Kotlin可空变量赋值

Question

在Kotlin中,此代码的语法是否更短:

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

首先我试过这个:

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

但后来我意识到这不是一个任务,所以代码什么都不做.然后我试过:

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

哪个有效,但有点冗长.我希望这样的事情:

swipeView ?= view.find<MeasureTypePieChart>

但不幸的是,这不起作用.有没有办法用短语法完成这个？

我知道我可以这样做:

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

Answer 1

更短的语法将是避免swipeView永远存在null.

局部变量

如果swipeView是局部变量,则可以在最初分配时将其声明为非null:

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

函数参数

如果swipeView是函数参数,那么您可以使用默认参数来确保它永远不会null:

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

类属性

只读

如果swipeView是只读类属性(即val),则可以使用Kotlin的内置Lazy:

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

易变的

如果swipeView是一个可变类属性(即var),那么你可以定义自己的委托类似Lazy但可变.例如,以下是基于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."
}

用法:

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

该initializer如果将只能被称为swipeView是读取尚未初始化(从以前的读或写).