fill_async
Description:
public virtual async ssize_t fill_async (ssize_t count, int io_priority = DEFAULT, Cancellable? cancellable = null) throws Error
  
  Reads data into this's buffer asynchronously, up to count size.
 io_priority can be used to prioritize reads. For the synchronous version of this function, see 
      fill.
If count is -1 then the attempted read size is equal to the number of bytes that are required to fill the buffer.
Parameters:
| this | |
| count | the number of bytes that will be read from the stream | 
| io_priority | the I/O priority of the request | 
| cancellable | optional Cancellable object | 
| callback | |
| user_data | a pointer |