Add WeakRef

mirai-core/src/androidMain/kotlin/net/mamoe/mirai/utils/WeakRef.kt

@@ -0,0 +1,5 @@
+package net.mamoe.mirai.utils
+
+import java.lang.ref.WeakReference
+
+actual typealias WeakRef<T> = WeakReference<T>

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/utils/WeakRef.kt

@@ -0,0 +1,59 @@
+@file:Suppress("unused")
+
+package net.mamoe.mirai.utils
+
+import kotlin.reflect.KProperty
+
+/**
+ * WeakRef that `getValue` for delegation throws an [IllegalStateException] if the referent is released by GC. Therefore it returns notnull value only
+ */
+inline class UnsafeWeakRef<T>(private val weakRef: WeakRef<T>) {
+    fun get(): T = weakRef.get() ?: error("WeakRef is released")
+    fun clear() = weakRef.clear()
+
+    operator fun getValue(thisRef: Any?, property: KProperty<*>): T? = this.get()
+}
+
+/**
+ * Weak Reference.
+ * In JVM, this is implemented as a typealias to `WeakReference` from JDK.
+ *
+ * Reference details:
+ * In JVM, instances of objects are stored in the Heap and are accessed via references.
+ * GC can automatically collect and release the memory used by objects that are not directly referred by any other.
+ * WeakReference is not direct reference, therefore it does no influence on garbage collection.
+ * Using weak reference can help GC with that.
+ *
+ * @see weakRef provides a WeakRef
+ * @see unsafeWeakRef provides a UnsafeWeakRef
+ */
+expect class WeakRef<T>(referent: T) {
+    fun get(): T?
+    fun clear()
+}
+
+/**
+ * Provides a weak reference to [this]
+ * The `getValue` for delegation returns [this] when [this] is not released by GC
+ */
+fun <T> T.weakRef(): WeakRef<T> = WeakRef(this)
+
+/**
+ * Provides a weak reference to [this].
+ * The `getValue` for delegation throws an [IllegalStateException] if the referent is released by GC. Therefore it returns notnull value only
+ */
+fun <T> T.unsafeWeakRef(): UnsafeWeakRef<T> = UnsafeWeakRef(this.weakRef())
+
+/**
+ * Provides delegate value.
+ *
+ * ```kotlin
+ * val bot: Bot? by param.weakRef()
+ * ```
+ */
+operator fun <T> WeakRef<T>.getValue(thisRef: Any?, property: KProperty<*>): T? = this.get()
+
+/**
+ * Call the block if the referent is absent
+ */
+inline fun <T, R> WeakRef<T>.ifAbsent(block: (T) -> R): R? = this.get()?.let(block)

mirai-core/src/jvmMain/kotlin/net/mamoe/mirai/utils/WeakRef.kt

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+package net.mamoe.mirai.utils
+
+import java.lang.ref.WeakReference
+
+actual typealias WeakRef<T> = WeakReference<T>