How to...in playgrounds

9. Decode a JSON string

In order to create a simple test for your decodable code:

import Foundation

// in this case the show name is {"A"-Team}, hence the \\ to espace the {"}
let jsonString = """
{
  "id": "345team",
  "name": "\\"A\\"-Team",
}
"""

struct Show: Decodable {
  let id: String
  let name: String
}

guard let rawData = jsonString.data(using: .utf8),
  let card = try? JSONDecoder().decode(Show.self, from: rawData) else {
    fatalError("couldn't parse the Raw JSON string, the json content must be malformed")
}

print(Show.name)

8. Decode a JSON file

Useful to provide test data to ViewControllers, when you don't have an internet connection.

Given that you have a sample-data.json file in your Resources folder

import Foundation

guard
  let jsonFileURL = Bundle.main.url(forResource: "sample-data", withExtension: "json"),
  let jsonData = try? Data(contentsOf: jsonFileURL),
  let decodedResult = try? JSONDecoder().decode(MyModel.self, from: jsonData) else {
    fatalError("coudln't read the file from the main bundle")
}

7. Use images in markdown

First, you need to add an image to the Resource folder of a playground named design.png, then from the code, you can simply add:

/*:
  ## Inserting a markdown image
  
  ![sample image](design.png)
*/

and it will render correctly

6. Programatically create an instance of NSViewController

Another one for mac, in playgrounds

import Cocoa
import PlaygroundSupport

class MyViewController: NSViewController {
  override func loadView() {
    self.view = NSView(
      frame: CGRect(x: 0, y: 0, width: 300, height: 200)
    )
    self.view.wantsLayer = true
    self.view.layer?.backgroundColor = NSColor.blue.cgColor
  }
}

let viewController = MyViewController()

PlaygroundPage.current.liveView = viewController

The important part, is that the method loadView must be implemented, as using the empty initializer, the code will throw an error missing the view if loadView hasn't been implemented

5. Open a image for iOS

Given an image added to the Resources folder, like tutorial-01.jpg, I would open it from a playground like:

let image = UIImage(named: "tutorial-01.jpg")

4. Open a image for macOS

Given an image added to the Resources folder, like tutorial-01.jpg, I would open it from a playground like:

let fileURL = Bundle.main.url(forResource: "tutorial-01", withExtension: "jpg")!
let image = NSImage(contentsOf: fileURL)

3. Write unit tests

Tip taken from john sundell's Writing unit tests in Swift playgrounds post

import Foundation
import XCTest

class MyTestClass: XCTestCase {
  func testSomething() {
    // write test here
  }
}

class TestObserver: NSObject, XCTestObservation {
  func testCase(_ testCase: XCTestCase,
                  didFailWithDescription description: String,
                  inFile filePath: String?,
                  atLine lineNumber: Int) {
    assertionFailure(description, line: UInt(lineNumber))
  }
}

let testObserver = TestObserver()
XCTestObservationCenter.shared.addTestObserver(testObserver)
MyTestClass.defaultTestSuite.run()

2. Test an instance of UIView

When attempting to preview an instance of UIView, it's important to call view.layoutIfNeeded() otherwise the view would never layout on Playgrounds.

let view = MyCustomView()
view.frame = CGRect(x: 0, y: 0, width: 300, height: 150)
view.layoutIfNeeded()

1. Use a custom font

First, add the my-custom-font.otf file to the Resources folder of the playground, then within the playground, you can use the font as follows:

let fontName = "my-custom-font"
let fontURL = Bundle.main.url(forResource: fontName, withExtension: "otf")
CTFontManagerRegisterFontsForURL(fontURL! as CFURL, .process, nil)

let uiFontName = "MyCustomFont-Bold"
let customFont = UIFont(name: uiFontName, size: 16.0)