Reusable

A Swift mixin to use UITableViewCells, UICollectionViewCells and UIViewControllers in a type-safe way, without the need to manipulate their String-typed reuseIdentifiers. This library also supports arbitrary UIView to be loaded via a XIB using a simple call to loadFromNib()

Introduction

This library aims to make it super-easy to create, dequeue and instantiate Reusable views anywhere this pattern is used: From the obvious UITableViewCell and UICollectionViewCell to custom UIViews, even supporting UIViewControllers from Storyboards.
All of that symply by marking your classes as conforming to a protocols, without having to add any code, and creating a type-safe API with no more String-based API.

// Example of what Reusable allows you to do
final class MyCustomCell: UITableViewCell, Reusable { /* And that's it! */ }
tableView.registerReusableCell(MyCustomCell)
let cell: MyCustomCell = tableView.dequeueReusableCell(indexPath: indexPath)

This concept, called a Mixin (a protocol with default implementation for all its methods), is explained here in my blog post in details.

Table of Contents

• Type-safe cells
• Type-safe XIB-based reusable views
• Type-safe ViewControllers from Storyboards
• Additional tips
• Example project
• License

Type-safe UITableViewCell / UICollectionViewCell

✍️ Examples and explanations below use UITableView and UITableViewCell, but the exact same examples and explanations apply for UICollectionView and UICollectionViewCell.

1. Declare your cells to conform to Reusable or NibReusable

• Use the Reusable protocol if they don't depend on a NIB (this will use registerClass(…) to register the cell)
• Use the NibReusable protocol if they use a XIB file for their content (this will use registerNib(…) to register the cell)

final class CustomCell: UITableViewCell, Reusable { /* And that's it! */ }

✍️ Notes

• For cells embedded in a Storyboard's tableView, either one of those two protocols will work (as you won't register the cell them manually anyway)
• If you create a XIB-based cell, don't forget to set its Reuse Identifier field in Interface Builder to the same string as the name of the cell class itself.

final class CodeBasedCustomCell: UITableViewCell, Reusable {
  // By default this cell will have a reuseIdentifier of "CodeBasedCustomCell"
  // unless you provide an alternative implementation of `var reuseIdentifier`
  
  // No need to add anything to conform to Reusable. you can just keep your normal cell code
  @IBOutlet private weak var label: UILabel!
  func fillWithText(text: String?) { label.text = text }
}

final class NibBasedCustomCell: UITableViewCell, NibReusable {
  // Here we provide a nib for this cell class (which, if we don't override the protocol's
  // default implementation of `nib`, will use a XIB of the same name as the class)
  
  // No need to add anything to conform to Reusable. you can just keep your normal cell code
  @IBOutlet private weak var pictureView: UIImageView!
  func fillWithImage(image: UIImage?) { pictureView.image = image }
}

// A UICollectionViewCell which doesn't need a XIB to register
// Either because it's all-code, or because it's registered via Storyboard
final class CodeBasedCollectionViewCell: UICollectionViewCell, Reusable {
  // The rest of the cell code goes here
}

// A UICollectionViewCell using a XIB to define it's UI
// And that will need to register using that XIB
final class NibBasedCollectionViewCell: UICollectionViewCell, NibReusable {
  // The rest of the cell code goes here
}

2. Register your cells

Unless you've prototyped your cell in a Storyboard, you'll have to register the cell class or Nib by code.

