kotlin - Kotlin可为空的变量赋值

在Kotlin中，此代码是否有任何较短的语法:

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

首先我尝试了这个:

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

但后来我意识到这不是一项任务，因此该代码不执行任何操作。然后我尝试了:

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

哪个可行，但是有点冗长。我期望这样的事情:

swipeView ?= view.find<MeasureTypePieChart>

但不幸的是，这行不通。有什么办法可以用简短的语法来做到这一点？

我知道我可以这样做:

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

最佳答案

较短的语法是避免swipeView成为null。

局部变量

如果swipeView是局部变量，则在初始分配它时可以将其声明为非null:

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

函数参数

如果swipeView是函数参数，则可以使用default argument来确保它永远不是null:

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

类属性

只读

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

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

可变的

如果swipeView是可变的类属性(即var)，那么您可以定义自己的类似于Lazy的委托(delegate)，但是可变。例如以下基于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) }

仅当initializer被读取且尚未初始化(从先前的读取或写入)时，才会调用swipeView。