added pathForAppGroup iOS function (#328)

fs.js

 
 }
 
+/**
+ * Returns the path for the app group.
+ * @param  {string} groupName Name of app group
+ * @return {Promise}
+ */
+function pathForAppGroup(groupName:string):Promise {
+  return RNFetchBlob.pathForAppGroup(groupName);
+}
+
 /**
  * Wrapper method of readStream.
  * @param  {string} path Path of the file.
   writeStream,
   writeFile,
   appendFile,
+  pathForAppGroup,
   readFile,
   exists,
   createFile,

ios/RNFetchBlob/RNFetchBlob.m

 
 }
 
+#pragma mark - fs.pathForAppGroup
+RCT_EXPORT_METHOD(pathForAppGroup:(NSString *)groupName
+                  resolver:(RCTPromiseResolveBlock)resolve
+                  rejecter:(RCTPromiseRejectBlock)reject)
+{
+    NSString * path = [RNFetchBlobFS getPathForAppGroup:groupName];
+
+    if(path) {
+        resolve(path);
+    } else {
+        reject(@"RNFetchBlob file not found", @"could not find path for app group", nil);
+    }
+}
+
 #pragma mark - fs.exists
 RCT_EXPORT_METHOD(exists:(NSString *)path callback:(RCTResponseSenderBlock)callback) {
     [RNFetchBlobFS exists:path callback:callback];

ios/RNFetchBlobFS.h

 + (NSString *) getDocumentDir;
 + (NSString *) getTempPath:(NSString*)taskId withExtension:(NSString *)ext;
 + (NSString *) getPathOfAsset:(NSString *)assetURI;
++ (NSString *) getPathForAppGroup:(NSString *)groupName;
 + (void) getPathFromUri:(NSString *)uri completionHandler:(void(^)(NSString * path, ALAssetRepresentation *asset)) onComplete;
 
 // fs methods

ios/RNFetchBlobFS.m

     return tempPath;
 }
 
++ (NSString *) getPathForAppGroup:(NSString *)groupName {
+    NSFileManager* fileManager = [NSFileManager defaultManager];
+    NSURL* containerURL = [fileManager containerURLForSecurityApplicationGroupIdentifier:groupName];
+    if(containerURL) {
+        return [containerURL path];
+    } else {
+        return nil;
+    }
+}
+
 #pragma margk - readStream
 
 + (void) readStream:(NSString *)uri