How to pass or make public variables or fields accessible to Tthread?

Question

I am writing and building my software for Mono using Delphi Prism. So, I decided that my serial communication will be handled by a thread. Since global variables strictly are not allowed unless you enable the global variable option for the project, I decided to follow the Delphi Prism convention. So, then how do you pass or make the public variables or fields accessible to a thread?

Here is my test mainform code:

MainForm = partial class(System.Windows.Forms.Form)

  private
    method SignalBtn_Click(sender: System.Object; e: System.EventArgs);
    method CommBtn_Click(sender: System.Object; e: System.EventArgs);
    method button1_Click(sender: System.Object; e: System.EventArgs);
    method button2_Click(sender: System.Object; e: System.EventArgs);
    method button4_Click(sender: System.Object; e: System.EventArgs);
    method button5_Click(sender: System.Object; e: System.EventArgs);
    method MainForm_Load(sender: System.Object; e: System.EventArgs); 
    method ShutdownBtn_Click(sender: System.Object; e: System.EventArgs);
    method MySerialData(sender: System.Object; e:SerialDataReceivedEventArgs);
    method LoginBtn_Click(sender: System.Object; e: System.EventArgs);
  protected
    method Dispose(disposing: Boolean); override;
  public
    RX:Array[0..5] of byte;
    TX:Array[0..6] of byte;
    serialPort1:System.IO.Ports.SerialPort;
    thr:Thread;
    stoploop:Boolean;
    mcommand:Byte;
    thechannel:Integer;
    constructor;
    method FillTable;
  end;

Here is the Thread for serial communication:

  ThreadComm = class(MainForm)
  public
    class procedure mythread; static;
  end;

Here is how ThreadComm runs:

class procedure ThreadComm.mythread;
begin
    while true do
    begin
        TX[0]:=$FF;
        TX[1]:=$01;
        TX[2]:=$01;
        TX[3]:=$04;
        TX[4]:=$A2;
        TX[5]:=(TX[2] xor TX[3] xor TX[4]);

        SerialPort1.Write(TX,0,6);
        while SerialPort1.BytesToWrite>0 do;
        Thread.Sleep(100);

        if (stoploop) then
            break;
    end;
end;

Every time I compile the code, it raises 30 or so similar error messages stating the following:

Cannot call instance member "SerialPort1" without an instance reference

I know what the error means, but the only way to solve it is by creating an instance of the mainform. If you do that, then it won't be the same instance as the main program's instance. If this is the case, then you will have to create new instance of mainform all the time when you need to access its fields or public variables. Right?

class method Program.Main(args: array of string);
begin
  Application.EnableVisualStyles();
  Application.SetCompatibleTextRenderingDefault(false);
  Application.ThreadException += OnThreadException;
  **using lMainForm := new MainForm do
    Application.Run(lMainForm);**
end;

I want to use all them variables are in the thread and they happen to be within the mainform public area.

Thanks

Does this thing compile: `ThreadComm = class(MainForm)`? I'm asking because it looks very strange to me; But I've never worked with delphi-prism, only with true Delphi and C#. — Cosmin Prund, Jul 12 '11 at 13:31
@Cosmin, I try to make ThreadComm inherit Mainform thinking that it will expose all the fields to the thread and apparently it actually compiles when I comment out all the lines of code within the MyThread. That's what I am confused about. It seems to be inheriting mainform but it can't see the fields. — ThN, Jul 12 '11 at 13:34
Is `class procedure NameOfProc; static;` valid delphi prism syntax? That gives me a headache, because it essentially specifies the same thing *twice*! Once with the `class` modifier, then again with the `static` modifier. — Cosmin Prund, Jul 12 '11 at 13:54
@Cosmin, I understand what you are saying and I felt the same when I took the thread example from Embarcadero website and applied it to my test program. If their code is wrong, then my code is wrong. — ThN, Jul 12 '11 at 14:27
Here is the link to embarcadero thread tutorial. http://conferences.embarcadero.com/article/32225#TThreadMap — ThN, Jul 12 '11 at 14:34

score 1 · Answer 1 · answered Jul 12 '11 at 13:39

1

Pass the things your thread needs as parameters to its constructor. Store references to those objects as fields in your thread class. Use them while your thread runs.

And don't make your thread class a descendant of your main form. That makes no sense. If your thread needs access to the entire form, then pass a MainForm reference to the thread's constructor. But if all your thread needs is a serial-port object, then just pass that, not the entire form.

Do not create additional instances of MainForm. That will create more forms; it won't give you access to the field values of your real main form.

answered Jul 12 '11 at 13:39

Rob Kennedy

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Taking references in a thread is often a recipe for disaster. Take copies if you can. – David Heffernan Jul 12 '11 at 13:46
@Rob, "it won't give you access to the field values of the real main form." That's what I was getting at in my question. Thanks. – ThN Jul 12 '11 at 13:47
1

You can't make a copy of a thread, @David. If you did, you've have two threads, the same problem as creating multiple `MainForm` instances. Instead, make both the main form and the thread keep references to the same shared object. That should be much easier to manage in Prism than in normal Delphi because .Net lightens the burden of managing object ownership. – Rob Kennedy Jul 12 '11 at 13:53
1

@Rob I meant to take a copy of the *data* that you pass to the thread. I was not clear. – David Heffernan Jul 12 '11 at 13:55

score 1 · Accepted Answer · answered Jul 12 '11 at 13:52

Your thread procedure (class procedure mythread; static;) seems to be "static" or "class", while the the fields you want to access are regular fields of a MainForm instance. That can't work, because there's one and only one procedure mythread, but there may be multiple instances of MainForm. While in practice there will probably only be one instance of MainForm, the compiler doesn't know that.

You should start by removing both class prefix and static sufix from that procedure, to make it a regular instance procedure, that can read instance fields. Even that's not going to be enough, because you derived from your MainForm, and I assume you create a new instance of ThreadCom and start the thread: Once you make the method a regular instance method the code will compile, but the result isn't going to be the one you expect, because you'll be accessing members of a different instance of MainForm.

Solutions:

Make the mythread procedure a instance method of your MainForm. It'll gain access to all required fields and access them from the proper instance!
Follow Rob's suggestion: create a new class, not derived from MainForm, pass it an instance of MainForm in the constructor, you that from the thread procedure. You'll (again) need to avoid static/class fields and procedures, because those can't be linked to one instance of MainForm.

at Cosmin: It works great. Thanks. I made mythread as part of the mainform class within public section and created a thread using mythread procedure like so : thr := new Thread(@Mythread); thr.start; I always thought you have to create a separate class for you to create thread of any kind. Shows how often I work with thread. :) — ThN, Jul 12 '11 at 15:30

How to pass or make public variables or fields accessible to Tthread?

2 Answers2