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+! Copyright (C) 2004 Chris Double.
+! Copyright (C) 2006, 2008 Slava Pestov.
+! See http://factorcode.org/license.txt for BSD license.
+USING: http http.server io kernel math namespaces
+continuations calendar sequences assocs hashtables
+accessors arrays alarms quotations combinators fry
+http.server.redirection furnace assocs.lib urls ;
+IN: furnace.callbacks
+
+SYMBOL: responder
+
+TUPLE: callback-responder responder callbacks ;
+
+: <callback-responder> ( responder -- responder' )
+    H{ } clone callback-responder boa ;
+
+TUPLE: callback cont quot expires alarm responder ;
+
+: timeout 20 minutes ;
+
+: timeout-callback ( callback -- )
+    [ alarm>> cancel-alarm ]
+    [ dup responder>> callbacks>> delete-at ]
+    bi ;
+
+: touch-callback ( callback -- )
+    dup expires>> [
+        dup alarm>> [ cancel-alarm ] when*
+        dup '[ , timeout-callback ] timeout later >>alarm
+    ] when drop ;
+
+: <callback> ( cont quot expires? -- callback )
+    f callback-responder get callback boa
+    dup touch-callback ;
+
+: invoke-callback ( callback -- response )
+    [ touch-callback ]
+    [ quot>> request get exit-continuation get 3array ]
+    [ cont>> continue-with ]
+    tri ;
+
+: register-callback ( cont quot expires? -- id )
+    <callback> callback-responder get callbacks>> set-at-unique ;
+
+: forward-to-url ( url -- * )
+    ! When executed inside a 'show' call, this will force a
+    ! HTTP 302 to occur to instruct the browser to forward to
+    ! the request URL.
+    <temporary-redirect> exit-with ;
+
+: cont-id "factorcontid" ;
+
+: forward-to-id ( id -- * )
+    ! When executed inside a 'show' call, this will force a
+    ! HTTP 302 to occur to instruct the browser to forward to
+    ! the request URL.
+    <url>
+        swap cont-id set-query-param forward-to-url ;
+
+: restore-request ( pair -- )
+    first3 exit-continuation set request set call ;
+
+SYMBOL: post-refresh-get?
+
+: redirect-to-here ( -- )
+    ! Force a redirect to the client browser so that the browser
+    ! goes to the current point in the code. This forces an URL
+    ! change on the browser so that refreshing that URL will
+    ! immediately run from this code point. This prevents the
+    ! "this request will issue a POST" warning from the browser
+    ! and prevents re-running the previous POST logic. This is
+    ! known as the 'post-refresh-get' pattern.
+    post-refresh-get? get [
+        [
+            [ ] t register-callback forward-to-id
+        ] callcc1 restore-request
+    ] [
+        post-refresh-get? on
+    ] if ;
+
+SYMBOL: current-show
+
+: store-current-show ( -- )
+    ! Store the current continuation in the variable 'current-show'
+    ! so it can be returned to later by 'quot-id'. Note that it
+    ! recalls itself when the continuation is called to ensure that
+    ! it resets its value back to the most recent show call.
+    [ current-show set f ] callcc1
+    [ restore-request store-current-show ] when* ;
+
+: show-final ( quot -- * )
+    [ redirect-to-here store-current-show ] dip
+    call exit-with ; inline
+
+: resuming-callback ( responder request -- id )
+    url>> cont-id query-param swap callbacks>> at ;
+
+M: callback-responder call-responder* ( path responder -- response )
+    '[
+        , ,
+
+        [ callback-responder set ]
+        [ request get resuming-callback ] bi
+
+        [
+            invoke-callback
+        ] [
+            callback-responder get responder>> call-responder
+        ] ?if
+    ] with-exit-continuation ;
+
+: show-page ( quot -- )
+    [ redirect-to-here store-current-show ] dip
+    [
+        [ ] t register-callback swap call exit-with
+    ] callcc1 restore-request ; inline
+
+: quot-id ( quot -- id )
+    current-show get swap t register-callback ;
+
+: quot-url ( quot -- url )
+    quot-id f swap cont-id associate derive-url ;