Add SuspendLazy

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

+@file:Suppress("unused", "FunctionName")
+
+package net.mamoe.mirai.utils
+
+import kotlinx.coroutines.CompletableDeferred
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Deferred
+import kotlinx.coroutines.async
+import net.mamoe.mirai.contact.QQ
+
+/**
+ * 创建一个在当前 [CoroutineScope] 下执行的 [SuspendLazy]
+ */
+fun <R> CoroutineScope.SuspendLazy(initializer: suspend () -> R): SuspendLazy<R> = SuspendLazy(this, initializer)
+
+/**
+ * 挂起初始化的 [lazy], 是属性不能 `suspend` 的替代品.
+ *
+ * 本对象代表值初始化时将会通过 [CoroutineScope.async] 运行 [initializer]
+ *
+ * [SuspendLazy] 是:
+ * - 线程安全
+ * - 只会被初始化一次.
+ * - `SuspendLazy<R>.value` 返回 `Deferred<R>`
+ * - 可以用作属性代表 (`val profile: by SuspendLazy(GlobalScope) { calculateValue() }`)
+ *
+ * @sample QQ.profile
+ */
+class SuspendLazy<R>(scope: CoroutineScope, val initializer: suspend () -> R) : Lazy<Deferred<R>> {
+    private val valueUpdater: Deferred<R> by lazy { scope.async { initializer() } }
+
+    @Suppress("EXPERIMENTAL_API_USAGE")
+    override val value: Deferred<R>
+        get() = if (isInitialized()) {
+            CompletableDeferred(valueUpdater.getCompleted())
+        } else {
+            CompletableDeferred<R>().apply {
+                valueUpdater.invokeOnCompletion {
+                    this.complete(valueUpdater.getCompleted())
+                }
+            }
+        }
+
+    override fun isInitialized(): Boolean = valueUpdater.isCompleted
+}
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