[Bridge] Add support for JS async functions to RCT_EXPORT_METHOD

Summary:
Adds support for JS async methods and helps guide people writing native modules w.r.t. the callbacks. With this diff, on the native side you write:

```objc
RCT_EXPORT_METHOD(getValueAsync:(NSString *)key
                       resolver:(RCTPromiseResolver)resolve
                       rejecter:(RCTPromiseRejecter)reject)
{
  NSError *error = nil;
  id value = [_nativeDataStore valueForKey:key error:&error];

  // "resolve" and "reject" are automatically defined blocks that take
  // any object (nil is OK) and an NSError, respectively
  if (!error) {
    resolve(value);
  } else {
    reject(error);
  }
}
```

On the JS side, you can write:

```js
var {DemoDataStore} = require('react-native').NativeModules;
DemoDataStore.getValueAsync('sample-key').then((value) => {
  console.log('Got:', value);
}, (error) => {
  console.error(error);
  // "error" is an Error object whose message is the NSError's description.
  // The NSError's code and domain are also set, and the native trace i
Closes https://github.com/facebook/react-native/pull/1232
Github Author: James Ide <ide@jameside.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/BatchedBridge/BatchingImplementation/BatchedBridgeFactory.js

@@ -19,9 +19,17 @@ var slice = Array.prototype.slice;
 
 var MethodTypes = keyMirror({
   remote: null,
+  remoteAsync: null,
   local: null,
 });
 
+type ErrorData = {
+  message: string;
+  domain: string;
+  code: number;
+  nativeStackIOS?: string;
+};
+
 /**
  * Creates remotely invokable modules.
  */
@@ -36,21 +44,40 @@ var BatchedBridgeFactory = {
    */
   _createBridgedModule: function(messageQueue, moduleConfig, moduleName) {
     var remoteModule = mapObject(moduleConfig.methods, function(methodConfig, memberName) {
-      return methodConfig.type === MethodTypes.local ? null : function() {
+      switch (methodConfig.type) {
+        case MethodTypes.remote:
+          return function() {
             var lastArg = arguments.length > 0 ? arguments[arguments.length - 1] : null;
             var secondLastArg = arguments.length > 1 ? arguments[arguments.length - 2] : null;
-        var hasSuccCB = typeof lastArg === 'function';
+            var hasErrorCB = typeof lastArg === 'function';
-        var hasErrorCB = typeof secondLastArg === 'function';
+            var hasSuccCB = typeof secondLastArg === 'function';
-        hasErrorCB && invariant(
+            hasSuccCB && invariant(
-          hasSuccCB,
+              hasErrorCB,
               'Cannot have a non-function arg after a function arg.'
             );
             var numCBs = (hasSuccCB ? 1 : 0) + (hasErrorCB ? 1 : 0);
             var args = slice.call(arguments, 0, arguments.length - numCBs);
-        var onSucc = hasSuccCB ? lastArg : null;
+            var onSucc = hasSuccCB ? secondLastArg : null;
-        var onFail = hasErrorCB ? secondLastArg : null;
+            var onFail = hasErrorCB ? lastArg : null;
-        return messageQueue.call(moduleName, memberName, args, onFail, onSucc);
+            messageQueue.call(moduleName, memberName, args, onSucc, onFail);
           };
+
+        case MethodTypes.remoteAsync:
+          return function(...args) {
+            return new Promise((resolve, reject) => {
+              messageQueue.call(moduleName, memberName, args, resolve, (errorData) => {
+                var error = _createErrorFromErrorData(errorData);
+                reject(error);
+              });
+            });
+          };
+
+        case MethodTypes.local:
+          return null;
+
+        default:
+          throw new Error('Unknown bridge method type: ' + methodConfig.type);
+      }
     });
     for (var constName in moduleConfig.constants) {
       warning(!remoteModule[constName], 'saw constant and method named %s', constName);
@@ -59,7 +86,6 @@ var BatchedBridgeFactory = {
     return remoteModule;
   },
 
-
   create: function(MessageQueue, modulesConfig, localModulesConfig) {
     var messageQueue = new MessageQueue(modulesConfig, localModulesConfig);
     return {
@@ -80,4 +106,14 @@ var BatchedBridgeFactory = {
   }
 };
 
+function _createErrorFromErrorData(errorData: ErrorData): Error {
+  var {
+    message,
+    ...extraErrorInfo,
+  } = errorData;
+  var error = new Error(message);
+  error.framesToPop = 1;
+  return Object.assign(error, extraErrorInfo);
+}
+
 module.exports = BatchedBridgeFactory;

Libraries/Utilities/MessageQueue.js

@@ -441,14 +441,14 @@ var MessageQueueMixin = {
   },
 
   /**
-   * @param {Function} onFail Function to store in current thread for later
-   * lookup, when request fails.
