add read method with lenght and position params

FS.common.js

@@ -260,6 +260,36 @@ var RNFS = {
     return readFileGeneric(filepath, encodingOrOptions, RNFSManager.readFile);
   },
 
+  read(filepath: string, length = 0, position = 0, encodingOrOptions?: any): Promise<string> {
+  	var options = {
+      encoding: 'utf8'
+    };
+
+    if (encodingOrOptions) {
+      if (typeof encodingOrOptions === 'string') {
+        options.encoding = encodingOrOptions;
+      } else if (typeof encodingOrOptions === 'object') {
+        options = encodingOrOptions;
+      }
+    }
+
+    return RNFSManager.read(normalizeFilePath(filepath), length, position).then((b64) => {
+      var contents;
+
+      if (options.encoding === 'utf8') {
+        contents = utf8.decode(base64.decode(b64));
+      } else if (options.encoding === 'ascii') {
+        contents = base64.decode(b64);
+      } else if (options.encoding === 'base64') {
+        contents = b64;
+      } else {
+        throw new Error('Invalid encoding type "' + String(options.encoding) + '"');
+      }
+
+      return contents;
+    });
+  },
+
   // Android only
   readFileAssets(filepath: string, encodingOrOptions?: any): Promise<string> {
     if (!RNFSManager.readFileAssets) {

README.md

@@ -363,6 +363,12 @@ Reads the file at `path` and return contents. `encoding` can be one of `utf8` (d
 
 Note: you will take quite a performance hit if you are reading big files
 
+### `read(filepath: string, length = 0, position = 0, encodingOrOptions?: any): Promise<string>`
+
+Reads length bits from the given position of the file and returns contents. `encoding` can be one of `utf8` (default), `ascii`, `base64`. Use `base64` for reading binary files.
+
+Note: reading big files piece by piece using this method may be useful in terms of performance.
+
 ### `readFileAssets(filepath:string, encoding?: string): Promise<string>`
 
 Reads the file at `path` in the Android app's assets folder and return contents. `encoding` can be one of `utf8` (default), `ascii`, `base64`. Use `base64` for reading binary files.

RNFSManager.m

@@ -261,6 +261,51 @@ - (dispatch_queue_t)methodQueue
   resolve(base64Content);
 }
 
+RCT_EXPORT_METHOD(read:(NSString *)filepath
+                  length: (NSInteger *)length
+                  position: (NSInteger *)position
+                  resolver:(RCTPromiseResolveBlock)resolve
+                  rejecter:(RCTPromiseRejectBlock)reject)
+{
+    BOOL fileExists = [[NSFileManager defaultManager] fileExistsAtPath:filepath];
+
+    if (!fileExists) {
+        return reject(@"ENOENT", [NSString stringWithFormat:@"ENOENT: no such file or directory, open '%@'", filepath], nil);
+    }
+
+    NSError *error = nil;
+
+    NSDictionary *attributes = [[NSFileManager defaultManager] attributesOfItemAtPath:filepath error:&error];
+
+    if (error) {
+        return [self reject:reject withError:error];
+    }
+
+    if ([attributes objectForKey:NSFileType] == NSFileTypeDirectory) {
+        return reject(@"EISDIR", @"EISDIR: illegal operation on a directory, read", nil);
+    }
+
+    // Open the file handler.
+    NSFileHandle *file = [NSFileHandle fileHandleForReadingAtPath:filepath];
+    if (file == nil) {
+        return reject(@"EISDIR", @"EISDIR: Could not open file for reading", nil);
+    }
+
+    // Seek to the position if there is one.
+    [file seekToFileOffset: (int)position];
+
+    NSData *content;
+    if ((int)length > 0) {
+        content = [file readDataOfLength: (int)length];
+    } else {
+        content = [file readDataToEndOfFile];
+    }
+
+    NSString *base64Content = [content base64EncodedStringWithOptions:NSDataBase64EncodingEndLineWithLineFeed];
+
+    resolve(base64Content);
+}
+
 RCT_EXPORT_METHOD(hash:(NSString *)filepath
                   algorithm:(NSString *)algorithm
                   resolver:(RCTPromiseResolveBlock)resolve

android/src/main/java/com/rnfs/RNFSManager.java

@@ -150,6 +150,35 @@ public void readFile(String filepath, Promise promise) {
     }
   }
 
+  @ReactMethod
+  public void read(String filepath, int length, int position, Promise promise){
+    try {
+      File file = new File(filepath);
+
+      if (file.isDirectory()) {
+        rejectFileIsDirectory(promise);
+        return;
+      }
+
+      if (!file.exists()) {
+        rejectFileNotFound(promise, filepath);
+        return;
+      }
+
+      FileInputStream inputStream = new FileInputStream(filepath);
+      byte[] buffer = new byte[length];
+      inputStream.skip(position);
+      inputStream.read(buffer,0,length);
+
+      String base64Content = Base64.encodeToString(buffer, Base64.NO_WRAP);
+
+      promise.resolve(base64Content);
+    } catch (Exception ex) {
+        ex.printStackTrace();
+        reject(promise, filepath, ex);
+    }
+  }
+
   @ReactMethod
   public void readFileAssets(String filepath, Promise promise) {
     InputStream stream = null;