If you're reading this you've probably heard a lot about Kotlin Multiplatform, but aren't sure where to start. The platform is evolving fast, which means a lot of the docs are out of date, and/or can be a little overwhelming. But nobody likes reading the directions, so I'm writing a series of posts that take about 5 minutes each to run through, and introduce the basics on Kotlin Multiplatform.
Part 1 gets you starting from nothing. This is part 2, which focuses on expect
/actual
statements. Kotlin's native interop is one of its most distinguishing features. Unlike other "cross platform" ecosystems, Kotlin is designed to embrace the native platform it is integrating with. expect
/actual
statements are one of the main mechanisms it provides to do that.
Five minutes? Well, yes, if you've already gone through the previous Kotlin Multiplatform article and completed it. We're starting where Part 1 left off.
One area that requires platform-specific logic is dealing with dates. We're going to create a simple expect
class, KMPDate
, which takes a date format string and lets you create a formatted string representing today's date.
First, open /app/src/commonMain/kotlin/sample/Sample.kt
. Underneath the Greeting
class, create a new expect
class:
expect class KMPDate(formatString: String) {
fun asString(): String
}
The expect
keyword declares that the common code can expect different actual
implementations for each platform. In this case, expect
applies to a class. However, it can also apply to functions, objects, annotations, etc. See here for more.
Go to /app/src/main/java/sample/SampleAndroid.kt
to create the actual
Android implementation. Underneath the MainActivity
class, add the following:
actual class KMPDate actual constructor(formatString: String) { // 1
private val dateFormat = SimpleDateFormat(formatString) // 2
actual fun asString(): String {
return dateFormat.format(Date()) // 3
}
}
The expect declaration of
KMPDate
stated that it takes aformatString
, so anactual constructor
implementation accompanies theactual class
implementation.Create a
SimpleDateFormat
object using theformatString
. iOS has anNSDateFormatter
class, which is similar toSimpleDateFormat
, but we'll get to later. There is currently no common Kotlin implementation, but for most things, Android and iOS have equivalent features available.The
SimpleDateFormat
object formatsDate()
- which produces aDate
object representing today's date - into a string.
Next, go to app/src/iosMain/kotlin/sample/SampleIos.kt
, which is currently empty. Paste in the following:
actual class KMPDate actual constructor(formatString: String) { // 1
private val dateFormatter = NSDateFormatter().apply { // 2
this.dateFormat = formatString
}
actual fun asString(): String {
return formatter.stringFromDate(NSDate()) // 3
}
}
Identical to the
actual class
declaration on the Android side.Pass
formatString
into an iOSNSDateFormatter
object, which - as mentioned earlier - achieves the same result asSimpleDateFormat
on Android.The
NSDateFormatter
object formats a string based onNSDate()
- which produces anNSDate
object representing today.
Finally, go back to /app/src/commonMain/kotlin/sample/Sample.kt
. Inside of the Greeting
class, create a KMPDate
object that takes "MMM dd" as its date format:
private val date = KMPDate("MMM dd")
And last but not least, use string interpolation to insert the formatted date string from each platform into the value of hello()
:
fun hello(): String = "Hello, Android and iOS worlds, today is ${date.asString()}!"
All done! Run you Android and iOS apps and see the dates displayed on each platform!
If you come from an iOS background and want a deeper explanation of how to use Kotlin Multiplatform to make a more involved app, then you might want to check out this article I wrote.
As you can see, it's all about starting small with Kotlin Multiplatform. We recently launched The Touchlab Refactory; an incremental refactoring service with KMP which focuses on a single feature at a time with no disruption to your current development workflow. Learn more here.
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