    * @param {Function} onSucc Function to store in current thread for later
    * lookup, when request succeeds.
+   * @param {Function} onFail Function to store in current thread for later
+   * lookup, when request fails.
    * @param {Object?=} scope Scope to invoke `cb` with.
    * @param {Object?=} res Resulting callback ids. Use `this._POOLED_CBIDS`.
    */
-  _storeCallbacksInCurrentThread: function(onFail, onSucc, scope) {
+  _storeCallbacksInCurrentThread: function(onSucc, onFail, scope) {
     invariant(onFail || onSucc, INTERNAL_ERROR);
     this._bookkeeping.allocateCallbackIDs(this._POOLED_CBIDS);
     var succCBID = this._POOLED_CBIDS.successCallbackID;
@@ -506,7 +506,7 @@ var MessageQueueMixin = {
     return ret;
   },
 
-  call: function(moduleName, methodName, params, onFail, onSucc, scope) {
+  call: function(moduleName, methodName, params, onSucc, onFail, scope) {
     invariant(
       (!onFail || typeof onFail === 'function') &&
       (!onSucc || typeof onSucc === 'function'),
@@ -514,10 +514,10 @@ var MessageQueueMixin = {
     );
     // Store callback _before_ sending the request, just in case the MailBox
     // returns the response in a blocking manner.
-    if (onSucc) {
+    if (onSucc || onFail) {
-      this._storeCallbacksInCurrentThread(onFail, onSucc, scope, this._POOLED_CBIDS);
+      this._storeCallbacksInCurrentThread(onSucc, onFail, scope, this._POOLED_CBIDS);
+      onSucc && params.push(this._POOLED_CBIDS.successCallbackID);
       onFail && params.push(this._POOLED_CBIDS.errorCallbackID);
-      params.push(this._POOLED_CBIDS.successCallbackID);
     }
     var moduleID = this._remoteModuleNameToModuleID[moduleName];
     if (moduleID === undefined || moduleID === null) {

React/Base/RCTBridge.m

@@ -47,6 +47,11 @@ typedef NS_ENUM(NSUInteger, RCTBridgeFields) {
   RCTBridgeFieldFlushDateMillis
 };
 
+typedef NS_ENUM(NSUInteger, RCTJavaScriptFunctionKind) {
+  RCTJavaScriptFunctionKindNormal,
+  RCTJavaScriptFunctionKindAsync,
+};
+
 #ifdef __LP64__
 typedef struct mach_header_64 *RCTHeaderValue;
 typedef struct section_64 RCTHeaderSection;
@@ -204,6 +209,27 @@ static NSArray *RCTBridgeModuleClassesByModuleID(void)
   return RCTModuleClassesByID;
 }
 
+// TODO: Can we just replace RCTMakeError with this function instead?
