Author: Lou Adler
I was surprised to find that I didn't have an article covering this topic - I wrote
a couple of 'em for inquiry.com a couple of years ago. Oh well... they're in
another place and time... In any case, what I'll give you here are two techniques
for handling previous instances of your applications.
Answer:
Oh! Were that only the phrase I used when I was single! Alas, like most, it
unfortunately wasn't. But at least I could apply that phrase in my professional
life; of course, in a totally different way. Okay, enough of the play on words.
There are many applications that lend themselves to having more than one copy
running at any time. But in my experience, most of the applications I build only
lend themselves to a single instance. For instance, it isn't practical to run more
than one instance of a data-entry application; especially when it updates local
data. In cases such as this, I limit the execution of another instance of a program
by executing some simple code.
What I'm going to show you is two different functions that do exactly the same
thing. The only difference between the two is that the first function can only be
run in Win32, and the other function can run in either Win16 or Win32. Here's the
code:
1
2 // ===================================================
3 // Called by your project file, prevents a 2nd
4 // instance of the program from executing and
5 // instead activates the already executing instance.
6 // Returns TRUE if a previous instance of the
7 // program is already running. Win32 ONLY
8 // ===================================================
9
10 function IsPrevInst: Boolean;
11 var
12 semName,
13 appClass: PChar;
14 hSem: THandle;
15 hWndMe: HWnd;
16 appTitle: array[0..MAX_PATH] of Char;
17 begin
18 // Init
19 Result := FALSE;
20 GetMem(semName, 15);
21 GetMem(appClass, 15);
22 StrPCopy(semName, 'SemaphoreName');
23 StrPCopy(appClass, 'TApplication');
24 StrPCopy(appTitle, ExtractFileName(Application.Title));
25
26 // Create a Semaphore in memory. If this is the
27 // first instance, then hSem's value should be 0.
28 hSem := CreateSemaphore(nil, 0, 1, semName);
29
30 // Check to see if the semaphore exists
31 if (hSem <> 0) and (GetLastError() =
32 ERROR_ALREADY_EXISTS) then
33 begin
34 CloseHandle(hSem);
35
36 // Get the current window's handle then change
37 // its title so we can look for the other instance
38 hWndMe := FindWindow(appClass, appTitle);
39 SetWindowText(hWndMe, 'ZZZZZZZ');
40
41 // Search for other instance of this window then bring
42 // it to the top of the Z-order stack. We find it by
43 // matching the Application Class and
44 // Application Title.
45 hWndMe := FindWindow(appClass, appTitle);
46 if (hWndMe <> 0) then
47 begin
48 BringWindowToTop(hWndMe);
49 ShowWindow(hWndMe, SW_SHOWNORMAL);
50 end;
51
52 Result := TRUE;
53 end;
54
55 // Destroy PChars
56 FreeMem(semName, 15);
57 FreeMem(appClass, 15);
58 end;
59
60 //This is a different twist on the previous example.
61 //It uses a mutex (MUTually EXclusive) instead of a sema-
62 //phore.
63
64 procedure CheckPrevInstEx(MainFormClassName,
65 MainFormCaption: string);
66 var
67 PrevWnd: HWnd;
68 Mutex: THandle;
69 begin
70 {$IFDEF Win32}
71 Mutex := CreateMutex(nil, False, 'InstanceMutex');
72 if WaitForSingleObject(Mutex, 10000) = WAIT_TIMEOUT then
73 Application.Terminate;
74 {$ELSE}
75 if HPrevInst = 0 then
76 Application.Terminate;
77 {$ENDIF}
78
79 PrevWnd := FindWindow(PChar(MainFormClassName),
80 PChar(MainFormCaption));
81 if PrevWnd <> 0 then
82 PrevWnd := GetWindow(PrevWnd, GW_OWNER);
83 if PrevWnd <> 0 then
84 begin
85 if IsIconic(PrevWnd) then
86 ShowWindow(PrevWnd, SW_SHOWNORMAL)
87 else
88 {$IFDEF Win32}
89 SetForegroundWindow(PrevWnd);
90 {$ELSE}
91 BringWindowToTop(PrevWnd);
92 {$ENDIF}
93 Application.Terminate;
94 end;
95 ReleaseMutex(Mutex);
96 CloseHandle(Mutex);
97 end;
98
99 {To use the functions above, you can either embed them in the project file, or
100 better yet, place them in a globally accessible library for use in all your
101 applications that need them. Here's some example code for implementing them:}
102
103 //This example uses the IsPrevInst function
104 program RxProto;
105
106 uses
107 Forms,
108 Main in 'Main.pas' {MainForm},
109 Proc in 'Proc.pas',
110 //This is my global library
111 UTIL32 in '..\Lib\UTIL\Util32.pas',
112 LoopPnThr in '..\Packages\LoopPnThr.pas';
113
114 {$R *.RES}
115
116 begin
117 if not IsPrevInst then
118 begin
119 Application.Initialize;
120 Application.CreateForm(TMainForm, MainForm);
121 Application.Run;
122 end
123 else
124 Application.Terminate;
125 end.
126 //Here's the other way...
127 program RxProto;
128
129 uses
130 Forms,
131 Main in 'Main.pas' {MainForm},
132 Proc in 'Proc.pas',
133 UTIL32 in '..\Lib\UTIL\Util32.pas',
134 LoopPnThr in '..\Packages\LoopPnThr.pas';
135
136 {$R *.RES}
137
138 begin
139 CheckPrevInstEx('TApplication', 'My Application');
140 //This code won't do anything if CheckPrevInstEx doesn't
141 //pass muster
142 Application.Initialize;
143 Application.CreateForm(TMainForm, MainForm);
144 Application.Run;
145 end.
As you can see, pretty simple stuff. Have fun with it!
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