This style guide is different from others you may see, because the focus is centered on readability for print and the web. We created this style guide to keep the code in our tutorials consistent.

Our overarching goals are conciseness, readability and simplicity.

You should also check out our other style guides:

• Swift
• Objective-C
• Java

This style-guide is somewhat of a mash-up between the existing Kotlin language style guides, and a tutorial-readability focused Swift style-guide. The language guidance is drawn from:

• The Android Kotlin style guide
• The Kotlin Coding Conventions
• The Android contributors style guide
• The Google Java Style Guide.

Alterations to support additional readability in tutorials were inspired by the raywenderlich.com Swift style guide.

It is possible to get Android Studio to adhere to these style guidelines, via a rather complex sequence of menus. To make it easier, we've provided a coding style that can be imported into Android Studio.

The file can be found here.

To install the file, open Android Studio Settings and go to Editor > Code Style > Kotlin, then click the gear menu and choose Import Scheme....

From now on, projects you create should follow the correct style guidelines.

• Nomenclature
  • Packages
  • Classes & Interfaces
  • Methods
  • Fields
  • Variables & Parameters
  • Misc
• Declarations
  • Visibility Modifiers
  • Fields & Variables
  • Classes
  • Data Type Objects
  • Enum Classes
• Spacing
  • Indentation
  • Line Length
  • Vertical Spacing
• Semicolons
• Getters & Setters
• Brace Style
• When Statements
• Annotations
• Types
  • Type Inference
  • Constants vs. Variables
  • Companion Objects
  • Optionals
• XML Guidance
• Language
• Copyright Statement
• Smiley Face
• Credit

On the whole, naming should follow Java standards, as Kotlin is a JVM-compatible language.

Package names are similar to Java: all lower-case, multiple words concatenated together, without hypens or underscores:

BAD:

com.RayWenderlich.funky_widget

GOOD:

com.raywenderlich.funkywidget

Written in UpperCamelCase. For example RadialSlider.

Written in lowerCamelCase. For example setValue.

Generally, written in lowerCamelCase. Fields should not be named with Hungarian notation, as Hungarian notation is erroneously thought to be recommended by Google, and must have an empty line.

Example field names:

class MyClass {

  var publicField: Int = 0
  
  val person = Person()
  
  private var privateField: Int?
  
}

Constant values in the companion object should be written in uppercase, with an underscore separating words and has an empty line before and after companion's brackets.

companion object {

  const val THE_ANSWER = 42
  
}

Written in lowerCamelCase.

Single character values must be avoided, except for temporary looping variables.

In code, acronyms should be treated as words. For example:

BAD:

XMLHTTPRequest
URL: String? 
findPostByID

GOOD:

XmlHttpRequest
url: String
findPostById

Only include visibility modifiers if you need something other than the default of public.

BAD:

public val wideOpenProperty = 1
private val myOwnPrivateProperty = "private"

GOOD:*

val wideOpenProperty = 1
private val myOwnPrivateProperty = "private"

Access level modifiers should be explicitly defined for classes, methods and member variables.

Prefer single declaration per line.

GOOD:

username: String
twitterHandle: String

Exactly one class per source file, although inner classes are encouraged where scoping appropriate.

Prefer data classes for simple data holding objects.

BAD:

class Person(val name: String) {
  override fun toString() : String {
    return "Person(name=$name)"
  }
}

GOOD:

data class Person(val name: String)

TODO: UPDATE FOR KOTLIN (This is java-only and may not be true with Kotlin)

Enum classes should be avoided where possible, due to a large memory overhead. Static constants are preferred. See http://developer.android.com/training/articles/memory.html#Overhead for further details.

Enum classes without methods may be formatted without line-breaks, as follows:

private enum CompassDirection { EAST, NORTH, WEST, SOUTH }

Spacing is especially important in raywenderlich.com code, as code needs to be easily readable as part of the tutorial.

Indentation is using spaces - never tabs.

Indentation for blocks uses 2 spaces (not the default 4):

BAD:

for (i in 0..9) {
    Log.i(TAG, "index=" + i);
}

GOOD:

for (i in 0..9) {
  Log.i(TAG, "index=" + i)
}

Indentation for line wraps should use 4 spaces (not the default 8):

BAD:

val widget: CoolUiWidget =
        someIncrediblyLongExpression(that, reallyWouldNotFit, on, aSingle, line)

GOOD:

val widget: CoolUiWidget =
    someIncrediblyLongExpression(that, reallyWouldNotFit, on, aSingle, line)

Lines should be no longer than 120 characters long.

There should be exactly one blank line between methods to aid in visual clarity and organization. Whitespace within methods should separate functionality, but having too many sections in a method often means you should refactor into several methods.

Semicolons are dead to us should be avoided wherever possible in Kotlin.

