Hey Ian,
It presents a simpler means of synchronization, allows the client to
block in an OS-independent way, and mimics the API of the MQTT
client libraries in other languages. A wait() function as the sole
blocking call is a common pattern in other modern asynchronous
libraries: Microsoft Async, C++11 threads/futures. the Paho Java
client library, etc. I'm not a GUI guy, I'm an RTOS guy, and do this
pattern a lot, where a completed action always signals an OS
synchronization object, but also fires an optional callback if the
user requested it.
In particular, as I originally mentioned, I'm trying to make the C++
API mimic the Java one as much as possible, like:
mqtt::itoken tok = client.publish("some topic",
myMessage);
// Do something else for a while
tok.wait_for_completion();
I can work around this in the C++ library by intercepting the
callbacks, but two issues make this a little more complicated due to
to the outgoing "send" thread in the C lib:
(1) There doesn't appear to be a way to get the MQTTAsync_token of a
message when you send it. And thus it's difficult to tie a token to
the message.
(2) Upon return of the send/sendMessage calls, the message has
"disappeared" into the queue of the send thread. It doesn't appear
in the list of outgoing messages (MQTTAsync_getPendingTokens) until
some time later. So it is easy for the app to think that all
messages were sent when some have yet to even be queued for
transmission.
Thanks for your time,
Frank
On 05/20/2013 05:30 AM, Ian Craggs
wrote:
A deliberate choice. Why have one
blocking call in an asynchronous API which is designed for GUI
use, amongst others? This is an event-oriented API - use the
deliveryComplete callback. To receive messages with MQTTAsync,
you have to use callbacks. You have to use callbacks to
determine the success of API calls.
For a blocking, simpler to use API, with less use of callbacks,
there is MQTTClient.
Ian
On 19/05/13 19:06, Frank Pagliughi wrote:
Well, while I'm throwing sand in the works, I might as well
continue...
One thing that appears to be "missing" from the C async API is
the ability to block a thread on a token while waiting for an
action to complete. Something like:
int MQTTAsync_waitForCompletion(MQTTAsync handle,
MQTTAsync_token token, int timeout);
This would prevent the app from wasting CPU cycles while
spinning on a flag from a callback, and can simplify client apps
that don't need the full power of a callback function by
eliminating the need for them for basic synchronization. Plus it
would allow the user to write more portable client apps, since
the library would hide the OS-specific thread functionality.
It appears that the library already wraps condition variables in
Linux. Maybe Windows could use a Manual Reset Event to do this?
Frank
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