To do this, instead of calling registerClass(…) or registerNib(…) using a String-based reuseIdentifier, just call:

tableView.registerReusableCell(theCellClass)

class MyViewController: UIViewController {
  @IBOutlet private weak var tableView: UITableView!
  
  override func viewDidLoad() {
    super.viewDidLoad()
    tableView.registerReusableCell(CodeBasedCustomCell) // This will register using the class without using a UINib
    tableView.registerReusableCell(NibBasedCustomCell) // This will register using NibBasedCustomCell.xib
  }
}

3. Dequeue your cells

To dequeue a cell (typically in your cellForRowAtIndexPath implementation), simply call dequeueReusableCell(indexPath:):

// Either
let cell = tableView.dequeueReusableCell(indexPath: indexPath) as MyCustomCell
// Or
let cell: MyCustomCell = tableView.dequeueReusableCell(indexPath: indexPath)

As long as Swift can use type-inference to understand that you'll want a cell of type MyCustomCell (either using as MyCystomCell or explicitly typing the receiving variable cell: MyCustomCell), it will magically infer both the cell class to use and thus its reuseIdentifier needed to dequeue the cell, and which exact type to return to save you a type-cast.

• No need for you to manipulate reuseIdentifiers Strings manually anymore!
• No need to force-casting the returned UITableViewCell instance down to your MyCustomCell class either!

extension MyViewController: UITableViewDataSource {
  func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    if indexPath.section == 0 {
      let cell = tableView.dequeueReusableCell(indexPath: indexPath) as CodeBasedCustomCell
      // Customize the cell here. You can call any type-specific methods here without the need for type-casting
      cell.fillWithText("Foo")
      return cell
    } else {
      let cell = tableView.dequeueReusableCell(indexPath: indexPath) as NibBasedCustomCell
      // Customize the cell here. no need to downcasting here either!
      cell.fillWithImage(UIImage(named:"Bar"))
      return cell
    }
  }
}

Now all you have is a beautiful code and type-safe cells, with compile-type checking, and no more String-based API!

💡 If the cell class is computed at runtime in a variable, you won't be able to use as theVariable or let cell: theVariable obviously… but instead you can use the optional parameter cellType (which otherwise gets infered by the return type and is thus not necessary to provide explicitly)

📑 Example with a cell type determined at runtime

class ParentCell: UITableViewCell, Reusable {}
class Child1Cell: ParentCell {}
class Child2Cell: ParentCell {}

func cellType(for indexPath: NSIndexPath) -> ParentCell.Type {
  return (indexPath.row % 2 == 0) ? Child1Cell.self : Child2Cell.self
}

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
  let cellClass = self.cellType(for: indexPath)
  // As `self.cellType(for:)` always returns a `ParentCell` (sub-)class, the type
  // of the variable `cell` below is infered to be `ParentCell` too. So only methods
  // declared in the parent `ParentCell` class will be accessible on the `cell` variable.
  let cell = tableView.dequeueReusableCell(indexPath: indexPath, cellType: cellClass)
  return cell  
}

Type-safe XIB-based reusable views

Reusable also allows you to create reusable custom views designed in Interface Builder to reuse them in other XIBs or by code, like creating custom UI widgets used in multiple places in your app.

1. Declare your views to conform to NibLoadable or NibOwnerLoadable

In your swift source declaring your custom view class:

• Use the NibLoadable protocol if the XIB you're using don't use its "File's Owner" and the reusable view you're designing is the root view of the XIB
• Use the NibOwnerLoadable protocol if you used a "File's Owner" of the XIB being of the class of your reusable view, and the root view(s) of the XIB is to be set as a subview providing its content.

// a XIB-based custom UIView, used as root of the XIB
final class NibBasedRootView: UIView, NibLoadable { /* and that's it! */ }

// a XIB-based custom UIView, used as the XIB's "File's Owner"
final class NibBasedFileOwnerView: UIView, NibOwnerLoadable { /* and that's it! */ }

💡 You should use the second approach if you plan to use your custom view in another XIB or Storyboard.
This will allow you to just drop a UIView in a XIB/Storyboard and change its class to the class of your custom XIB-based view to use it. That custom view will then automagically load its own content from the associated XIB when instantiated by the storyboard containing it, without having to write additional code to load the content of the ustom view manually every time.

