chore: core 多模块骨架

core/domain/src/main/kotlin/com/mingyuan/android_client/core/domain/UseCase.kt

@@ -0,0 +1,11 @@
+package com.mingyuan.android_client.core.domain
+
+/**
+ * Base contract for a single-action use case.
+ *
+ * Keeping use cases as plain interfaces with a `suspend operator invoke` keeps domain logic
+ * framework-agnostic and trivially testable on the JVM (see [com.mingyuan.android_client.core.domain.UseCaseTest]).
+ */
+interface UseCase<in Input, out Output> {
+    suspend operator fun invoke(input: Input): Output
+}

core/domain/src/test/kotlin/com/mingyuan/android_client/core/domain/UseCaseTest.kt

@@ -0,0 +1,17 @@
+package com.mingyuan.android_client.core.domain
+
+import com.google.common.truth.Truth.assertThat
+import kotlinx.coroutines.test.runTest
+import org.junit.Test
+
+class UseCaseTest {
+
+    @Test
+    fun `invoke delegates to the implementation`() = runTest {
+        val double = object : UseCase<Int, Int> {
+            override suspend fun invoke(input: Int): Int = input * 2
+        }
+
+        assertThat(double(21)).isEqualTo(42)
+    }
+}