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On Mar 9, 2012, at 8:14 PM, Alex Zavatone wrote:
> I've been reading (and trying out) a few approaches on HTTP communication
> using GCD to do a dispatch_sync or dispatch_async to a dispatch queue or
> using an NSURLRequest.
>
> Which of these is the preferred method for ingesting strings from HTTP URLs
> on iOS? Are there any plusses to one over the other?
This certainly depends how large your data is which you want to receive and how
expensive it is (CPU wise) to process the data.
I would use NSURLConnection / NSURLRequest in an asynchronous manner. You
handle each chunk of incoming data in the connection's delegate method:
- (void) connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
[self.dataProcessor handleDataChunk:data];
}
Usually, your "data chunk processing method" will process the data
synchronously. For smaller data, this approach is sufficient.
If the data received by the connection is large, the handling of data may take
considerably time, and if the delegates are scheduled on the main thread your
main thread may become busy, and the UI actions may stutter. A simple approach
is to save the data to a temporary file, then process it. However, on devices
with faster hardware and more than one CPU, this approach is suboptimal
performance wise.
Another naive approach to alleviate the above problem, would be to
asynchronously schedule the method handleDataChunk: onto a dispatch queue.
However, this approach is not scaleable for large data and may not work on
restricted devices with limited memory and slow CPUs. While the main thread
passes the work to another thread and remains responsive, all the received data
chunks will be queued up in the dispatch queue waiting there to be processed.
If the data comes in fast, and the processing is slow and if you have large
data sets, well this may tie up too much memory on iOS and you crash.
If you have to implement a scaleable solution, which shall be able to handle
large data and yielding optimal performance a more elaborated solution is
required. This would essentially require to block the thread where the
connection receives data which in turn will force the underlaying connection to
stop reading further data from the socket.
You can active this as above, except that you need to design your
handleDataChunk: such that it blocks if it received a data chunk and is
currently processing the data. Then you need also to start the NSURLConnection
on a secondary thread so that your delegate will be executed on this secondary
thread.
Andreas
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