BAD:

val horseGiftedByTrojans = true;
if (horseGiftedByTrojans) {
  bringHorseIntoWalledCity();
}

GOOD:

val horseGiftedByTrojans = true
if (horseGiftedByTrojans) {
  bringHorseIntoWalledCity()
}

Unlike Java, direct access to fields in Kotlin is preferred.

If custom getters and setters are required, they should be declared following Kotlin conventions rather than as separate methods.

Only trailing closing-braces are awarded their own line. All others appear the same line as preceding code:

BAD:

class MyClass
{
  fun doSomething()
  {
    if (someTest)
    {
      // ...
    }
    else
    {
      // ...
    }
  }
}

GOOD:

class MyClass {

  fun doSomething() {
    if (someTest) {
      // ...
    } else {
      // ...
    }
  }
  
}

Conditional statements are always required to be enclosed with braces and new line, irrespective of the number of lines required.

BAD:

if (someTest)
  doSomething()
if (someTest) doSomethingElse()

GOOD:

if (someTest) {
  doSomething()
}
if (someTest) { 
  doSomethingElse() 
}

Unlike switch statements in Java, when statements do not fall through. Separate cases using commas if they should be handled the same way. Always include the else case and new line with brackets.

BAD:

when (anInput) {
  1 -> doSomethingForCaseOne()
  2 -> doSomethingForCaseOneOrTwo()
  3 -> doSomethingForCaseThree()
}

GOOD:

when (anInput) {
  1, 2 -> { 
    doSomethingForCaseOneOrTwo()
  }
  3 -> {
    doSomethingForCaseThree()
  }
  else -> {
    println("No case satisfied")
  }
}

Standard annotations should be used - in particular override. This should appear on the same line as the function declaration.

BAD:

fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState);
}

GOOD:

override fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState);
}

Always use Kotlin's native types when available. Kotlin is JVM-compatible so [TODO: more info]

Type inference should be preferred where possible to explicitly declared types.

BAD:

val something: MyType = MyType()
val meaningOfLife: Int = 42

GOOD:

val something = MyType()
val meaningOfLife = 42

Constants are defined using the val keyword, and variables with the var keyword. Always use val instead of var if the value of the variable will not change.

Tip: A good technique is to define everything using val and only change it to var if the compiler complains!

** TODO: A bunch of stuff about companion objects **

Declare variables and function return types as nullable with ? where a null value is acceptable.

Use implicitly unwrapped types declared with !! only for instance variables that you know will be initialized before use, such as subviews that will be set up in onCreate for an Activity or onCreateView for a Fragment.

When naming nullable variables and parameters, avoid naming them like nullableString or maybeView since their nullability is already in the type declaration.

When accessing a nullable value, use the safe call operator if the value is only accessed once or if there are many nullables in the chain:

editText?.setText("foo")

TODO: Update the rest of this section from Swift

Use optional binding when it's more convenient to unwrap once and perform multiple operations:

if let textContainer = self.textContainer {
  // do many things with textContainer
}

For optional binding, shadow the original name when appropriate rather than using names like unwrappedView or actualLabel.

Preferred:

var subview: UIView?
var volume: Double?

// later on...
if let subview = subview, let volume = volume {
  // do something with unwrapped subview and volume
}

Not Preferred:

var optionalSubview: UIView?
var volume: Double?

if let unwrappedSubview = optionalSubview {
  if let realVolume = volume {
    // do something with unwrappedSubview and realVolume
  }
}

Since Android uses XML extensively in addition to Kotlin and Java, we have some rules specific to XML. These can be found in our Java code standards

Use en-US English spelling. 🇺🇸

BAD:

val colourName = "red"

GOOD:

colorName = "red"

BAD:

display

GOOD: use show word instead

show

The following copyright statement should be included at the top of every source file:

/*
 * Copyright (c) 2019 Razeware LLC
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * Notwithstanding the foregoing, you may not use, copy, modify, merge, publish, 
 * distribute, sublicense, create a derivative work, and/or sell copies of the 
 * Software in any work that is designed, intended, or marketed for pedagogical or 
 * instructional purposes related to programming, coding, application development, 
 * or information technology.  Permission for such use, copying, modification,
 * merger, publication, distribution, sublicensing, creation of derivative works, 
 * or sale is expressly withheld.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

Smiley faces are a very prominent style feature of the raywenderlich.com site! It is very important to have the correct smile signifying the immense amount of happiness and excitement for the coding topic. The closing square bracket ] is used because it represents the largest smile able to be captured using ASCII art. A closing parenthesis ) creates a half-hearted smile, and thus is not preferred.

Bad:

:)

Good:

:]

This style guide is a collaborative effort from the most stylish raywenderlich.com team members:

• Darryl Bayliss
• Tom Blankenship
• Sam Davies
• Mic Pringle
• Ellen Shapiro
• Ray Wenderlich
• Joe Howard