2. Design your view in Interface Builder

For example if you named your class MyCustomWidget and made it NibOwnerLoadable:

• Set the File's Owner's class to MyCustomWidget
• Design the content of the view via the root view of that XIB (which is a standard UIView with no custom class) and its subviews
• Connect any @IBOutlets and @IBActions between the File's Owner (the MyCustomWidget) and its content

final class MyCustomWidget: UIView, NibOwnerLoadable {
  @IBOutlet private var rectView: UIView!
  @IBOutlet private var textLabel: UILabel!

  @IBInspectable var rectColor: UIColor? {
    didSet {
      self.rectView.backgroundColor = self.rectColor
    }
  }
  @IBInspectable var text: String? {
    didSet {
      self.textLabel.text = self.text
    }
  }
…
}

Then that widget can be integrated in a Storyboard Scene (or any other XIB) by simply dropping a UIView on the Storyboard, and changing its class to MyCustomWidget.

3a. Auto-loading the content of a NibOwnerLoadable view

If you used NibOwnerLoadable and made your custom view the File's Owner of your XIB, you should then override init?(coder:) so that it load it's associated XIB as subviews and add constraints automatically:

final class MyCustomWidget: UIView, NibOwnerLoadable {
  …
  required init?(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
    MyCustomWidget.loadFromNib(owner: self)
  }
  override init(frame: CGRect) {
    super.init(frame: frame)
  }
}

Overriding init?(coder:) allows your MyCustomWidget custom view to load its content from the associated XIB MyCustomWidget.xib and add it as subviews of itself.

💡 Note: overriding init(frame:), even just to call super.init(frame: frame) might seems pointless, but seems necessary in some cases due to a strange issue with Swift and dynamic dispatch not being able to detect and call the superclass implementation all by itself: I've sometimes seen crashes when not implementing it explicitly, so better safe than sorry.

3b. Instantiating a NibLoadable view

If you used NibLoadable and made your custom view the root view of your XIB (not using the File's Owner at all), these are not designed to be used in other Storyboards or XIBs like NibOwnerLoadable is, as they won't be able to auto-load their content.

Instead, you will instantiate those NibLoadable views by code, which is as simple as calling loadFromNib() on your custom class:

let view1 = NibBasedRootView.loadFromNib() // Create one instance
let view2 = NibBasedRootView.loadFromNib() // Create another one
let view3 = NibBasedRootView.loadFromNib() // and another one
…

💡 You could also use MyCustomWidget.loadFromNib() on a NibOwnerLoadable — the same way we just did on NibLoadable views above — to load them by code if needs be too.

Type-safe ViewControllers from Storyboards

Reusable also allows you to mark your UIViewController classes as StoryboardBased or StoryboardSceneBased to easily instantiate them from their associated Storyboard in a type-safe way.

1. Declare your UIViewController to conform to StoryboardBased or StoryboardSceneBased

In your swift source declaring your custom UIViewController class:

• Use the StoryboardBased protocol if the *.storyboard file has the same name as the ViewController's class, and its scene is the "initial scene" of the storyboard.
  • This is typically ideal if you use one Storyboard per ViewController, for example.
• Use the StoryboardSceneBased protocol if scene in your storyboard has the same sceneIdentifier as the name of the ViewController's class, but the *.storyboard file name doesn't necessary match the ViewController's class name.
  • This is typically ideal for secondary scenes in bigger storyboards
  • You'll then be required to implement the storyboard type property to indicate the storyboard it belongs to.

final class CustomVC: UIViewController: StoryboardBased { /* and that's it! */ }

final class SecondaryVC: UIViewController: StoryboardSceneBased {
  static let storyboard = UIStoryboard(name: "CustomVC", bundle: nil)
  /* and that's it! */
}

2. Instantiate your UIViewControllers

Simply call instantiate() on your custom class. This will automatically know which storyboard to load it from, and which scene (initial or not) to use to instantiate it.

