Add #162 IOS implementation

src/ios/RNFetchBlob/RNFetchBlob.m

     }
 })
 
+RCT_EXPORT_METHOD(df:(RCTResponseSenderBlock)callback
+{
+    [RNFetchBlobFS df:callback];
+})
+
 - (UIViewController *) documentInteractionControllerViewControllerForPreview: (UIDocumentInteractionController *) controller {
     UIWindow *window = [UIApplication sharedApplication].keyWindow;
     return window.rootViewController;

src/ios/RNFetchBlobFS.h

 //+ (void) writeFileFromFile:(NSString *)src toFile:(NSString *)dest append:(BOOL)append;
 + (void) writeAssetToPath:(ALAssetRepresentation * )rep dest:(NSString *)dest;
 + (void) readStream:(NSString *)uri encoding:(NSString * )encoding bufferSize:(int)bufferSize tick:(int)tick streamId:(NSString *)streamId bridgeRef:(RCTBridge *)bridgeRef;
++ (void) df:(RCTResponseSenderBlock)callback;
 
 // constructor
 - (id) init;

src/ios/RNFetchBlobFS.m

     }
 }
 
+#pragma mark - get disk space
+
++(void) df:(RCTResponseSenderBlock)callback
+{
+    uint64_t totalSpace = 0;
+    uint64_t totalFreeSpace = 0;
+    NSError *error = nil;
+    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
+    NSDictionary *dictionary = [[NSFileManager defaultManager] attributesOfFileSystemForPath:[paths lastObject] error: &error];
+    
+    if (dictionary) {
+        NSNumber *fileSystemSizeInBytes = [dictionary objectForKey: NSFileSystemSize];
+        NSNumber *freeFileSystemSizeInBytes = [dictionary objectForKey:NSFileSystemFreeSize];
+        totalSpace = [fileSystemSizeInBytes unsignedLongLongValue];
+        totalFreeSpace = [freeFileSystemSizeInBytes unsignedLongLongValue];
+        callback(@[[NSNull null], @{
+                  @"free" : [NSNumber numberWithInt:totalFreeSpace],
+                  @"total" : [NSNumber numberWithInt:totalSpace]
+                }]);
+    } else {
+        callback(@[@"failed to get storage usage."]);
+    }
+    
+}
+
 + (void) writeAssetToPath:(ALAssetRepresentation * )rep dest:(NSString *)dest
 {
     int read = 0;