A summary of all the kotlin tips from Google's Android Developer Twitter account with the hastag #31DaysofKotlin
A Twitter Moment summary of all the tweets are here -> https://twitter.com/i/moments/980488782406303744
- Add code samples in this repo for all 31 days
- Create an app for quick reference
- Create ref app in the Play Store
Let’s run with some standard Kotlin functions! Short and powerful, let, apply, with and run all have a receiver (this), may have an argument (it) and may have a return value. See the differences
Extending existing APIs with default arguments usually makes everyone happy. Android KTX lets you set the padding on one side of a view using default parameters.
A one line function that saves so much code!
Code: https://github.com/android/android-ktx/blob/master/src/main/java/androidx/core/view/View.kt#L117
Love the speed of Parcelable, but don’t like writing all that code? Say hello to Parcelize
Spec: https://github.com/Kotlin/KEEP/blob/master/proposals/extensions/android-parcelable.md
Powerful but hard to use - that’s how the text styling Spans API feels. Android KTX adds extension functions for some of the most common spans and makes the API easier to use
Android KTX: https://github.com/android/android-ktx/blob/master/src/main/java/androidx/core/text/SpannableStringBuilder.kt
Lambdas are sweet. With last parameter call syntax, you can cleanup callbacks, Callable, and Runnable.
For example, Android KTX sweetens postDelayed with a small wrapper.
Docs: https://kotlinlang.org/docs/reference/lambdas.html Android KTX: https://github.com/android/android-ktx/blob/master/src/main/java/androidx/core/os/Handler.kt#L38
Bundle up, and get ready for the concise bundle creator in Android KTX. No more calls to putString, putInt, or any of their 20 friends.
One call will make you a new bundle, and it’ll even handle Arrays!
Code: https://github.com/android/android-ktx/blob/master/src/main/java/androidx/core/os/Bundle.kt#L27
[1/4] Specifically terrific? Domain specific languages can be made by using type safe builders. They make for clean APIs; and you can build them yourself too.
Extension Lambdas: https://kotlinlang.org/docs/reference/lambdas.html#function-literals-with-receiver Type Safe Builders: https://kotlinlang.org/docs/reference/type-safe-builders.html
Combine the power of Content Values with the brevity of Kotlin. Use the Android KTX Content Values creator and just pass a Pair<StringKey, Value>.
Android KTX: https://github.com/android/android-ktx/blob/master/src/main/java/androidx/core/content/ContentValues.kt#L21
Iterators in interesting places? Android KTX adds iterators to ViewGroup and SparseArray
To define iterator extensions use the operator keyword. Foreach loops will use the extensions!
Docs: https://kotlinlang.org/docs/reference/control-flow.html#for-loops Android KTX: https://github.com/android/android-ktx/blob/master/src/main/java/androidx/core/view/ViewGroup.kt#L66
[1/2] Utility methods for a class? Add them to the top level of the source file. In Java, they are compiled as static methods of that class.
Doc: https://kotlinlang.org/docs/reference/basic-syntax.html
Write Kotlin (time * 2) faster with operator overloading. Objects like Path, Range or SpannableStrings naturally allow for operations like addition or subtraction. With Kotlin, you can implement your own operators
https://goo.gl/EGipGz https://goo.gl/iy8tZ9
[1/2] Sequences are lists that never existed. A Sequence is a cousin of Iterator, lazily generating one value at a time. This matters when using map and filter - they’ll create Sequences instead of copying the list for every step!
Docs: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/index.html
If you ever converted a Drawable to a Bitmap then you know how much boilerplate you need.
Android KTX has a great set of functions to make your code more concise when working with classes from the graphics package:
https://github.com/android/android-ktx/tree/master/src/main/java/androidx/core/graphics/drawable
[1/2] No more Util classes! Extend the functionality of a class by using extension functions. Put the name of the class you’re extending before the name of the method you’re adding.
Doc: https://kotlinlang.org/docs/reference/extensions.html#extension-functions
Example: https://github.com/android/android-ktx/blob/master/src/main/java/androidx/core/net/Uri.kt#L28
Delegate your work to another class with by. Favor composition over inheritance with class delegation and reuse property accessor logic with delegator properties.
