B4J Tutorial [server] Automatic Reconnecting WebSocket

Discussion in 'B4J Tutorials' started by Erel, Jan 6, 2016.

  1. Erel

    Erel Administrator Staff Member Licensed User

    When working with WebSocket handlers the client maintains an online connection. The connection is expected to be kept alive for as long as needed.

    However there are cases where the connection can be temporary broken. Especially when the client is a mobile browser.

    Using the Reconnecting-WebSocket open source project the client can automatically try to reconnect.

    See this video for an example. Pay attention to the server time updates.

    In order to enable this feature you need to:
    1. Use b4j_ws.js v0.91+ (open the file with a text editor to see the version)
    2. Copy reconnecting-websocket.js to the www folder.
    3. Add <script src="/reconnecting-websocket.js"></script> to the html file. Note that you don't need to add this to all handlers. Only to those that should use a reconnecting websocket.

    Both files are available in the attached project (Objects\www).

    The server doesn't distinguish between an automatic reconnection to a regular connection. In both cases a new WebSocket handler instance is created.
    You can use the HttpSession to restore the previous state.

    For example:
    Private Sub WebSocket_Connected (WebSocket1 As WebSocket)
       ws = WebSocket1
    Dim session As HttpSession = ws.UpgradeRequest.GetSession
    If session.HasAttribute("state") = False Then
         state.Number = 
    "state", state) 'sets a reference to the state object.
         Log($"Creating new state. Number = ${state.Number}"$)
         state = session.GetAttribute(
    Log($"Reusing previous state. Number = ${state.Number}"$)
    End If
    End Sub
    Remember that sessions are stored in the server side.

    Attached Files:

  2. alwaysbusy

    alwaysbusy Expert Licensed User

    Note: Just for the ABMaterial users. ABMaterial does already use the reconnecting-websocket.js and has a modified version of b4j_ws.js needed for ABMaterial to run.
  3. vfafou

    vfafou Active Member Licensed User

    Hello, Erel! Is this applicable to Custom WebSocket Framework?
    I ask you because I see that you refer only to browser connections and not to device connections.

    Thank you in advance.
    Last edited: Jan 21, 2016
  4. Erel

    Erel Administrator Staff Member Licensed User

    It is only relevant for browsers. The client in the "custom websocket framework" already implement a similar feature where a timer is used to reconnect.
  5. vfafou

    vfafou Active Member Licensed User

    Some times, it's occuring the following problem:
    It is possible to have an internet line problem to my server.
    The connections with the tablets, may totally be recovered but there is an option that may will be semi-recovered.
    I have noticed that after a server internet disconnection, I have communication from tablets to server, but the server cannot send anything to tablets!
    Do you have any suggestion? :)
  6. Erel

    Erel Administrator Staff Member Licensed User

    This is not relevant to this thread. Please start a new one instead.
  7. ValhallSW

    ValhallSW Member Licensed User

    I would like to see this example, Guess My Number, as a native B4A-application, not just in a browser
  8. vfafou

    vfafou Active Member Licensed User

  9. Erel

    Erel Administrator Staff Member Licensed User

    Next time please start a new thread for a new question.
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