Delphi TTask get data from main thread - multithreading

Question

From what I have understood reading Nick Hodges, this code should be fine:
TTask.Run(
procedure
var
resp, tmp: string;
req: boolean;
bwriter: TBinaryWriter;
myfile: TFileStream;
begin
//tell the user to wait
TThread.Queue(TThread.CurrentThread,
procedure
begin
LoginButton.Text := 'Please wait...';
end
);
//some checks
try
resp := GetURL('... here I get a result from the server...');
if (resp = fOKstatus) then
begin
req := true;
myfile := TFileStream.Create(TPath.Combine(TPath.GetHomePath, 'docs.mkb'), fmCreate);
try
bwriter := TBinaryWriter.Create(myfile, TEncoding.Unicode, false);
try
bwriter.Write(UsernameEdit.Text);
bwriter.Write(AppIDEdit.Text);
bwriter.Close;
finally
bwriter.Free;
end;
finally
myfile.Free;
end;
end
else
begin
req := false;
end;
except
req := false;
end;
//final
TThread.Queue(TThread.CurrentThread,
procedure
begin
if (req = true) then
begin
LoginButton.Text := 'Success!';
ShowMessage('Close the app to complete the registration.');
end
else
begin
LoginButton.Text := 'Login failed.';
end;
end
);
end
);
This runs in a separated thread, and it is linked to the main thread with the calls to Queue(). In fact, at the beginning I am updating the Text of a Button using this method.
QUESTION. Look at these 2 lines:
bwriter.Write(UsernameEdit.Text);
bwriter.Write(AppIDEdit.Text);
I need to retrieve the username and AppID (which is a random code) from two Edit controls that are in the main thread UI. Is this correct?
I guess that I should call Queue(), but so far the program is working well.
Can I take the values in this way safely? I am not updating anything, and I just need to grab the data, but I am not sure if mixing contents from 2 different tasks can be dangerous/bad practice.

Answer 1

The 2 lines of code you are concerned about are NOT thread-safe. You must synchronize with the main thread for all UI access, both reading and writing. TThread.Queue() is asynchronous, so it is not suitable for the purpose of retrieving values from the UI. Use TThread.Synchronize() instead, which is synchronous:
TTask.Run(
procedure
var
resp, tmp, username, appid: string;
req: boolean;
bwriter: TBinaryWriter;
myfile: TFileStream;
begin
//tell the user to wait
TThread.Queue(nil,
procedure
begin
LoginButton.Text := 'Please wait...';
end
);
//some checks
try
resp := GetURL('... here I get a result from the server...');
if resp = fOKstatus then
begin
req := true;
TThread.Synchronize(nil,
procedure
begin
username := UsernameEdit.Text;
appid := AppIDEdit.Text;
end
);
myfile := TFileStream.Create(TPath.Combine(TPath.GetHomePath, 'docs.mkb'), fmCreate);
try
bwriter := TBinaryWriter.Create(myfile, TEncoding.Unicode, false);
try
bwriter.Write(username);
bwriter.Write(appid);
bwriter.Close;
finally
bwriter.Free;
end;
finally
myfile.Free;
end;
end
else
begin
req := false;
end;
except
req := false;
end;
//final
TThread.Queue(nil,
procedure
begin
if req then
begin
LoginButton.Text := 'Success!';
ShowMessage('Close the app to complete the registration.');
end
else
begin
LoginButton.Text := 'Login failed.';
end;
end
);
end
);
Alternatively, assuming the main UI thread is the one starting the TTask, you can read the 2 values before starting the TTask and let the anonymous procedure capture them:
var
username, appid: string;
begin
username := UsernameEdit.Text;
appid := AppIDEdit.Text;
TTask.Run(
procedure
var
resp, tmp: string;
req: boolean;
bwriter: TBinaryWriter;
myfile: TFileStream;
begin
//tell the user to wait
TThread.Queue(nil,
procedure
begin
LoginButton.Text := 'Please wait...';
end
);
//some checks
try
resp := GetURL('... here I get a result from the server...');
if resp = fOKstatus then
begin
req := true;
myfile := TFileStream.Create(TPath.Combine(TPath.GetHomePath, 'docs.mkb'), fmCreate);
try
bwriter := TBinaryWriter.Create(myfile, TEncoding.Unicode, false);
try
bwriter.Write(username);
bwriter.Write(appid);
bwriter.Close;
finally
bwriter.Free;
end;
finally
myfile.Free;
end;
end
else
begin
req := false;
end;
except
req := false;
end;
//final
TThread.Queue(nil,
procedure
begin
if req then
begin
LoginButton.Text := 'Success!';
ShowMessage('Close the app to complete the registration.');
end
else
begin
LoginButton.Text := 'Login failed.';
end;
end
);
end
);
end;