B4J Code Snippet Dynamic plain HTML form web-page

peacemaker · Aug 31, 2022

If you start building your first Server web app - this is the simplest example of the dynamically generated web-page with just a plain HTML form code, without any JavaScript, CSS... Just a bare functional.
It helps to pass your settings into the web-app and show back some info from the app.
Only code of the app handler (class module "grabber_settings" - note it's referenced in the HTML-form code) is shown where all is done - receiving form fields and reply to the user (to web-browser) by HTML-code.
The generated web-page is self-refreshed each 10 seconds to update the status on the screen.
Some links are to the variables in Main app module, ignore them (it's for main app function: grabber of RTSP video stream to .jpg-frames).

B4J web-app handler code:

'Class module
Sub Class_Globals
End Sub

Public Sub Initialize

End Sub

Sub Handle(req As ServletRequest, resp As ServletResponse)
    Try
        Dim Params As Map = req.ParameterMap
'        Log(CRLF & "------------" & CRLF & "Method: " & req.Method)
'        Log("ContentType: " & req.ContentType)
'        Log(req.GetSession.GetAttributesNames)
'        Log(Params)
 
        Dim form_result, ResponseText As StringBuilder
        form_result.Initialize
        ResponseText.Initialize
 
        Dim new_url As String = req.GetParameter("RTSP_URL")
        If new_url <> "" Then
            If new_url <> Main.RTSP_URL Then
                Main.Grabbing = False
                Main.RTSP_URL = new_url
                Main.Save_Settings
                form_result.Append("New URL is saved.").Append(CRLF)
            Else
                form_result.Append("URL was not changed.").Append(CRLF)
            End If
        End If
 
        Dim new_url As String = req.GetParameter("frames_path")
        If new_url <> "" Then
            If new_url <> Main.Frames_Path Then
                If File.Exists(new_url, "") Then
                    Main.Frames_Path = new_url
                    Main.Save_Settings
                    form_result.Append("New path is saved.").Append(CRLF)
                Else
                    form_result.Append("New path was WRONG: non-existent, ignored.").Append(CRLF)
                End If
            Else
                form_result.Append("Path was not changed.").Append(CRLF)
            End If
        End If
 
        If Params.ContainsKey("btnStart") Then
            Main.LastGrabbingError = ""
            Main.LastOutputFile = ""
            Main.Grabbing = True
            Main.Save_Settings
            form_result.Append("Service is started.").Append(CRLF)
        End If
        If Params.ContainsKey("btnStop") Then
            Main.Grabbing = False
            Main.Save_Settings
            form_result.Append("Service is stopped.").Append(CRLF)
        End If
 
        ResponseText.Append($"<html><body><meta http-equiv="refresh" content="10">"$)    'auto-refresh web-page
        ResponseText.Append("Server v." & Main.ver & " time: " & DateTime.Time(DateTime.Now))
        ResponseText.Append("<form method='POST' action='grabber_settings'>")
 
        Dim rtsp_form As String = $"Source IP-camera RTSP address: <input type="text" name="RTSP_URL" size="35" value="$ & Main.RTSP_URL & "></input>"
        rtsp_form = rtsp_form & $"<button type="Submit">Save URL</button>"$
        ResponseText.Append(rtsp_form)
 
        Dim rtsp_form As String = $"Grabbed frames path: <input type="text" name="frames_path" size="65" value="$ & Main.Frames_Path & "></input>"
        rtsp_form = rtsp_form & $"<button type="Submit">Save path</button>"$
        ResponseText.Append("<br>").Append(rtsp_form)
 
        ResponseText.Append("<br>").Append("Grabbing service: " & (IIf(Main.Grabbing, $"<button type="Submit" name="btnStop">STOP</button>"$, _
        $"<button type="Submit" name="btnStart">START !</button>"$)))
        ResponseText.Append("</form>")
        If form_result.Length > 0 Then
            ResponseText.Append("<br>").Append(form_result.ToString)    
        End If
       If Main.LastOutputFile <> "" Then
            Dim path As String = File.GetFileParent(Main.LastOutputFile)
            Dim FileExt As String = Main.LastOutputFile.SubString2(Main.LastOutputFile.Length - 4, Main.LastOutputFile.Length)
            Dim Files As List = File.ListFiles(path)
            Dim counter As Long
            For i = 0 To Files.Size - 1
                Dim fn As String = Files.Get(i)
                If fn.EndsWith(FileExt) Then
                    counter = counter + 1
                End If
            Next
 
            ResponseText.Append("<br>").Append("LastOutputFile: " & Main.LastOutputFile & "<br>" & FileExt & "-frames totally grabbed = " & counter)
        End If
        If Main.LastGrabbingError <> "" Then
            ResponseText.Append("<br>").Append("LastGrabbingError: " & Main.LastGrabbingError)
        End If
 
        ResponseText.Append("</body></html>")
 
        resp.Write(ResponseText.ToString)   'pass the reply HTML text to user's browser
    Catch
        resp.SendError(500, "Server internal error")
    End Try
    Log(form_result)
End Sub

Saving the parameters from fields
Error reporting

Runtime errors reporting:
Working and reporting about the status

NOTE: Sign "!" in any file path is forbidden, do not use it, it's a special symbol for Java programs.

B4J Code Snippet Dynamic plain HTML form web-page

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