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Pragmatic Kotlin Android Chapter September, 2019

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segunfamisa segunfamisa.com

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Introduction

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typealiases

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typealiases

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typealiases ● Nicknames for regular existing types

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// before fun applyDiscount(price: Double, discount: Double): Double = (100.0 - discount) / 100.0 * price

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// before fun applyDiscount(price: Double, discount: Double): Double = (100.0 - discount) / 100.0 * price Discount in €? Cents? Or Percentage??

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// before fun applyDiscount(price: Double, discount: Double): Double // fix #1

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// before fun applyDiscount(price: Double, discount: Double): Double // fix #1 /** * @param price - original price in Euros * @param discount - discount in percentage */ fun applyDiscount(price: Double, discount: Double): Double

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// before fun applyDiscount(price: Double, discount: Double): Double // fix #1 /** * @param price - original price in Euros * @param discount - discount in percentage */ fun applyDiscount(price: Double, discount: Double): Double KDoc comment

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// before fun applyDiscount(price: Double, discount: Double): Double // fix #1 /** * @param price - original price in Euros * @param discount - discount in percentage */ fun applyDiscount(price: Double, discount: Double): Double KDoc comment

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// before fun applyDiscount(price: Double, discount: Double): Double // fix #2

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// before fun applyDiscount(price: Double, discount: Double): Double // fix #2 fun applyDiscount(price: Double, discountInPercentage: Double): Double

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// before fun applyDiscount(price: Double, discount: Double): Double // fix #2 fun applyDiscount(price: Double, discountInPercentage: Double): Double Clearer param name

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// before fun applyDiscount(price: Double, discount: Double): Double // fix #3 // create typealias typealias Percentage = Double

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// before fun applyDiscount(price: Double, discount: Double): Double // fix #3 // create typealias typealias Percentage = Double // usage fun applyDiscount(price: Double, discount: Percentage): Double

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// before fun applyDiscount(price: Double, discount: Double): Double // fix #3 // create typealias typealias Percentage = Double // usage fun applyDiscount(price: Double, discount: Percentage): Double

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// before fun applyDiscount(price: Double, discount: Double): Double // fix #3 // create typealias typealias Percentage = Double // usage fun applyDiscount(price: Double, discount: Percentage): Double ● Short parameter name ● “type” of discount is clear ● No new type created

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// before fun applyDiscount(price: Double, discount: Double): Double // fix #3 // create typealias typealias Percentage = Double // usage fun applyDiscount(price: Double, discount: Percentage): Double ● Short parameter name ● “type” of discount is clear ● No new type created

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typealias ● Improves readability at no performance cost ● Does not create a new type ● Allows for easy refactoring

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inline classes

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inline classes ● New in Kotlin 1.3 ● Still experimental ● Useful for creating “wrappers” for primitive types

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// before fun applyDiscount(price: Double, discount: Double): Double = (100.0 - discount) / 100.0 * price

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// before fun applyDiscount(price: Double, discount: Double): Double = (100.0 - discount) / 100.0 * price Same problems as before ● Easy to mess up the order of the parameters ● Not clear what discount is - in Euros or in percentage?

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// before fun applyDiscount(price: Double, discount: Double): Double // wrapper classes class Price(val value: Double) class Percentage(val value: Double)

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// before fun applyDiscount(price: Double, discount: Double): Double // wrapper classes class Price(val value: Double) class Percentage(val value: Double) // Usage fun applyDiscount(price: Price, discount: Percentage): Price

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// before fun applyDiscount(price: Double, discount: Double): Double // wrapper classes class Price(val value: Double) class Percentage(val value: Double) // Usage fun applyDiscount(price: Price, discount: Percentage): Price

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// before fun applyDiscount(price: Double, discount: Double): Double // wrapper classes class Price(val value: Double) class Percentage(val value: Double) // Usage fun applyDiscount(price: Price, discount: Percentage): Price ● Less error-prone ● Clear intentions/expectations

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// before fun applyDiscount(price: Double, discount: Double): Double // wrapper classes class Price(val value: Double) class Percentage(val value: Double) // Usage fun applyDiscount(price: Price, discount: Percentage): Price ● Additional memory allocation

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// before fun applyDiscount(price: Double, discount: Double): Double // wrapper classes class Price(val value: Double) class Percentage(val value: Double) // Usage fun applyDiscount(price: Price, discount: Percentage): Price ● Additional memory allocation

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// before fun applyDiscount(price: Double, discount: Double): Double // wrapper classes inline class Price(val value: Double) inline class Percentage(val value: Double) // Usage fun applyDiscount(price: Price, discount: Percentage): Price

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// wrapper classes inline class Price(val value: Double) inline class Percentage(val value: Double) // Usage fun applyDiscount(price: Price, discount: Percentage): Price

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// Usage fun applyDiscount(price: Price, discount: Percentage): Price

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// Usage fun applyDiscount(price: Price, discount: Percentage): Price // Generated Java code public final double discountedPrice_lwIBXQA( double price, double discount ) { return Price.constructor-impl((100.0D - discount) / 100.0D * price); }

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// Usage fun applyDiscount(price: Price, discount: Percentage): Price // Generated Java code public final double discountedPrice_lwIBXQA( double price, double discount ) { return Price.constructor-impl((100.0D - discount) / 100.0D * price); }

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// Usage fun applyDiscount(price: Price, discount: Percentage): Price // Generated Java code public final double discountedPrice_lwIBXQA( double price, double discount ) { return Price.constructor-impl((100.0D - discount) / 100.0D * price); } Inline class variables are resolved to the primitive type they wrap!

