I've done extensive work in multi-threaded applications. And in my experience, there
have been times when a particular program I'm writing should be written as a
multi-threaded application, but using the TThread object just seems like overkill.
For instance, I write a lot of single function programs; that is, the entire functionality
(beside the user interface portion) of the program is contained in one single execution
procedure or function. Usually, this procedure contains a looping mechanism (e.g. FOR,
WHILE, REPEAT) that operates on a table or an incredibly large text file (for me, that's
on the order of 500MB-plus!). Since it's just a single procedure, using a TThread
is just too much work for my preferences.
For those experienced Delphi programmers, you know what happens to the user interface
when you run a procedure with a loop in it: The application stops receiving messages.
The most simple way of dealing with this situation is to make a call to Application.ProcessMessages
within the body of the loop so that the application can still receive messages from
external sources. And in many, if not most, cases, this is a perfectly valid thing to do.
However, if some or perhaps even one of the steps within the loop take more than a couple
of seconds to complete processing — as in the case of a query — Application.ProcessMessages
is practically useless because the application will only receive messages at the time the
call is made. So what you ultimately achieve is intermittent response at best. Using a
thread, on the other hand, frees up the interface because the process is running
completely separate from the main thread of the program where the interface resides. So
regardless of what you execute within a loop that is running in a separate thread, your
interface will never get locked up.
Don't confuse the discussion above with multi-threaded user interfaces. What I'm
talking about is executing long background threads that won't lock up your user interface
while they run. This is an important distinction to make because it's not really
recommended to write multi-user interfaces, because each thread that is created in the
system has its own message queue. Thus, a message loop must be created to fetch messages
out of the queue so they can be dispatched appropriately. The TApplication object that
controls the UI would be the natural place to set up message loops for background threads,
but it's not set up to detect when other threads are executed. The gist of all this is
that the sole reason you create threads is to distribute processing of independent
tasks. Since the UI and controls are fairly integrated, threads just don't make sense here
because in order to make the separate threads work together, you have to synchronize them
to work in tandem, which practically defeats threading altogether!
I mentioned above that the TThread object is overkill for really simple threaded
stuff. This is strictly an opinion, but experience has made me lean that way. In any case,
what is the alternative to TThread in Delphi?
The solution isn't so much an alternative as it is going a bit more low-level into the
Windows API. I've said this several times before: The VCL is essentially one giant wrapper
around the Windows API and all its complexities. But fortunately for us, Delphi provides a
very easy way to access lower-level functionality beyond the wrapper interface with
which it comes. And even more fortunate for us, we can create threads using a simple
Windows API function called CreateThread to bypass the TThread object
altogether. As you'll see below, creating threads in this fashion is incredibly easy to
do.
Setting Yourself Up
There are two distinct steps for creating a thread: 1)Create the thread itself, then 2)
Provide a function that will act as the thread entry point. The thread function or thread
entry point is the function (actually the address of the function) that tells your
thread where to start.
Unlike a regular function, there are some specific requirements regarding the thread
function that you have to obey:
- You can give the function any name you want, but it must be a function name (ie.
function MyThreadFunc) - The function must have a single formal parameter of type Pointer (I'll discuss
this below) - The function return type is always LongInt
- Its declaration must always be preceded by the stdcall directive. This tells the
compiler that the function will be passing parameters in the standard Windows convention.
Whew! That seems like a lot but it's really not as complicated as it might seem from
the description above. Here's an example declaration:
function MyThreadFunc(Ptr : Pointer) : LongInt; stdcall;
That's it! Hope I didn't get you worried. The CreateThread call is a bit more
involved, but it too is not very complicated once you understand how to call it. Here's
its declaration, straight out of the help file:
function CreateThread
(lpThreadAttributes: Pointer; //Address of thread security attributes
dwStackSize: DWORD; //Thread stack size
lpStartAddress: TFNThreadStartRoutine;//Address of the thread function
lpParameter: Pointer; //Input parameter for the thread
dwCreationFlags: DWORD; //Creation flags
var lpThreadId: DWORD): //ThreadID reference
THandle; stdcall; //Function returns a handle to the thread
This is not as complicated as it seems. First of all, you rarely have to set security
attributes, so that can be set to nil. Secondly, in most cases, your stack size can
be 0 (actually, I've never found an instance where I have to set this to a value higher
than zero). You can optionally pass a parameter through the lpParameter argument as
a pointer to a structure or address of a variable, but I've usually opted to use global
variables instead (I know, this breaking a cardinal rule of structured programming, but it
sure eases things). Lastly, I've rarely had to set creation flags unless I want my thread
to start in a suspended state so I can do some preprocessing. For the most part, I set
this value as zero.
