Calling JS function from ASP.NET codebehind

Question

I have two javascript functions in my aspx page. They use some fabric.js functions.

function saveCanvas() {
    js = JSON.stringify(canvas.toDatalessJSON());
    $get('<%= txtJSON.ClientID%>').value = js;

}
function loadCanvas() {
    js = $get('<%= txtJSON.ClientID%>').value;
    canvas.clear();
    canvas.loadFromDatalessJSON(js);
    canvas.renderAll();
}

And in the codebehind:

 Protected Sub SaveJSON()
    Dim scriptKey As String = "123"
    Dim javaScript As String = "<script type='text/javascript'>saveCanvas();</script>"
    ClientScript.RegisterStartupScript(Me.GetType(), scriptKey, javaScript)
 End Sub

Protected Sub LoadJSON()
    Dim scriptKey As String = "456"
    Dim javaScript As String = "<script type='text/javascript'>loadCanvas();</script>"
    ClientScript.RegisterStartupScript(Me.GetType(), scriptKey, javaScript)
End Sub

Now my question: Why does loadCanvas work while saveCanvas does not? txtJSON is not populated with the JSON-string. Calling the saveCanvas function from the aspx page works fine.

Answer 1

The problem is that you call saveCanvas after the postback is done and at that point the canvas data is long gone.

If you have a button "Save" then you need to call saveCanvas when it is clicked so that the data is saved before the browser posts the page back to the server:

<asp:Button runat="server" Text="Save" OnClickClick="saveCanvas()" />