Run suspending calls within Dispatchers.Default

This addresses a need for off-main-thread invocation of
Call.Factory.newCall to support lazy HttpClient initialization.

Author: jingibus

retrofit/kotlin-test/src/test/java/retrofit2/KotlinSuspendTest.kt

@@ -17,6 +17,7 @@ package retrofit2
 
 import kotlinx.coroutines.CoroutineDispatcher
 import kotlinx.coroutines.GlobalScope
+import kotlinx.coroutines.Runnable
 import kotlinx.coroutines.async
 import kotlinx.coroutines.runBlocking
 import kotlinx.coroutines.withContext
@@ -353,6 +354,32 @@ class KotlinSuspendTest {
     }
   }
 
+  @Test fun usesCoroutineContextForCallFactory() {
+    val okHttpClient = OkHttpClient()
+    var callFactoryThread: Thread? = null
+    val outerContextThread: Thread
+    val retrofit = Retrofit.Builder()
+      .baseUrl(server.url("/"))
+      .callFactory {
+        callFactoryThread = Thread.currentThread()
+        okHttpClient.newCall(it)
+      }
+      .addConverterFactory(ToStringConverterFactory())
+      .build()
+    val example = retrofit.create(Service::class.java)
+
+    server.enqueue(MockResponse().setBody("Hi"))
+
+    runBlocking {
+      outerContextThread = Thread.currentThread()
+      example.body()
+    }
+
+    assertThat(callFactoryThread).isNotNull
+    assertThat(outerContextThread).isNotEqualTo(callFactoryThread)
+  }
+
+
   @Suppress("EXPERIMENTAL_OVERRIDE")
   private object DirectUnconfinedDispatcher : CoroutineDispatcher() {
     override fun isDispatchNeeded(context: CoroutineContext): Boolean = false

retrofit/src/main/java/retrofit2/KotlinExtensions.kt

@@ -20,7 +20,7 @@ package retrofit2
 
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.suspendCancellableCoroutine
-import java.lang.reflect.ParameterizedType
+import kotlinx.coroutines.withContext
 import kotlin.coroutines.intrinsics.COROUTINE_SUSPENDED
 import kotlin.coroutines.intrinsics.intercepted
 import kotlin.coroutines.intrinsics.suspendCoroutineUninterceptedOrReturn
@@ -30,57 +30,63 @@ import kotlin.coroutines.resumeWithException
 inline fun <reified T: Any> Retrofit.create(): T = create(T::class.java)
 
 suspend fun <T : Any> Call<T>.await(): T {
-  return suspendCancellableCoroutine { continuation ->
-    continuation.invokeOnCancellation {
-      cancel()
-    }
-    enqueue(object : Callback<T> {
-      override fun onResponse(call: Call<T>, response: Response<T>) {
-        if (response.isSuccessful) {
-          val body = response.body()
-          if (body == null) {
-            val invocation = call.request().tag(Invocation::class.java)!!
-            val method = invocation.method()
-            val e = KotlinNullPointerException("Response from " +
+  // TODO: a better solution for off-main-thread call factories than this.
+  return withContext(Dispatchers.Default) {
+    suspendCancellableCoroutine { continuation ->
+      continuation.invokeOnCancellation {
+        cancel()
+      }
+      enqueue(object : Callback<T> {
+        override fun onResponse(call: Call<T>, response: Response<T>) {
+          if (response.isSuccessful) {
+            val body = response.body()
+            if (body == null) {
+              val invocation = call.request().tag(Invocation::class.java)!!
+              val method = invocation.method()
+              val e = KotlinNullPointerException("Response from " +
                 method.declaringClass.name +
                 '.' +
                 method.name +
                 " was null but response body type was declared as non-null")
-            continuation.resumeWithException(e)
+              continuation.resumeWithException(e)
+            } else {
+              continuation.resume(body)
+            }
           } else {
-            continuation.resume(body)
+            continuation.resumeWithException(HttpException(response))
           }
-        } else {
-          continuation.resumeWithException(HttpException(response))
         }
-      }
 
-      override fun onFailure(call: Call<T>, t: Throwable) {
-        continuation.resumeWithException(t)
-      }
-    })
+        override fun onFailure(call: Call<T>, t: Throwable) {
+          continuation.resumeWithException(t)
+        }
+      })
+    }
   }
 }
 
 @JvmName("awaitNullable")
 suspend fun <T : Any> Call<T?>.await(): T? {
-  return suspendCancellableCoroutine { continuation ->
-    continuation.invokeOnCancellation {
-      cancel()
-    }
-    enqueue(object : Callback<T?> {
-      override fun onResponse(call: Call<T?>, response: Response<T?>) {
-        if (response.isSuccessful) {
-          continuation.resume(response.body())
-        } else {
-          continuation.resumeWithException(HttpException(response))
-        }
+  // TODO: a better solution for off-main-thread call factories than this.
+  return withContext(Dispatchers.Default) {
+    suspendCancellableCoroutine { continuation ->
+      continuation.invokeOnCancellation {
+        cancel()
       }
+      enqueue(object : Callback<T?> {
+        override fun onResponse(call: Call<T?>, response: Response<T?>) {
+          if (response.isSuccessful) {
+            continuation.resume(response.body())
+          } else {
+            continuation.resumeWithException(HttpException(response))
+          }
+        }
 
-      override fun onFailure(call: Call<T?>, t: Throwable) {
-        continuation.resumeWithException(t)
-      }
-    })
+        override fun onFailure(call: Call<T?>, t: Throwable) {
+          continuation.resumeWithException(t)
+        }
+      })
+    }
   }
 }
 
@@ -91,19 +97,22 @@ suspend fun Call<Unit>.await() {
 }
 
 suspend fun <T> Call<T>.awaitResponse(): Response<T> {
-  return suspendCancellableCoroutine { continuation ->
-    continuation.invokeOnCancellation {
-      cancel()
-    }
-    enqueue(object : Callback<T> {
-      override fun onResponse(call: Call<T>, response: Response<T>) {
-        continuation.resume(response)
+  // TODO: a better solution for off-main-thread call factories than this.
+  return withContext(Dispatchers.Default) {
+    suspendCancellableCoroutine { continuation ->
+      continuation.invokeOnCancellation {
+        cancel()
       }
+      enqueue(object : Callback<T> {
+        override fun onResponse(call: Call<T>, response: Response<T>) {
+          continuation.resume(response)
+        }
 
-      override fun onFailure(call: Call<T>, t: Throwable) {
-        continuation.resumeWithException(t)
-      }
-    })
+        override fun onFailure(call: Call<T>, t: Throwable) {
+          continuation.resumeWithException(t)
+        }
+      })
+    }
   }
 }