another option is to use Visual Format Language (VFL), which is a bit less long winded.
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However the ASCII type syntax has its own pitfalls and its also a bit harder to animate as `NSLayoutConstraint.constraintsWithVisualFormat` returns an array.
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## Prepare to meet your Maker!
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Heres the same constraints created using ConstraintMaker
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```swift
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let padding = UIEdgeInsetsMake(10, 10, 10, 10)
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view1.snp_makeConstraints { make in
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make.top.equalTo(superview.snp_top).with.offset(padding.top) // with is an optional semantic filler
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make.left.equalTo(superview.snp_left).with.offset(padding.left)
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make.bottom.equalTo(superview.snp_bottom).with.offset(-padding.bottom)
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make.right.equalTo(superview.snp_right).with.offset(-padding.right)
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}
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```
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Or even shorter
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```swift
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view1.snp_makeConstraints { make in
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make.edges.equalTo(superview).with.insets(padding)
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return // this return is a fix for implicit returns in Swift and is only required for single line constraints
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}
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```
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Also note in the first example we had to add the constraints to the superview `superview.addConstraints`.
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Snappy however will automagically add constraints to the appropriate view.
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Snappy will also call `view1.setTranslatesAutoresizingMaskIntoConstraints(false)` for you.
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## Not all things are created equal
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> `.equalTo` equivalent to **NSLayoutRelation.Equal**
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> `.lessThanOrEqualTo` equivalent to **NSLayoutRelation.LessThanOrEqual**
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> `.greaterThanOrEqualTo` equivalent to **NSLayoutRelation.GreaterThanOrEqual**
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These three equality constraints accept one argument which can be any of the following:
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#### 1. ViewAttribute
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```swift
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make.centerX.lessThanOrEqualTo(view2.snp_left)
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```
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ViewAttribute | NSLayoutAttribute
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------------------------- | --------------------------
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view.snp_left | NSLayoutAttribute.Left
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view.snp_right | NSLayoutAttribute.Right
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view.snp_top | NSLayoutAttribute.Top
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view.snp_bottom | NSLayoutAttribute.Bottom
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view.snp_leading | NSLayoutAttribute.Leading
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view.snp_trailing | NSLayoutAttribute.Trailing
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view.snp_width | NSLayoutAttribute.Width
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view.snp_height | NSLayoutAttribute.Height
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view.snp_centerX | NSLayoutAttribute.CenterX
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view.snp_centerY | NSLayoutAttribute.CenterY
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view.snp_baseline | NSLayoutAttribute.Baseline
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#### 2. UIView/NSView
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if you want view.left to be greater than or equal to label.left :
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```swift
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// these two constraints are exactly the same
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make.left.greaterThanOrEqualTo(label)
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make.left.greaterThanOrEqualTo(label.snp_left)
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```
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#### 3. Strict Checks
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Auto Layout allows width and height to be set to constant values.
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if you want to set view to have a minimum and maximum width you could pass a primitive to the equality blocks:
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```swift
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// width >= 200 && width <= 400
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make.width.greaterThanOrEqualTo(200)
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make.width.lessThanOrEqualTo(400)
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```
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However Auto Layout does not allow alignment attributes such as left, right, centerY etc to be set to constant values.
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So if you pass a primitive for these attributes Snappy will turn these into constraints relative to the view’s superview ie:
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```swift
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// creates view.left <= view.superview.left + 10
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make.left.lessThanOrEqualTo(10)
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```
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You can also use other primitives and structs to build your constraints, like so:
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```swift
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make.top.equalTo(42)
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make.height.equalTo(20)
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make.size.equalTo(CGSizeMake(50, 100))
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make.edges.equalTo(UIEdgeInsetsMake(10, 0, 10, 0))
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make.left.equalTo(view).offset(UIEdgeInsetsMake(10, 0, 10, 0))
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```
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## Learn to prioritize
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> `.prority` allows you to specify an exact priority
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> `.priorityHigh` equivalent to **UILayoutPriority.DefaultHigh**
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> `.priorityMedium` is half way between high and low
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> `.priorityLow` equivalent to **UILayoutPriority.DefaultLow**
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Priorities are can be tacked on to the end of a constraint chain like so:
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```swift
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make.left.greaterThanOrEqualTo(label.snp_left).with.priorityLow();
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make.top.equalTo(label.snp_top).with.priority(600);
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```
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## Composition, composition, composition
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Snappy also gives you a few convenience methods which create multiple constraints at the same time.
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#### edges
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```swift
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// make top, left, bottom, right equal view2
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make.edges.equalTo(view2);
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// make top = superview.top + 5, left = superview.left + 10,
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// bottom = superview.bottom - 15, right = superview.right - 20
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make.edges.equalTo(superview).insets(UIEdgeInsetsMake(5, 10, 15, 20))
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```
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#### size
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```swift
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// make width and height greater than or equal to titleLabel
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make.size.greaterThanOrEqualTo(titleLabel)
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// make width = superview.width + 100, height = superview.height - 50
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make.size.equalTo(superview).offset(CGSizeMake(100, -50))
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```
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#### center
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```swift
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// make centerX and centerY = button1
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make.center.equalTo(button1)
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// make centerX = superview.centerX - 5, centerY = superview.centerY + 10
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make.center.equalTo(superview).offset(CGPointMake(-5, 10))
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```
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You can chain view attributes for increased readability:
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```swift
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// All edges but the top should equal those of the superview
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make.left.right.and.bottom.equalTo(superview)
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make.top.equalTo(otherView)
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```
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## Hold on for dear life
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Sometimes you need modify existing constraints in order to animate or remove/replace constraints.
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In Snappy there are a few different approaches to updating constraints.
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#### 1. References
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You can hold on to a reference of a particular constraint by assigning the result of a constraint make expression to a local variable or a class property.
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You could also reference multiple constraints by storing them away in an array.
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```swift
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var topConstraint: Constraint? = nil
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...
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// when making constraints
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view1.snp_makeConstraints { make in
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self.topConstraint = make.top.equalTo(superview).with.offset(padding.top)
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make.left.equalTo(superview).with.offset(padding.left)
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}
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...
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// then later you can call
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self.topConstraint.uninstall()
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```
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### 2. snp_remakeConstraints
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`snp_remakeConstraints` is similar to `snp_makeConstraints`, but will first remove all existing constraints installed by Snappy.
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```swift
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func changeButtonPosition() {
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self.button.snp_remakeConstraints { make in
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make.size.equalTo(self.buttonSize)
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if topLeft {
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make.top.left.equalTo(10)
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} else {
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make.bottom.equalTo(self.view).offset(-10)
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make.right.equalTo(self.view).offset(-10)
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}
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}
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}
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```
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## TODO
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* Eye candy
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* Example projects
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* Tests
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