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// Usage fun applyDiscount(price: Price, discount: Percentage): Price // Generated Java code public final double discountedPrice_lwIBXQA( double price, double discount ) { return Price.constructor-impl((100.0D - discount) / 100.0D * price); }

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// Usage fun applyDiscount(price: Price, discount: Percentage): Price // Generated Java code public final double discountedPrice_lwIBXQA( double price, double discount ) { return Price.constructor-impl((100.0D - discount) / 100.0D * price); } “-” is not a valid character for functions in Java, so for now, inline classes don’t work in Java

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inline classes ● Don’t work with Java for now ● Kotlin compiler is not always able to inline ● inline classes cannot have more than one “param”

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Another example ● Android Resource wrapper inline class StringResource(@StringRes val resId: Int)

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inline classes typealiases

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inline classes typealiases No new type is introduced

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inline classes New type is introduced typealiases No new type is introduced

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inline classes typealiases Can assign typealias to underlying type: typealias Password = String fun login(password: String) val pwd: Password = "kisHuy8q172" login(password = pwd)

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inline classes Cannot assign inline class to the wrapped type: inline class Password(val value: String) fun login(password: String) val pwd = Password("kisHuy8q172") login(password = pwd) // won't compile typealiases Can assign typealias to underlying type: typealias Password = String fun login(password: String) val pwd: Password = "kisHuy8q172" login(password = pwd)

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property delegates

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property delegates ● Special kinds of properties that are implemented differently ● E.g lazy properties, observable properties etc.

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Lazy Initialized only once.

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fun computeExpensiveStringThatDoesntChange() = "stringggg" val expensiveValue : String by lazy { println("Log expensiveValue created") computeExpensiveStringThatDoesntChange() }

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fun computeExpensiveStringThatDoesntChange() = "stringggg" val expensiveValue : String by lazy { println("Log expensiveValue created") computeExpensiveStringThatDoesntChange() } // Usage println(expensiveValue) println(expensiveValue)

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fun computeExpensiveStringThatDoesntChange() = "stringggg" val expensiveValue : String by lazy { println("Log expensiveValue created") computeExpensiveStringThatDoesntChange() } // Usage println(expensiveValue) println(expensiveValue) // Output "Log expensiveValue created" "stringggg" "stringggg"

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fun computeExpensiveStringThatDoesntChange() = "stringggg" val expensiveValue : String by lazy { println("Log expensiveValue created") computeExpensiveStringThatDoesntChange() } // Usage println(expensiveValue) println(expensiveValue) // Output "Log expensiveValue created" "stringggg" "stringggg" First access to the variable prints this

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fun computeExpensiveStringThatDoesntChange() = "stringggg" val expensiveValue : String by lazy { println("Log expensiveValue created") computeExpensiveStringThatDoesntChange() } // Usage println(expensiveValue) println(expensiveValue) // Output "Log expensiveValue created" "stringggg" "stringggg" Subsequent access doesn’t go into the lazy block

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Another sample - viewModel creation class MyActivity : Activity { @Inject lateinit var viewModelFactory: ViewModelFactory private val viewModel by viewModels { viewModelFactory } }

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Another sample - viewModel creation class MyActivity : Activity { @Inject lateinit var viewModelFactory: ViewModelFactory private val viewModel by viewModels { viewModelFactory } } implementation "androidx.activity:activity-ktx:$libs.ktxActivityVersion"

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class ViewModelLazy ( private val viewModelClass: KClass, private val storeProducer: () -> ViewModelStore, private val factoryProducer: () -> ViewModelProvider.Factory ) : Lazy { }

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class ViewModelLazy ( private val viewModelClass: KClass, private val storeProducer: () -> ViewModelStore, private val factoryProducer: () -> ViewModelProvider.Factory ) : Lazy { }

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class ViewModelLazy ( ... ) : Lazy { private var cached: VM? = null override val value: VM get() { val viewModel = cached return if (viewModel == null) { val factory = factoryProducer() val store = storeProducer() ViewModelProvider(store,factory) .get(viewModelClass.java).also { cached = it } } else { viewModel } } override fun isInitialized() = cached != null }

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Putting it all together: inline fun ComponentActivity.viewModels( noinline factoryProducer: (() -> Factory)? = null ): Lazy { val factoryPromise = factoryProducer ?: { val application = application ?: throw IllegalArgumentException() AndroidViewModelFactory.getInstance(application) } return ViewModelLazy(VM::class, { viewModelStore }, factoryPromise) }

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Putting it all together: inline fun ComponentActivity.viewModels( noinline factoryProducer: (() -> Factory)? = null ): Lazy { val factoryPromise = factoryProducer ?: { val application = application ?: throw IllegalArgumentException() AndroidViewModelFactory.getInstance(application) } return ViewModelLazy(VM::class, { viewModelStore }, factoryPromise) } Extension function to make access easier!