Now that I've thoroughly confused you, let's look at an example function that creates a
thread:
procedure TForm1.Button1Click(Sender: TObject);
var
thr : THandle;
thrID : DWORD;
begin
FldName := ListBox1.Items[ListBox1.ItemIndex];
thr := CreateThread(nil, 0, @CreateRecID, nil, 0, thrID);
if (thr = 0) then
ShowMessage('Thread not created');
end;
Embarrassingly simple, right? It is. To make the thread in the function above, I
declared two variables, thr and thrID, which stand for the handle of the
thread and its identifier, respectively. I set a global variable that the thread function
will access immediately before the call to CreateThread, then make the declaration,
assigning the return value of the function to thr and inputting the address of my
thread function, and the thread ID variable. The rest of the parameters I set to nil or 0.
Not much to it.
Notice that the procedure that actually makes the call is an OnClick handler for a
button on a form. You can pretty much create a thread anywhere in your code as long as you
set up properly. Here's the entire unit code for my program; you can use it for a
template. This program is actually fairly simple. It adds an incremental numeric key value
to a table called RecID, based on the record number (which makes things really easy).
Browse the code; we'll discuss it below:
unit main;
interface
uses
Windows, Messages, SysUtils, Classes,
Graphics, Controls, Forms, Dialogs, DB, DBTables, StdCtrls, ComCtrls,
Buttons;
type
TForm1 = class(TForm)
Edit1: TEdit;
Label1: TLabel;
OpenDialog1: TOpenDialog;
SpeedButton1: TSpeedButton;
Label2: TLabel;
StatusBar1: TStatusBar;
Button1: TButton;
ListBox1: TListBox;
procedure SpeedButton1Click(Sender: TObject);
procedure Button1Click(Sender: TObject);
end;
var
Form1: TForm1;
TblName : String;
FldName : String;
implementation
{$R *.DFM}
function CreateRecID(P : Pointer) : LongInt; stdcall;
var
tbl : TTable;
I : Integer;
ses : TSession;
msg : String;
begin
Randomize; //Initialize random number generator
I := 0;
{Disable the Execute button so another thread can't be executed
while this one is running}
EnableWindow(Form1.Button1.Handle, False);
{If you're going to access any data in a thread, you have to create a
separate }
ses := TSession.Create(Application);
ses.SessionName := 'MyRHSRecIDSession' + IntToStr(Random(1000));
tbl := TTable.Create(Application);
with tbl do begin
Active := False;
SessionName := ses.SessionName;
DatabaseName := ExtractFilePath(TblName); //TblName is a global variable set
TableName := ExtractFileName(TblName); //in the SpeedButton's OnClick handler
Open;
First;
try
{Start looping structure}
while NOT EOF do begin
if (State <> dsEdit) then
Edit;
msg := 'Record ' + IntToStr(RecNo) + ' of ' + IntToStr(RecordCount);
{Display message in status bar}
SendMessage(Form1.StatusBar1.Handle, WM_SETTEXT, 0, LongInt(PChar(msg)));
FieldByName(FldName).AsInteger := RecNo;
Next;
end;
finally
Free;
ses.Free;
EnableWindow(Form1.Button1.Handle, True);
end;
end;
msg := 'Operation Complete!';
SendMessage(Form1.StatusBar1.Handle, WM_SETTEXT, 0, LongInt(PChar(msg)));
end;
procedure TForm1.SpeedButton1Click(Sender: TObject);
var
tbl : TTable;
I : Integer;
begin
with OpenDialog1 do
if Execute then begin
Edit1.Text := FileName;
TblName := FileName;
tbl := TTable.Create(Application);
with tbl do begin
Active := False;
DatabaseName := ExtractFilePath(TblName);
TableName := ExtractFileName(TblName);
Open;
LockWindowUpdate(Self.Handle);
for I := 0 to FieldCount - 1 do begin
ListBox1.Items.Add(Fields[I].FieldName);
end;
LockWindowUpdate(0);
Free;
end;
end;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
thr : THandle;
thrID : DWORD;
begin
FldName := ListBox1.Items[ListBox1.ItemIndex];
thr := CreateThread(nil, 0, @CreateRecID, nil, 0, thrID);
if (thr = 0) then
ShowMessage('Thread not created');
end;
end.
