Kotlin For Android - Properties (part 4 of 7)

Kotlin for Android
May 24, 2018
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Properties, Annotations and Nullability
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Kotlin Properties
PROPERTIES
interface Factory<T: Any> {
var mock: T?
}
class Provider<T: Any>(init: () -> T): Factory<T> {
override var mock: T? = null
val instance: T by lazy(init)
}
provider.mock = mock(...)
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● Properties replace java fields
● Declared in primary constructor or at the top of a class
● Declaration is like variables inside a function
● Can be declared inside interfaces

Kotlin Properties
PROPERTIES
var mock: T?
}
private set
}
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● Declaration is like variables inside a type
● Properties compile into:
○ A field + getter for val
○ A field + getter & setter for var
○ The field is private
○ Getter has same visibility
○ Setter defaults to same visibility

private final String string;
String getString() // Java getter
val string: String // Kotlin val
private CharSequence text;
CharSequence getText() // Java getter
void setText(CharSequence sss) // Java setter
var text: CharSequence // Kotlin var
private Boolean isMarried;
Boolean isMarried() // Java getter
void setMarried(Boolean married) // Java setter
var isMarried: Boolean // Kotlin var
Java Synthetic
Properties
PROPERTIES
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Kotlin Properties
PROPERTIES
var mock: T?
}
@get:JvmName("it")
private set
}
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● Declaration is like variables inside a type
● Properties compile into several members
● Use qualifiers to annotate a part of a property

Annotation Qualifiers
PROPERTIES
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Qualifier @Foo applies to
@file:Foo Entire .kt file
@param:Foo Parameter of a single-param fun
@receiver:Foo Receiver of a fun with receiver
Below are exclusive or most relevant to properties
@property:Foo The property ¯_(ツ)_/¯
@field:Foo Field of a property
@get:Foo Getter of a property
@set:Foo Setter of a property
@setparam:Foo The parameter of the setter of a property
@delegate:Foo The delegate of a property

// Suppress unused warnings in the file
@file:Suppress("unused")
// Get a string through context
fun @receiver:StringRes Int.getString() = …
class Example(
// annotate Java field
@field:DrawableRes val foo: Int,
// annotate Java getter
@get:VisibleForTesting val bar: Boolean,
// annotate Java constructor parameter
@param:Mock val quux)
Annotation Qualifiers
PROPERTIES
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● Delegate the implementation of a property
● Has to implement getValue() for val
● Additionally setValue() for var
Property Delegates
PROPERTIES
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Property Delegates
PROPERTIES
interface Delegate<T> {
operator fun getValue(
thisRef: Any?,
property: KProperty<T>): T
operator fun setValue(
thisRef: Any?,
property: KProperty<T>,
value: T)
}
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● Delegate the implementation of a property
● Has to implement getValue() for val
● Additionally setValue() for var

class C {
var p: T by MyDelegate()
}
// Compiles down to:
class C {
private val p$delegate = MyDelegate()
var p: T
get() = p$delegate.getValue(this, this::p)
set(v: T) = p$delegate.setValue(this, this::p, v)
}
Property Delegates
PROPERTIES
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class Foo {
val foo: String by lazy { "moo" }
val bar: Int by lazy { 5 }
}
Property Delegates
PROPERTIES
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open class BackedByMap {
protected val _map = mutableMapOf<String, Any?>()
}
class Foo : BackedByMap() {
var foo: String? by _map
var bar: Int? by _map
}
Property Delegates
Magic
PROPERTIES
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}
}
class User(val map: Map<String, Any?>) {
val name: String by map
val age: Int by map
}
Property Delegates
Magic
PROPERTIES
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Property Delegates
Magic
PROPERTIES
}
}
val f = Foo()
f.foo = "moo"
f.bar = 5
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Property Delegates
Magic
PROPERTIES
val map get() = _map.toMap()
}
}
val f = Foo()
println(f.map) // {}
f.foo = "moo"
f.bar = 5
println(f.map) // {foo=moo, bar=5}
f.foo = null
println(f.map) // {foo=null, bar=5}
f.bar = null
println(f.map) // {foo=null, bar=null}
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Property Delegates
Magic
PROPERTIES
val map get() = _map.filterValues { it != null }
}
}
val f = Foo()
f.foo = "moo"
f.bar = 5
println(f.map) // {foo=moo, bar=5}
f.foo = null
println(f.map) // {bar=5}
f.bar = null
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● Kind of like lazy
● Limited to var
● No additional code
● Compiler will not check and may crash at runtime
● Only makes sense for non-null properties
lateinit
PROPERTIES
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● Every type has a nullable version
○ Any vs Any?Nullability
PROPERTIES
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Nullability
PROPERTIES
val any: Any = Any()
val maybe: Any? = null
fun show(maybe: Any?) = print(maybe)
fun reallyShow(any: Any) = print(any)
show(maybe)
show(any)
reallyShow(any)
reallyShow(maybe) // compiler error
maybe?.let(::reallyShow) // Coming Soon™
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● Every type has a nullable version - Any?
● Can supply non-nullable argument to a nullable
parameter
○ Vice-versa is a compiler error

Nullability
PROPERTIES
maybe.hashCode() // Compiler error
maybe?.hashCode() // OK
maybe?.hashCode().compareTo(42) // Compiler error
maybe?.hashCode()?.compareTo(42) // OK
if (maybe != null) {
maybe.hashCode() // Compiler knows value is nonnull
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parameter
● Safe invocation

Nullability
PROPERTIES
fun Any?.hashCode() = this?.hashCode() ?: 0
maybe.hashCode() // OK
maybe.hashCode().compareTo(42) // OK
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parameter
● Safe invocation
● You can extend nullable types

Nullability
PROPERTIES
val any: Any = Any()
fun Any?.hashCode() = this?.hashCode() ?: 0
maybe.hashCode() // OK
any.hashCode() // OK
maybe.hashCode().compareTo(42) // OK
y/kotlin-workbook
parameter
● Safe invocation
● You can extend nullable types

Questions?
Next Up: Extension Functions in kotlin-std-lib

Kotlin For Android - Properties (part 4 of 7)

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