Asynchronous Calls: what changed

Revision 1 to 2. Removed lines have a minus sign; added lines have a plus sign.

Old	New		Line
1	1		## Problem statement
2	2
3	3		When handling some event, you need to do something, but you can't do it right away. For example, you need to free an object, but it is or will be referenced somewhere in the parent (or its parent etc.) later on. Or you need to update GUI elements accessed by several threads in a thread safe manner.
4	4
5	5		## Solution
6	6
7	7		Call Application.[[doc:lcl/forms/tapplication.queueasynccall.html\|QueueAsyncCall]]:
8	8
9	9		[[doc:lcl/forms/tdataevent.html\|TDataEvent]]` = procedure (Data: PtrInt) of object;`\
10		-	\
	10	+
11	11		`procedure `[[doc:lcl/forms/tapplication.queueasynccall.html\|QueueAsyncCall]]`(AMethod: TDataEvent; Data: PtrInt);`\
12	12
13	13		This will "queue" the given method with the given parameter for execution in the main event loop, when all other events have been processed. In the example above, the reference to the object you wanted to free has gone, since the then-parent has finished execution, and the object you wanted to free can be freed safely.
14	14
15	15		Note that this is a more generic version of [[doc:lcl/forms/tapplication.releasecomponent.html\|ReleaseComponent]], and ReleaseComponent calls this method.
16	16
17	17		## Examples
18	18
19	19		### Simple data passed to async function
20	20
21	21		The following program shows the use of [[doc:lcl/forms/tapplication.queueasynccall.html\|QueueAsyncCall]]. If you press on the CallButton, 'Click 1', 'Click 2' and 'Async 1' is added to the LogListBox. Note that Async call is only executed after the CallButtonClick event has finished.
22	22
23	23		``` pascal
24	24		unit TestQueueAsyncCall;
25	25
26	26		{$mode objfpc}{$H+}
27	27
28	28		interface
29	29
30	30		uses
31	31		Classes, SysUtils, LResources, Forms, Controls, Graphics, Dialogs, Buttons,
32	32		StdCtrls;
33	33
34	34		type
35	35
36	36		{ TQueueAsyncCallForm }
37	37
38	38		TQueueAsyncCallForm = class(TForm)
39	39		CallButton: TButton;
40	40		LogListBox: TListBox;
41	41		procedure CallButtonClick(Sender: TObject);
42	42		private
43	43		{ private declarations }
44	44		FCounter: PtrInt;
45	45		procedure Async(Data: PtrInt);
46	46		public
47	47		{ public declarations }
48	48		end;
49	49
50	50		var
51	51		QueueAsyncCallForm: TQueueAsyncCallForm;
52	52
53	53		implementation
54	54
55	55		{$R *.lfm}
56	56
57	57		{ TQueueAsyncCallForm }
58	58
59	59		procedure TQueueAsyncCallForm.CallButtonClick(Sender: TObject);
60	60		begin
61	61		LogListBox.Items.Add('Click 1');
62	62		FCounter := FCounter+1;
63	63		Application.QueueAsyncCall(@Async,FCounter);
64	64		LogListBox.Items.Add('Click 2');
65	65		end;
66	66
67	67		procedure TQueueAsyncCallForm.Async(Data: PtrInt);
68	68		begin
69	69		LogListBox.Items.Add('Async '+ IntToStr(Data));
70	70		end;
71	71
72	72		end.
73	73		```
74	74
75	75		### Record passed to async function
76	76
77	77		Here is some code extracted from MultiLog's MemoChannel unit used for thread safe logging messages to Memo control. Thanks to QueueAsyncCall() all writings to Memo are serialized. There are no locks and no conflicts, even with hundreds of threads calling Write() method.
78	78
79	79		```delphi
80	80		...
81	81		type
82	82		TLogMsgData = record // record can hold much more then a simple string :-)
83	83		Text: string;
84	84		end;
85	85		PLogMsgData = ^TLogMsgData;
86	86		...
87	87		procedure TMemoChannel.Write(const AMsg: string);
88	88		var
89	89		LogMsgToSend: PLogMsgData;
90	90		begin
91	91		New(LogMsgToSend);
92	92		LogMsgToSend^.Text:= AMsg;
93	93		Application.QueueAsyncCall(@WriteAsyncQueue, PtrInt(LogMsgToSend)); // put log msg into queue that will be processed from the main thread after all other messages
94	94		end;
95	95		...
96	96		procedure TMemoChannel.WriteAsyncQueue(Data: PtrInt);
97	97		var // called from main thread after all other messages have been processed to allow thread safe TMemo access
98	98		ReceivedLogMsg: TLogMsgData;
99	99		begin
100	100		ReceivedLogMsg := PLogMsgData(Data)^;
101	101		try
102	102		if (FMemo <> nil) and (not Application.Terminated) then
103	103		begin
104	104		...
105	105		FMemo.Append(ReceivedLogMsg.Text) // <<< fully thread safe
106	106		end;
107	107		finally
108	108		Dispose(PLogMsgData(Data));
109	109		end;
110	110		end;
111	111		```
112	112
113	113		## See also
114	114
115	115		- [TThread.Queue, which provides the same functionality in a Delphi compatible way and with anonymous method support in FPC 3.3.x and later](https://www.freepascal.org/docs-html/rtl/classes/tthread.queue.html)
116	116		- [New 0.9.26 features. Part 1. SendMessage and PostMessage](http://lazarus-dev.blogspot.com/2008/01/new-0926-features-part-1-sendmessage.html)
117	117		- [[Main Loop Hooks]]
118	118		- [[Multithreaded Application Tutorial]]
119	119
120	120		---
121	121		From the Free Pascal wiki, page "Asynchronous Calls", as it stood on 2023-09-07 after a change by Marcov, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).

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