Property Delegates

Kotlin is packed with great language features, and delegated properties are a powerful way to specify how a property works and create re-usable policies for those properties. On top of the ones that exist in Kotlin's standard library, TornadoFX provides a few more property delegates that are particularly helpful for JavaFX development.

Single Assign

It is often ideal to initialize properties immediately upon construction. But inevitably there are times when this simply is not feasible. When a property needs to delay its initialization until it is first called, a lazy delegate is typically used. You specify a lambda instructing how the property value is initialized when its getter is called the first time.

val fooValue by lazy { buildExpensiveFoo() }

But there are situations where the property needs to be assigned later not by a value-supplying lambda, but rather some external entity at a later time. When we leverage type-safe builders we may want to save a Button to a class-level property so we can reference it later. If we do not want myButton to be nullable, we need to use the lateinit modifier.

class MyView: View() {

    lateinit var myButton: Button

    override val root = vbox {
        myButton = button("New Entry")
    }
}

The problem with lateinit is it can be assigned multiple times by accident, and it is not necessarily thread safe. This can lead to classic bugs associated with mutability, and you really should strive for immutability as much as possible ( Effective Java by Bloch, Item #13).

By leveraging the singleAssign() delegate, you can guarantee that property is only assigned once. Any subsequent assignment attempts will throw a runtime error, and so will accessing it before a value is assigned. This effectively gives us the guarantee of immutability, although it is enforced at runtime rather than compile time.

class MyView: View() {

    var myButton: Button by singleAssign()

    override val root = vbox {
        myButton = button("New Entry")
    }
}

Even though this single assignment is not enforced at compile time, infractions can be captured early in the development process. Especially as complex builder designs evolve and variable assignments move around, singleAssign() is an effective tool to mitigate mutability problems and allow flexible timing for property assignments.

By default, singleAssign() synchronizes access to its internal value. You should leave it this way especially if your application is multithreaded. If you wish to disable synchronization for whatever reason, you can pass a SingleAssignThreadSafetyMode.NONE value for the policy.

var myButton: Button by singleAssign(SingleAssignThreadSafetyMode.NONE)

JavaFX Property Delegate

Do not confuse the JavaFX Property with a standard Java/Kotlin "property". The Property is a special type in JavaFX that maintains a value internally and notifies listeners of its changes. It is proprietary to JavaFX because it supports binding operations, and will notify the UI when it changes. The Property is a core feature of JavaFX and has its own JavaBeans-like pattern.

This pattern is pretty verbose however, and even with Kotlin's syntax efficiencies it still is pretty verbose. You have to declare the traditional getter/setter as well as the Property item itself.

 class Bar {
    private val fooProperty by lazy { SimpleObjectProperty<T>() }
    fun fooProperty() = fooProperty
    var foo: T
        get() = fooProperty.get()
        set(value) = fooProperty.set(value)
}

Fortunately, TornadoFX can abstract most of this away. By delegating a Kotlin property to a JavaFX property(), TornadoFX will get/set that value against a new Property instance. To follow JavaFX's convention and provide the Property object to UI components, you can create a function that fetches the Property from TornadoFX and returns it.

class Bar {
    var foo by property<String>()
    fun fooProperty() = getProperty(Bar::foo)
}

Especially as you start working with TableView and other complex controls, you will likely find this pattern helpful when creating model classes, and this pattern is used in several places throughout this book.

Note you do not have to specify the generic type if you have an initial value to provide to the property. In the below example, it will infer the type as `String.

class Bar {
    var foo by property("baz")
    fun fooProperty() = getProperty(Bar::foo)
}

Alternative Property Syntax

There is also an alternative syntax which produces almost the same result:

import tornadofx.getValue
import tornadofx.setValue

class Bar {
    val fooProperty = SimpleStringProperty()
    var foo by fooProperty
}

Here you define the JavaFX property manually and delegate the getters and setters directly from the property. This might look cleaner to you, and so you are free to choose whatever syntax you are most comfortable with. However, the first alternative creates a JavaFX compliant property in that it exposes the Property via a function called fooProperty(), while the latter simply exposes a variable called fooProperty. For TornadoFX there is no difference, but if you interact with legacy libraries that require a property function you might need to stick with the first one.

Null safety of Properties

By default properties will have a Platform Type with uncertain nullability and completely ignore the null safety of Kotlin:

class Bar {
    var foo by property<String>()
    fun fooProperty() = getProperty(Bar::foo)

    val bazProperty = SimpleStringProperty()
    var baz by bazProperty

    init {
        foo = null
        foo.length // Will throw NPE during runtime

        baz = null
        baz.length // Will throw NPE during runtime
    }
}

To remedy this you can set the type of your property on the var (not on the Property-Object itself!). But keep in mind to set a default value on the property object when you set the var to be nullable or you will get an NPE anyways:

class Bar {
    var foo:String by property<String>("") // Non-nullable String with default value
    fun fooProperty() = getProperty(Bar::foo)

    val bazProperty = SimpleStringProperty() // No default needed
    var baz: String? by bazProperty // Nullable String

    init {
        foo = null // Will no longer compile
        foo.length

        baz = null
        baz.length // Will no longer compile
    }
}

FXML Delegate

If you have a given MyView View with a neighboring FXML file MyView.fxml defining the layout, the fxid() property delegate will retrieve the control defined in the FXML file. The control must have an fx:id that is the same name as the variable.

<Label fx:id="counterLabel">

Now we can inject this Label into our View class:

val counterLabel : Label by fxid()

Otherwise, the ID must be specifically passed to the delegate call.

val myLabel : Label by fxid("counterLabel")

Please read Chapter 10 to learn more about FXML.