+static NSDictionary *RCTJSErrorFromNSError(NSError *error)
+{
+  NSString *errorMessage;
+  NSArray *stackTrace = [NSThread callStackSymbols];
+  NSMutableDictionary *errorInfo =
+  [NSMutableDictionary dictionaryWithObject:stackTrace forKey:@"nativeStackIOS"];
+
+  if (error) {
+    errorMessage = error.localizedDescription ?: @"Unknown error from a native module";
+    errorInfo[@"domain"] = error.domain ?: RCTErrorDomain;
+    errorInfo[@"code"] = @(error.code);
+  } else {
+    errorMessage = @"Unknown error from a native module";
+    errorInfo[@"domain"] = RCTErrorDomain;
+    errorInfo[@"code"] = @-1;
+  }
+
+  return RCTMakeError(errorMessage, nil, errorInfo);
+}
+
 @class RCTBatchedBridge;
 
 @interface RCTBridge ()
@@ -239,6 +265,7 @@ static NSArray *RCTBridgeModuleClassesByModuleID(void)
 @property (nonatomic, copy, readonly) NSString *moduleClassName;
 @property (nonatomic, copy, readonly) NSString *JSMethodName;
 @property (nonatomic, assign, readonly) SEL selector;
+@property (nonatomic, assign, readonly) RCTJavaScriptFunctionKind functionKind;
 
 @end
 
@@ -400,6 +427,48 @@ case _value: { \
           }
         } else if ([argumentName isEqualToString:@"RCTResponseSenderBlock"]) {
           addBlockArgument();
+        } else if ([argumentName isEqualToString:@"RCTPromiseResolveBlock"]) {
+          RCTAssert(i == numberOfArguments - 2,
+                    @"The RCTPromiseResolveBlock must be the second to last parameter in -[%@ %@]",
+                    _moduleClassName, objCMethodName);
+          RCT_ARG_BLOCK(
+            if (RCT_DEBUG && ![json isKindOfClass:[NSNumber class]]) {
+              RCTLogError(@"Argument %tu (%@) of %@.%@ must be a promise resolver ID", index,
+                          json, RCTBridgeModuleNameForClass(_moduleClass), _JSMethodName);
+              return;
+            }
+
+            // Marked as autoreleasing, because NSInvocation doesn't retain arguments
+            __autoreleasing RCTPromiseResolveBlock value = (^(id result) {
+              NSArray *arguments = result ? @[result] : @[];
+              [bridge _invokeAndProcessModule:@"BatchedBridge"
+                                       method:@"invokeCallbackAndReturnFlushedQueue"
+                                    arguments:@[json, arguments]
+                                      context:context];
+            });
+          )
+          _functionKind = RCTJavaScriptFunctionKindAsync;
+        } else if ([argumentName isEqualToString:@"RCTPromiseRejectBlock"]) {
+          RCTAssert(i == numberOfArguments - 1,
+                    @"The RCTPromiseRejectBlock must be the last parameter in -[%@ %@]",
+                    _moduleClassName, objCMethodName);
+          RCT_ARG_BLOCK(
+            if (RCT_DEBUG && ![json isKindOfClass:[NSNumber class]]) {
+              RCTLogError(@"Argument %tu (%@) of %@.%@ must be a promise rejecter ID", index,
+                          json, RCTBridgeModuleNameForClass(_moduleClass), _JSMethodName);
+              return;
+            }
+
+            // Marked as autoreleasing, because NSInvocation doesn't retain arguments
+            __autoreleasing RCTPromiseRejectBlock value = (^(NSError *error) {
+              NSDictionary *errorJSON = RCTJSErrorFromNSError(error);
+              [bridge _invokeAndProcessModule:@"BatchedBridge"
+                                       method:@"invokeCallbackAndReturnFlushedQueue"
+                                    arguments:@[json, @[errorJSON]]
+                                      context:context];
+            });
+          )
+          _functionKind = RCTJavaScriptFunctionKindAsync;
         } else {
 
           // Unknown argument type
@@ -427,9 +496,18 @@ case _value: { \
 
     // Safety check
     if (arguments.count != _argumentBlocks.count) {
+      NSInteger actualCount = arguments.count;
+      NSInteger expectedCount = _argumentBlocks.count;
+
+      // Subtract the implicit Promise resolver and rejecter functions for implementations of async functions
+      if (_functionKind == RCTJavaScriptFunctionKindAsync) {
+        actualCount -= 2;
+        expectedCount -= 2;
+      }
+
       RCTLogError(@"%@.%@ was called with %zd arguments, but expects %zd",
                   RCTBridgeModuleNameForClass(_moduleClass), _JSMethodName,
-                  arguments.count, _argumentBlocks.count);
+                  actualCount, expectedCount);
       return;
     }
   }
@@ -535,7 +613,7 @@ static RCTSparseArray *RCTExportedMethodsByModuleID(void)
  *     },
  *     "methodName2": {
  *       "methodID": 1,
- *       "type": "remote"
+ *       "type": "remoteAsync"
  *     },
  *     etc...
