[Lazarus] Windows.PostMessage vs Application.QueueAsyncCall

[Michael Schnell]

On 03/18/2014 05:52 PM, Joao Morais wrote:
> I missed the "async" part, sorry. What about reuse App.QueueAsyncCall 
> with interface?
>
Combining the Interface idea with asynchronous signaling in fact is a 
nice idea.

The real issue is that in many cases you need to queue not only the call 
but the parameters for the call as well.

Here we have:

  - TThread.Queue only calls the call (but of course this is the 
procedure address and the "self" of the instance you use in the argument 
when calling TThread.Queue
  - Application.QeueAsyncCall additionally takes a pointer as a 
parameter that is queued and when scheduled given to the signaled 
procedure called in the main thread

But obviously it can be necessary to queue more information than just a 
pointer.

I did a testing program using TThread.Queue.

I defined a "TData" class that contains the data and a procedure with no 
parameter.

When placing the event, in the worker Thread do:
   - create an instance of the TData" class, just storing the reference 
in a local variable " Data: TData;  "Data := TData.Create; " ("Data" 
funnily is long forgotten when the call in fact is scheduled in the main 
thread)
   - Fill "Data" with the data to be worked on in the main thread
   - do " Queue(Data.QueuedCall); "

Now some time later the call is scheduled in the main thread and 
QueuedCall starts running.
    - You can correctly access all data in this instance of the TData 
class. Any other work in the WorkerThread would have created a new 
independent instance of same
    - After doing what needs to be done you just do " free; " as the 
last line of the QueuedCall procedure. Here the current instance it 
works on is freed. This does not (seem to) be a problem, as after the 
free you return to completely unrelated stuff.


I am not experienced with Interfaces, but I do know that interfaces can 
create and free class instances automatically "under the hood", and 
hence it might be better programming style to use same instead o 
manually do a " Create " and " Free; " in user code.

If it would be possible to do such an interface driven thingy in a 
library in a way that the actual structure of the data can be defined in 
user code, this would be a great  addition (IMHO better for the RTL than 
for the LCL).

-Michael