Add utility loadService

mirai-core-utils/src/androidMain/kotlin/Actuals.kt

@@ -13,6 +13,9 @@
 package net.mamoe.mirai.utils
 
 import android.util.Base64
+import java.util.*
+import kotlin.reflect.KClass
+import kotlin.reflect.full.createInstance
 
 
 public actual fun ByteArray.encodeBase64(): String {
@@ -40,3 +43,16 @@ public actual inline fun <reified E> Throwable.unwrap(): Throwable {
         ?.also { it.addSuppressed(e) }
         ?: this
 }
+
+public actual fun <T : Any> loadService(clazz: KClass<out T>, fallbackImplementation: String?): T {
+    var suppressed: Throwable? = null
+    return ServiceLoader.load(clazz.java).firstOrNull()
+        ?: runCatching {
+            Class.forName(fallbackImplementation).cast<Class<out T>>().kotlin.run { objectInstance ?: createInstance() }
+        }.onFailure {
+            suppressed = it
+        }.getOrNull()
+        ?: throw NoSuchElementException("Could not find an implementation for service class ${clazz.qualifiedName}").apply {
+            if (suppressed != null) addSuppressed(suppressed)
+        }
+}

mirai-core-utils/src/commonMain/kotlin/Services.kt

@@ -0,0 +1,16 @@
+/*
+ * Copyright 2019-2021 Mamoe Technologies and contributors.
+ *
+ * 此源代码的使用受 GNU AFFERO GENERAL PUBLIC LICENSE version 3 许可证的约束, 可以在以下链接找到该许可证.
+ * Use of this source code is governed by the GNU AGPLv3 license that can be found through the following link.
+ *
+ * https://github.com/mamoe/mirai/blob/dev/LICENSE
+ */
+
+package net.mamoe.mirai.utils
+
+import kotlin.reflect.KClass
+
+public expect fun <T : Any> loadService(clazz: KClass<out T>, fallbackImplementation: String? = null): T
+public inline fun <reified T : Any> loadService(fallbackImplementation: String? = null): T =
+    loadService(T::class, fallbackImplementation)

mirai-core-utils/src/jvmMain/kotlin/Actuals.kt

@@ -13,6 +13,8 @@
 package net.mamoe.mirai.utils
 
 import java.util.*
+import kotlin.reflect.KClass
+import kotlin.reflect.full.createInstance
 
 
 public actual fun ByteArray.encodeBase64(): String {
@@ -29,3 +31,16 @@ public actual inline fun <reified E> Throwable.unwrap(): Throwable {
         ?.also { it.addSuppressed(this) }
         ?: this
 }
+
+public actual fun <T : Any> loadService(clazz: KClass<out T>, fallbackImplementation: String?): T {
+    var suppressed: Throwable? = null
+    return ServiceLoader.load(clazz.java).firstOrNull()
+        ?: runCatching {
+            Class.forName(fallbackImplementation).cast<Class<out T>>().kotlin.run { objectInstance ?: createInstance() }
+        }.onFailure {
+            suppressed = it
+        }.getOrNull()
+        ?: throw NoSuchElementException("Could not find an implementation for service class ${clazz.qualifiedName}").apply {
+            if (suppressed != null) addSuppressed(suppressed)
+        }
+}