How to use asynchronous TNetHTTPClient-CodePudding

How to use asynchronous TNetHTTPClient? I tried following code, but it shows error.

procedure TForm1.Button1Click(Sender: TObject);
var
  httpclient: TNetHTTPClient;
  output: string;
begin
  httpclient := TNetHTTPClient.Create(nil);
  try
      httpclient.Asynchronous := True;
      output := httpclient.Get('https://google.com').ContentAsString;
  finally
    httpclient.Free;
  end;
end;

Error:

Error querying headers: The handle is in a wrong state for the requested operation

CodePudding user response：

In asynchronous mode, as the name implies, client runs request asynchronously in the background thread.

When following code is executed, ContentAsString fails because request is not completed at that point.

output := httpclient.Get('https://google.com').ContentAsString

If you want to use HTTP client in asynchronous mode, you will have to use completion handlers to run appropriate code after request is completed.

procedure TForm1.Button1Click(Sender: TObject);
var
  httpclient: TNetHTTPClient;
begin
  httpclient := TNetHTTPClient.Create(nil);
  httpclient.Asynchronous := True;
  httpclient.OnRequestCompleted := HTTPRequestCompleted;
  httpclient.Get('https://google.com');
end;

procedure TForm1.HTTPRequestCompleted(const Sender: TObject; const AResponse: IHTTPResponse);
var
  output: string;
begin
  output := AResponse.ContentAsString;
  Sender.Free;
end;

Using HTTP client in asynchronous mode is generally more complicated (especially from the memory management aspect) than using it in synchronous mode from the background thread.

Following is equivalent example using anonymous background thread:

procedure TForm1.Button1Click(Sender: TObject);
begin
  TThread.CreateAnonymousThread(
    procedure
    var
      httpclient: TNetHTTPClient;
      output: string;
    begin
      httpclient := TNetHTTPClient.Create(nil);
      try
        httpclient.Asynchronous := False;
        output := httpclient.Get('https://google.com').ContentAsString;
      finally
        httpclient.Free;
      end;
    end).Start;
end;

Of course, you can also use TTask or custom threads instead of anonymous threads.