Fix #89 IOS slice precision

Author: wkh237

src/ios/RNFetchBlob/RNFetchBlob.m

@@ -364,9 +364,7 @@ - (NSDictionary *)constantsToExport
 
 RCT_EXPORT_METHOD(slice:(NSString *)src dest:(NSString *)dest start:(nonnull NSNumber *)start end:(nonnull NSNumber *)end resolver:(RCTPromiseResolveBlock)resolve rejecter:(RCTPromiseRejectBlock)reject
 {
-     dispatch_sync(dispatch_get_main_queue(),^(void){
-         [RNFetchBlobFS slice:src dest:dest start:start end:end encode:@"" resolver:resolve rejecter:reject];
-     });
+    [RNFetchBlobFS slice:src dest:dest start:start end:end encode:@"" resolver:resolve rejecter:reject];
 })
 
 #pragma mark RNFetchBlob private methods

src/ios/RNFetchBlobFS.m

@@ -542,33 +542,38 @@ + (void)slice:(NSString *)path
     // abort for the source file not exists
     if([fm fileExistsAtPath:path] == NO)
     {
-        reject(@"RNFetchBlob slice failed", @"the file does not exists", path);
+        reject(@"RNFetchBlob slice failed : the file does not exists", path, nil);
         return;
     }
     long size = [fm attributesOfItemAtPath:path error:nil].fileSize;
-    // abort for the file size is less than start
-    if(size < [start longValue])
-    {
-        reject(@"RNFetchBlob slice failed", @"start is greater than file size", @"start is greater than file size");
-        return;
-    }
+    long max = MIN(size, [end longValue]);
+    
     if(![fm fileExistsAtPath:dest]) {
         [fm createFileAtPath:dest contents:@"" attributes:nil];
     }
     [handle seekToFileOffset:[start longValue]];
     while(read < expected)
     {
-
-        NSData * chunk = [handle readDataOfLength:10240];
-        long remain = expected - read;
-        if(remain < 10240)
+        
+        NSData * chunk;
+        long chunkSize = 0;
+        if([start longValue] + read + 10240 > max)
         {
-            [os write:[chunk bytes] maxLength:remain];
+            NSLog(@"read chunk %lu", max - read - [start longValue]);
+            chunkSize = max - read - [start longValue];
+            chunk = [handle readDataOfLength:chunkSize];
         }
         else
         {
-            [os write:[chunk bytes] maxLength:10240];
+            NSLog(@"read chunk %lu", 10240);
+            chunkSize = 10240;
+            chunk = [handle readDataOfLength:10240];
         }
+        if([chunk length] <= 0)
+            break;
+        long remain = expected - read;
+    
+        [os write:[chunk bytes] maxLength:chunkSize];
         read += [chunk length];
     }
     [handle closeFile];