func presentSecondary() {
  let vc = SecondaryVC.instantitate() // Init from the "SecondaryVC" scene of CustomVC.storyboard
  self.presentViewController(vc, animated: true) {}
}

Additional tips

Make your subclasses final

I advise your to mark your custom UITableViewCell, UICollectionViewCell, UIView and UIViewController subclasses as being final. This is because:

• In most cases, the custom cells and VCs you plan to instantiate are not intended to be subclassed themselves.
• more importantly, it helps the compiler a lot and gives you big optimizations
• it can be required in some cases when conforming to protocols that have Self requirements, like the ones used by this pod (Reusable, StoryboardBased, …).

In some cases you can avoid making your classes final, but in general it's a good practice, and in the case of this pod, usually your custom UIViewController or whatever won't be subclassed anyway:

• Either they are intended to be used and instantiated directly and never be subclassed, so final makes sense here
• In case your custom UIViewController, UITableViewCell, etc… is intended to be subclassed and be the parent class of many classes in your app, it makes more sense to add the protocol conformance (StoryboardBased, Reusable, …) to the child classes (and mark them final) than adding the protocol on the parent, abstract class.

Customize reuseIdentifier, nib, etc for non-conventional uses

The protocols in this pod, like Reusable, NibLoadable, NibReusable, NibOwnerLoadable, StoryboardBased… are what is usually called Mixins, which basically is a Swift protocol with a default implementation provided for all of its methods.

The main benefit is that you don't need to add any code: just conform to Reusable, NibOwnerLoadable or any of those protocol and you're ready to go with no additional code to write.

But of course, those provided implementations are just default implementations. That means that if you need you can still provide your own implementations in case for some reason some of your cells don't follow the classic configuration of using the same name for both the class, the reuseIdentifier and the XIB file.

final class VeryCustomNibBasedCell: UITableViewCell, NibReusable {
  // This cell use a non-standard configuration: its reuseIdentifier and XIB file
  // have a different name as the class itself. So we need to provide a custom implementation or `NibReusable`
  static var reuseIdentifier: String { return "VeryCustomReuseIdentifier" }
  static var nib: UINib { return UINib(nibName: "VeryCustomUI", bundle: nil) } // Use VeryCustomUI.xib
  
  // Then continue with the rest of your normal cell code 
}

The same is true for all the protocols of this pod, which always provide default implementations which could still be replaced by your own if you need some custom cases.

But the beauty is in 90% of cases the default implementation will match typical conventions and the default implementations will be exactly what you want!

Type-safety and fatalError

Reusable allows you to manipulate type-safe APIs and make you avoid typos. But things could still go wrong in case of a misconfguration, for example if you forgot to set the reuseIdentifier of your cell in its XIB, or you declared a FooViewController to be StoryboardBased but forgot to set the initial ViewController flag on that FooViewController scene in that Storyboard, etc.

In such cases, because those are developer errors that should be caught as early as possible in the development process, Reusable will call fatalError with an error message as descriptive as possible (instead of crashing with an obscure message about some force-cast or force-unwrap or whatnot) to help you configure it right.

For example, if Reusable fails to dequeue a cell, it will bail with a message like:

« Failed to dequeue a cell with identifier \(cellType.reuseIdentifier) matching type \(cellType.self). Check that the reuseIdentifier is set properly in your XIB/Storyboard and that you registered the cell beforehand. »

Hopefully, those explicit failure messages will allow you to understand what was misconfigured and help you fix it!

Example Project

This repository comes with an example project in the Example/ folder. Feel free to try it.

It demonstrates how Reusable works for:

• UITableViewCell and UICollectionViewCell subclasses,
• Cells whose UI template is either only provided by plain code, or provided by a XIB, or prototyped directly in a Storyboard.
• UICollectionView's SupplementaryViews (section Headers)
• Custom UIView designed in a XIB (NibOwnerLoadable)

License

This code is distributed under the MIT license. See the LICENSE file for more info.