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Observable fields Get notified whenever value changes. Need to supply initial value

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Sample usage - recyclerview adapter update & diffutil https://gist.github.com/segunfamisa/716401e58ead9e6eb1c848a51451f927 class ItemAdapter @Inject constructor() : RecyclerView.Adapter() { var items: List by observableList(initialValue = listOf()) ... }

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Sample usage - recyclerview adapter update & diffutil class ItemAdapter @Inject constructor() : RecyclerView.Adapter() { var items: List by observableList(initialValue = listOf()) ... }

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class ObservableListField( private val adapter: RecyclerView.Adapter, private val diffUtilCallbackProducer: (List, List) -> DiffUtil.Callback, initialValue: List ) : ObservableProperty>(initialValue = initialValue)

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class ObservableListField( private val adapter: RecyclerView.Adapter, private val diffUtilCallbackProducer: (List, List) -> DiffUtil.Callback, initialValue: List ) : ObservableProperty>(initialValue = initialValue) { override fun beforeChange( property: KProperty<*>, oldValue: List, newValue: List ): Boolean { return true } override fun afterChange(property: KProperty<*>, oldValue: List, newValue: List) { super.afterChange(property, oldValue, newValue) } }

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class ObservableListField( private val adapter: RecyclerView.Adapter, private val diffUtilCallbackProducer: (List, List) -> DiffUtil.Callback, initialValue: List ) : ObservableProperty>(initialValue = initialValue) { private var result: DiffUtil.DiffResult? = null override fun beforeChange( property: KProperty<*>, oldValue: List, newValue: List ): Boolean { val diffUtilCallback = diffUtilCallbackProducer?.invoke(oldValue, newValue) result = DiffUtil.calculateDiff(diffUtilCallback) return true } override fun afterChange(property: KProperty<*>, oldValue: List, newValue: List) { result?.dispatchUpdatesTo(adapter) super.afterChange(property, oldValue, newValue) } }

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class ObservableListField( private val adapter: RecyclerView.Adapter, private val diffUtilCallbackProducer: (List, List) -> DiffUtil.Callback, initialValue: List ) : ObservableProperty>(initialValue = initialValue) { private var result: DiffUtil.DiffResult? = null override fun beforeChange( property: KProperty<*>, oldValue: List, newValue: List ): Boolean { val diffUtilCallback = diffUtilCallbackProducer?.invoke(oldValue, newValue) result = DiffUtil.calculateDiff(diffUtilCallback) return true } override fun afterChange(property: KProperty<*>, oldValue: List, newValue: List) { result?.dispatchUpdatesTo(adapter) super.afterChange(property, oldValue, newValue) } }

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class ObservableListField( private val adapter: RecyclerView.Adapter, private val diffUtilCallbackProducer: (List, List) -> DiffUtil.Callback, initialValue: List ) : ObservableProperty>(initialValue = initialValue) { private var result: DiffUtil.DiffResult? = null override fun beforeChange( property: KProperty<*>, oldValue: List, newValue: List ): Boolean { val diffUtilCallback = diffUtilCallbackProducer?.invoke(oldValue, newValue) result = DiffUtil.calculateDiff(diffUtilCallback) return true } override fun afterChange(property: KProperty<*>, oldValue: List, newValue: List) { result?.dispatchUpdatesTo(adapter) super.afterChange(property, oldValue, newValue) } }

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class ObservableListField( private val adapter: RecyclerView.Adapter, private val diffUtilCallbackProducer: (List, List) -> DiffUtil.Callback, initialValue: List ) : ObservableProperty>(initialValue = initialValue) { private var result: DiffUtil.DiffResult? = null override fun beforeChange( property: KProperty<*>, oldValue: List, newValue: List ): Boolean { val diffUtilCallback = diffUtilCallbackProducer?.invoke(oldValue, newValue) result = DiffUtil.calculateDiff(diffUtilCallback) return true } override fun afterChange(property: KProperty<*>, oldValue: List, newValue: List) { result?.dispatchUpdatesTo(adapter) super.afterChange(property, oldValue, newValue) } }

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fun RecyclerView.Adapter.observableList( initialValue: List, diffUtilCallbackProducer: ((List, List) -> DiffUtil.Callback) ): ObservableProperty> { return ObservableListField( adapter = this, diffUtilCallbackProducer = diffUtilCallbackProducer, initialValue = initialValue ) } Putting it all together!

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More Kotlin features ● Generics ● Kotlin native ● Annotations

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Resources

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Resources https://kotlinlang.org/docs/reference/ https://gist.github.com/segunfamisa/716401e58ead9e6eb1c848a51451f927 AndroidX Source

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Thank you!