  *   },
@@ -559,7 +637,7 @@ static NSDictionary *RCTRemoteModulesConfig(NSDictionary *modulesByName)
       [methods enumerateObjectsUsingBlock:^(RCTModuleMethod *method, NSUInteger methodID, BOOL *_stop) {
         methodsByName[method.JSMethodName] = @{
           @"methodID": @(methodID),
-          @"type": @"remote",
+          @"type": method.functionKind == RCTJavaScriptFunctionKindAsync ? @"remoteAsync" : @"remote",
         };
       }];
 

React/Base/RCTBridgeModule.h

@@ -17,6 +17,20 @@
  */
 typedef void (^RCTResponseSenderBlock)(NSArray *response);
 
+/**
+ * Block that bridge modules use to resolve the JS promise waiting for a result.
+ * Nil results are supported and are converted to JS's undefined value.
+ */
+typedef void (^RCTPromiseResolveBlock)(id result);
+
+/**
+ * Block that bridge modules use to reject the JS promise waiting for a result.
+ * The error may be nil but it is preferable to pass an NSError object for more
+ * precise error messages.
+ */
+typedef void (^RCTPromiseRejectBlock)(NSError *error);
+
+
 /**
  * This constant can be returned from +methodQueue to force module
  * methods to be called on the JavaScript thread. This can have serious
@@ -37,7 +51,7 @@ extern const dispatch_queue_t RCTJSThread;
  * A reference to the RCTBridge. Useful for modules that require access
  * to bridge features, such as sending events or making JS calls. This
  * will be set automatically by the bridge when it initializes the module.
-* To implement this in your module, just add @synthesize bridge = _bridge;
+ * To implement this in your module, just add @synthesize bridge = _bridge;
  */
 @property (nonatomic, weak) RCTBridge *bridge;
 
@@ -70,6 +84,26 @@ extern const dispatch_queue_t RCTJSThread;
  * { ... }
  *
  * and is exposed to JavaScript as `NativeModules.ModuleName.doSomething`.
+ *
+ * ## Promises
+ *
+ * Bridge modules can also define methods that are exported to JavaScript as
+ * methods that return a Promise, and are compatible with JS async functions.
+ *
+ * Declare the last two parameters of your native method to be a resolver block
+ * and a rejecter block. The resolver block must precede the rejecter block.
+ *
+ * For example:
+ *
+ * RCT_EXPORT_METHOD(doSomethingAsync:(NSString *)aString
+ *                           resolver:(RCTPromiseResolveBlock)resolve
+ *                           rejecter:(RCTPromiseRejectBlock)reject
+ * { ... }
+ *
+ * Calling `NativeModules.ModuleName.doSomethingAsync(aString)` from
+ * JavaScript will return a promise that is resolved or rejected when your
+ * native method implementation calls the respective block.
+ *
  */
 #define RCT_EXPORT_METHOD(method) \
   RCT_REMAP_METHOD(, method)
@@ -118,7 +152,7 @@ extern const dispatch_queue_t RCTJSThread;
   RCT_EXTERN_REMAP_MODULE(, objc_name, objc_supername)
 
 /**
- * Similar to RCT_EXTERN_MODULE but allows setting a custom JavaScript name
+ * Like RCT_EXTERN_MODULE, but allows setting a custom JavaScript name.
  */
 #define RCT_EXTERN_REMAP_MODULE(js_name, objc_name, objc_supername) \
   objc_name : objc_supername \
@@ -136,7 +170,7 @@ extern const dispatch_queue_t RCTJSThread;
   RCT_EXTERN_REMAP_METHOD(, method)
 
 /**
- * Similar to RCT_EXTERN_REMAP_METHOD but allows setting a custom JavaScript name
+ * Like RCT_EXTERN_REMAP_METHOD, but allows setting a custom JavaScript name.
  */
 #define RCT_EXTERN_REMAP_METHOD(js_name, method) \
   - (void)__rct_export__##method { \