The most important function here, obviously, is the thread function, CreateRecID.
Let's take a look at it:
function CreateRecID(P : Pointer) : LongInt; stdcall;
var
tbl : TTable;
I : Integer;
ses : TSession;
msg : String;
begin
Randomize; //Initialize random number generator
I := 0;
{Disable the Execute button so another thread can't be executed
while this one is running}
EnableWindow(Form1.Button1.Handle, False);
{If you're going to access any data in a thread, you have to create a
separate }
ses := TSession.Create(Application);
ses.SessionName := 'MyRHSRecIDSession' + IntToStr(Random(1000));
tbl := TTable.Create(Application);
with tbl do begin
Active := False;
SessionName := ses.SessionName;
DatabaseName := ExtractFilePath(TblName); //TblName is a global variable set
TableName := ExtractFileName(TblName); //in the SpeedButton's OnClick handler
Open;
First;
try
{Start looping structure}
while NOT EOF do begin
if (State <> dsEdit) then
Edit;
msg := 'Record ' + IntToStr(RecNo) + ' of ' + IntToStr(RecordCount);
{Display message in status bar}
SendMessage(Form1.StatusBar1.Handle, WM_SETTEXT, 0, LongInt(PChar(msg)));
FieldByName(FldName).AsInteger := RecNo;
Next;
end;
finally
Free;
ses.Free;
EnableWindow(Form1.Button1.Handle, True);
end;
end;
msg := 'Operation Complete!';
SendMessage(Form1.StatusBar1.Handle, WM_SETTEXT, 0, LongInt(PChar(msg)));
end;
This is a pretty basic function. I'll leave it up to you to follow the flow of
execution. However, let's look at some very interesting things that are happening in the
thread function.
First of all, notice that I created a TSession object before I created the table I was
going to access. This is to ensure that the program will behave itself with the BDE. This
is required any time you access a table or other data source from within the context of a
thread. I've explained this in more detail in another article called How
Can I Run Queries in Threads? Directly above that, I made a call to the Windows
API function EnableWindow to disable the button that executes the code. I had to do
this because since the VCL is not thread-safe, there's no guarantee I'd be able to
successfully access the button's Enabled property safely. So I had to disable it
using the Windows API call that performs enabling and disabling of controls.
Moving on, notice how I update the caption of a status bar that's on the bottom of the
my form. First, I set the value of a text variable to the message I want displayed:
msg := 'Record ' + IntToStr(RecNo) + ' of ' + IntToStr(RecordCount);
Then I do a SendMessage, sending the WM_SETTEXT message to the status bar:
SendMessage(Form1.StatusBar1.Handle, WM_SETTEXT, 0, LongInt(PChar(msg)));
SendMessage will send a message directly to a control and bypass the window
procedure of the form that owns it.
Why did I go to all this trouble? For the very same reason that I used EnableWindow
for the button that creates the thread. But unfortunately, unlike the single call to EnableWindow,
there's no other way to set the text of a control other than sending it the WM_SETTEXT
message.
The point to all this sneaking behind the VCL is that for the most part, it's not safe
to access VCL properties or procedures in threads. In fact, the objects that are
particularly dangerous to access from threads are those descended from TComponent. These
comprise a large part of the VCL, so in cases where you have to perform some interaction
with them from a thread, you'll have to use a roundabout method. But as you can see from
the code above, it's not all that difficult.
Of the thousands of functions in the Windows API, CreateThread is one of the
most simple and straightforward. I spent a lot of time explaining things here, but there's
a lot of ground I didn't cover. Use this example as a template for your thread
exploration. Once you get the hang of it, you'll use threads in practically everything you
do.
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