Keeping state

We implement the generate method as shown below.

method generate {} {
    # Generate the next number in the sequence. The sequence 
    # generated so far is stored in the session.

    # Declare a member variable seq to hold the Fibonacci
    # sequence. All member variables are automatically 
    # available for use in the view templates.
    my variable seq

    # If we have already generated a sequence in this session
    # it will be stored in the session.
    if {[session exists fibonacci seq]} {
        # Yep, already in the process of building a sequence
        lappend seq [expr {[lindex $seq end-1]+[lindex $seq end]}]
    } else {
        # First call to generate a sequence
        set seq {0 1}
    }

    # Store it back in the session for the next request
    session set fibonacci $seq
}

The first time a user browses to the fibonacci/generate link, we want to display the starting two numbers in the sequence. Then for subsequent visits to the page within the same browser session, we want to include the next number in the sequence. To do that we need to be able store state on the server side. That is what a session does.

Sessions

A session maintains state between multiple requests from a single client. When the initial client request is received, a new session is created for it. Subsequent requests from the client are associated with that session and the state information associated with it is made available through the session object. Application code can store any Tcl values in the session object and retrieve them for subsequent requests. In general, the session object is not intended for storing large amounts of data, but for our purposes for storing the current sequence, it is adequate.

In the generate method above, we check if the session object contains the key fibonacci which holds the sequence generated so far. If so, we add the next number in the sequence and if not, we initialize the sequence with the first two numbers. We then store the new sequence back in the session so as to remember it for the next request. The object member variable seq also holds the sequence which makes it available in the view template.

We now need a way of displaying what we computed. We do that next.