Variable Not Kotlin

Question

In Kotlin, is there a shorter syntax for this code:

if(swipeView == null){ swipeView = view.find<MeasureTypePieChart>(R.id.swipeableView) } 

First I tried this:

  swipeView ?: view.find<MeasureTypePieChart>(R.id.swipeableView) 

but then I realized that this is not the destination, so the code does nothing. Then I tried:

 swipeView = swipeView ?: view.find<MeasureTypePieChart>(R.id.swipeableView) 

Which works, but it's a bit verbose. I would expect something like this:

 swipeView ?= view.find<MeasureTypePieChart> 

But, unfortunately, this does not work. Is there a way to accomplish this using short syntax?

I know I can do this:

 variable?.let { it = something } which works. 

Answer 1

A shorter syntax would be to avoid swipeView from ever null .

Local variable

If swipeView is a local variable, you can declare it non-empty when it first assigns it:

 val swipeView = ... ?: view.find<MeasureTypePieChart>(R.id.swipeableView) 

Function argument

If swipeView is a function argument, you can use the default argument so that it is never null :

 fun something(swipeView: View = view.find<MeasureTypePieChart>(R.id.swipeableView)) 

Class property

only for reading

If swipeView is a read-only class property (i.e. val ), you can use the built-in Kotlin Lazy :

 val swipeView by lazy { view.find<MeasureTypePieChart>(R.id.swipeableView) } 

Mutable

If swipeView is a property of a mutable class (i.e. var ), you can define your own delegate, similar to Lazy , but mutable. For example, the following is a description of kotlin / Lazy.kt :

 interface MutableLazy<T> : Lazy<T> { override var value: T } fun <T> mutableLazy(initializer: () -> T): MutableLazy<T> = SynchronizedMutableLazyImpl(initializer) fun <T> mutableLazy(lock: Any?, initializer: () -> T): MutableLazy<T> = SynchronizedMutableLazyImpl(initializer, lock) operator fun <T> MutableLazy<T>.getValue(thisRef: Any?, property: KProperty<*>): T = value operator fun <T> MutableLazy<T>.setValue(thisRef: Any?, property: KProperty<*>, value: T) { this.value = value } private object UNINITIALIZED_VALUE private class SynchronizedMutableLazyImpl<T>(initializer: () -> T, lock: Any? = null) : MutableLazy<T>, Serializable { private var initializer: (() -> T)? = initializer @Volatile private var _value: Any? = UNINITIALIZED_VALUE // final field is required to enable safe publication of constructed instance private val lock = lock ?: this override var value: T get() { val _v1 = _value if (_v1 !== UNINITIALIZED_VALUE) { @Suppress("UNCHECKED_CAST") return _v1 as T } return synchronized(lock) { val _v2 = _value if (_v2 !== UNINITIALIZED_VALUE) { @Suppress("UNCHECKED_CAST") (_v2 as T) } else { val typedValue = initializer!!() _value = typedValue initializer = null typedValue } } } set(value) { val _v1 = _value if (_v1 !== UNINITIALIZED_VALUE) { _value = value } else synchronized(lock) { _value = value initializer = null } } override fun isInitialized(): Boolean = _value !== UNINITIALIZED_VALUE override fun toString(): String = if (isInitialized()) value.toString() else "MutableLazy value not initialized yet." } 

Using:

 var swipeView by mutableLazy { view.find<MeasureTypePieChart>(R.id.swipeableView) } 

initializer is called only if the swipeView is swipeView read and not yet initialized (from a previous read or write).