Setting mouse cursor with a delay from another thread-CodePudding

I have some work to do in my main thread that sometimes last longer than a few seconds. I want to change the mouse cursor to a busy state (hourglass, over the whole main form) when that happens. But this must be done with a delay (say 250 ms), because most of the time the work is shorter and changing the cursor too often is annoying. I cannot do this from the main thread because it is obviously busy doing something else. So I thought to do it from another thread. But... surprise ! Setting Screen.Cursor:= crHourGlass from a thread does not work. I think the Screen object is meant to be accessed only from the main thread. I cannot synchronize anything with the main thread while it is busy... Any ideas on how to make it work ?

unit MouseCursor;

interface

uses
  System.Classes, Winapi.Windows;

type
  TMouseCursor = class(TThread)
  private type
    TCommand = (cmdSetBusy = $FB45BA57,cmdSetDone = $C75F1D29);
  private
    hResume: THandle;
    Command: TCommand;
    TimeOut: Cardinal;
  protected
    procedure Execute; override;
    procedure TerminatedSet; override;
  public
    constructor Create;
    destructor  Destroy; override;
    procedure Busy;
    procedure Done;
  end;

implementation

uses
  Vcl.Forms, Vcl.Controls;

const
  BusyDelay  = 250;

{ TMouseCursor }

constructor TMouseCursor.Create;
begin
 inherited Create(False);
 FreeOnTerminate:= True;
 hResume:= CreateEvent(nil, False, False, nil);
end;

destructor TMouseCursor.Destroy;
begin
 CloseHandle(hResume);
 inherited;
end;

procedure TMouseCursor.TerminatedSet;
begin
 Command:= cmdSetDone;
 inherited;
 SetEvent(hResume);
end;

procedure TMouseCursor.Busy;
begin
 Command:= cmdSetBusy;
 SetEvent(hResume);
end;

procedure TMouseCursor.Done;
begin
 Command:= cmdSetDone;
 SetEvent(hResume);
end;

procedure TMouseCursor.Execute;
var WaitRes: Cardinal;
begin
 TimeOut:= INFINITE;
 repeat
  WaitRes:= WaitForSingleObject(hResume, TimeOut);
  case WaitRes of
   WAIT_TIMEOUT : begin Screen.Cursor:= crHourGlass; TimeOut:= INFINITE; end;
   WAIT_OBJECT_0: case Command of
     cmdSetBusy: TimeOut:= BusyDelay;
     cmdSetDone: begin Screen.Cursor:= crDefault; TimeOut:= INFINITE; end;
   end;
  end;
 until Terminated;
end;

end.

CodePudding user response：

You will have to set the cursor using the thread that created the window. I would suggest changing your design and putting the work being done by the main thread into a worker thread which can be monitored by the main thread (perhaps using a timer or by the worker thread issuing Synchronize calls). Your main thread should not abandon the message loop that keeps the program running and responsive. So in general your main thread should be continuously listening for and handing the system messages that are sent to it. If any work may take some time put it into a worker thread.

For the Synchronise method to work the main thread has to be idle - the waiting Synchronise is picked up in the message loop.