Delegation: https://kotlinlang.org/docs/reference/delegation.html
Delegated properties: http://kotlinlang.org/docs/reference/delegated-properties.html
To make the concept of reified concrete an example is in order: Context.systemService() in Android KTX uses reified to pass a "real" type via generics. No more passing classes to getSystemService! https://github.com/android/android-ktx/blob/master/src/main/java/androidx/core/content/Context.kt#L37
Docs: https://kotlinlang.org/docs/reference/inline-functions.html#reified-type-parameters
Using Kotlin and Java in the same project? Do you have classes with top-level functions or properties?
By default, the compiler generates the class name as “YourFileKt”. Change it by annotating the file with @file:JvmName(“...“).
Doc: https://kotlinlang.org/docs/reference/java-to-kotlin-interop.html#package-level-functions
Can’t wait to use lambdas to make new APIs?
Get in line.
Kotlin lets you specify a function as inline - which means calls will be replaced with the function body. Breathe and make lambda-based APIs with zero overhead.
Docs: https://kotlinlang.org/docs/reference/inline-functions.html
[1/2] Are your function arguments valid? Check before using them, with require. If they’re not valid an IllegalArgumentException is thrown.
Doc: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/require.html
In Android, onCreate or other callbacks initialize objects. In Kotlin non-null vals must be initialized. What to do?
Enter lateinit. It's a promise: initialize me later! Use it to pinky-swear it will "eventually" be null safe
Docs: https://kotlinlang.org/docs/reference/properties.html#late-initialized-properties-and-variables
It’s good to be lazy! Defer the cost of expensive property initialization until they’re actually needed, by using lazy. The computed value is then saved and used for any future calls.
Lazy: https://kotlinlang.org/docs/reference/delegated-properties.html#lazy
[1/3] Kotlin sealed classes let you easily handle error data. When combined with LiveData you can use one LiveData to represent both the success path and the error path. Way better than using two variables.
Docs: https://kotlinlang.org/docs/reference/sealed-classes.html
Is the number of method overloads getting out of hand? Specify default parameter values in functions.
Make the code even more readable with named parameters.
Doc: https://kotlinlang.org/docs/reference/functions.html#default-arguments
In Kotlin, everything is public by default! Well, almost. Kotlin has a rich set of visibility modifiers you can use as well: private, protected, internal. Each of them reduces the visibility in a different way.
Docs: https://kotlinlang.org/docs/reference/visibility-modifiers.html
Creating classes with one role: to hold data? Mark them as “data” classes. The default implementation of equals() is generated (so are hashCode(), toString(), and copy()) and checks for structural equality.
Docs: https://kotlinlang.org/docs/reference/data-classes.html#data-classes https://kotlinlang.org/docs/reference/equality.html
In Kotlin, classes can have mutable and read-only properties, with getters and setters generated by default. You can also implement custom ones if required.
Docs: https://kotlinlang.org/docs/reference/properties.html
For loops get superpowers when used with two other Kotlin features: range expressions and destructuring.
Ranges: https://kotlinlang.org/docs/reference/ranges.html
Destructuring: https://kotlinlang.org/docs/reference/multi-declarations.html#destructuring-declarations
A switch statement with superpowers? Kotlin’s when expression can match on just about anything. Literal values, enums, ranges of numbers. You can even call arbitrary functions!
Docs: https://kotlinlang.org/docs/reference/control-flow.html#when-expression
Now with prisms? Android KTX uses destructuring to assign the component values of a color. You can use destructuring in your classes, or extend existing classes to add destructuring.
Docs: https://kotlinlang.org/docs/reference/multi-declarations.html
Formatting Strings? Refer to variables and expressions in string literals by putting $ in front of the variable name. Evaluate expressions using ${expression}.
Docs: https://kotlinlang.org/docs/reference/basic-types.html#string-templates
Handling nulls in style? Check out the elvis operator ?:, to cut your “null-erplate”. It’s just a small bit of syntax sugar to replace nulls with a default value or even return!
Docs: https://kotlinlang.org/docs/reference/null-safety.html#elvis-operator
Thanks to the Google Android Developer team for putting together this cool series of Kotlin info! 😄