How can I use the Clojure REPL together with Qt Jambi?

Question

I have not found a solution to use the Clojure REPL with Qt on the web. Basically the problem is that the REPL hangs as soon as you call QApplication/exec in order to get the UI to display. You cannot C-c C-c back into the REPL, and closing the active Qt window seems to kill the whole Clojure process.

Now simply calling QApplication/processEvents from within an agent is not possible, unless the agent runs in exactly the same thread in which you created your Qt widgets. It took me two days to figure this out and I have seen others have the same issue/problem but without a solution. So here is mine, in code:

(add-classpath "file:///usr/share/java/qtjambi.jar")
(ns qt4-demo
  (:import (com.trolltech.qt.gui QApplication QPushButton QFont QFont$Weight)
           (com.trolltech.qt.core QCoreApplication)
           (java.util Timer TimerTask)
           (java.util.concurrent ScheduledThreadPoolExecutor TimeUnit))
  (:require swank.core))

(defn init []
  (QApplication/initialize (make-array String 0)))

(def *gui-thread* (new java.util.concurrent.ScheduledThreadPoolExecutor 1))
(def *gui-update-task* nil)
(def *app* (ref nil))

(defn update-gui []
  (println "Updating GUI")
  (QApplication/processEvents))

(defn exec []
  (.remove *gui-thread* update-gui)
  (def *gui-update-task* (.scheduleAtFixedRate *gui-thread* update-gui 0 150 (. TimeUnit MILLISECONDS))))

(defn stop []
  (.remove *gui-thread* update-gui)
  (.cancel *gui-update-task*))

(defmacro qt4 [& rest]
  `(do
     (try (init) (catch RuntimeException e# (println e#)))
     ~@rest
     ))

(defmacro with-gui-thread [& body]
  `(.get (.schedule *gui-thread* (fn [] (do ~@body)) (long 0) (. TimeUnit MILLISECONDS))))

(defn hello-world []
  (with-gui-thread
    (qt4
     (let [app (QCoreApplication/instance)
           button (new QPushButton "Go Clojure Go")]
       (dosync (ref-set *app* app))
       (doto button
         (.resize 250 100)
         (.setFont (new QFont "Deja Vu Sans" 18 (.. QFont$Weight Bold value)))
         (.setWindowTitle "Go Clojure Go")
         (.show)))))
  (exec))

Basically it uses the ScheduledThreadPoolExecutor class in order to execute all Qt-code. You can use the with-gui-thread macro to make it easier to call functions from within the thread. This makes it possible to change the Qt UI on-the-fly, without recompiling.

Phosphocreatine

Yep, I had to do the same thing. – Homeland

I don't know anything about QT. But why do you want to do this? Clojure has access to Swing, which is a very powerful and versatile GUI framework. Are you connecting to a QT GUI that's already in place? – Selfabsorption

QT is arguably better than Swing in a lot of ways, including performance and native look-and-feel. – Homeland

Qt is also has a much cleaner API in my opinion. (It'd be hard to come up with an uglier API than Swing.) Qt apps also integrate far better with Linux desktop environments than Swing apps. – Testerman

Answer 1

If you want to mess with Qt widgets from the REPL, QApplication/invokeLater or QApplication/invokeAndWait are probably what you want. You can use them in conjunction with agents. Given this:

(ns qt4-demo
  (:import (com.trolltech.qt.gui QApplication QPushButton)
           (com.trolltech.qt.core QCoreApplication)))

(def *app* (ref nil))
(def *button* (ref nil))
(def *runner* (agent nil))

(defn init [] (QApplication/initialize (make-array String 0)))
(defn exec [] (QApplication/exec))

(defn hello-world [a]
  (init)
  (let [app (QCoreApplication/instance)
        button (doto (QPushButton. "Go Clojure Go") (.show))]
    (dosync (ref-set *app* app)
            (ref-set *button* button)))
  (exec))

Then from a REPL:

qt4-demo=> (send-off *runner* hello-world)
#<Agent@38fff7: nil>

;; This fails because we are not in the Qt main thread
qt4-demo=> (.setText @*button* "foo")
QObject used from outside its own thread, object=QPushButton(0x8d0f55f0) , objectThread=Thread[pool-2-thread-1,5,main], currentThread=Thread[main,5,main] (NO_SOURCE_FILE:0)

;; This should work though
qt4-demo=> (QApplication/invokeLater #(.setText @*button* "foo"))
nil
qt4-demo=> (QApplication/invokeAndWait #(.setText @*button* "bar"))
nil

Testerman

Answer 2

I've written about how to do this with SLIME on my blog (German) as well as on the Clojure mailing-list. The trick is to define appropriate functions on the Emacs side and tell SLIME to use those when making requests. Importantly, this frees you from having to do special incantations when invoking Qt code.

Quoting myself:

Given that we're talking Lisp here, anyway, the solution seemed to be obvious: Hack SLIME! So that's what I did. The code below, when dropped into your .emacs (at a point at which SLIME is already fully loaded), registers three new Emacs-Lisp functions for interactive use. You can bind them to whatever keys you like, or you may even just set the slime-send-through-qapplication variable to t after your application has started and not worry about key bindings at all. Either should make your REPL submissions and C-M-x-style interactive evaluations indirect through QCoreApplication/invokeAndWait.

Have fun!

(defvar slime-send-through-qapplication nil) 
(defvar slime-repl-send-string-fn (symbol-function 'slime-repl-send- 
string)) 
(defvar slime-interactive-eval-fn (symbol-function 'slime-interactive- 
eval)) 

(defun qt-appify-form (form) 
  (concatenate 'string    ;'
               "(let [return-ref (ref nil)] " 
               "  (com.trolltech.qt.core.QCoreApplication/invokeAndWait " 
               "   (fn [] " 
               "     (let [return-value (do " 
               form 
               "          )] " 
               "       (dosync (ref-set return-ref return-value))))) " 
               "  (deref return-ref))")) 

(defun slime-interactive-eval (string) 
  (let ((string (if slime-send-through-qapplication 
                    (qt-appify-form string) 
                    string))) 
    (funcall slime-interactive-eval-fn string))) 

(defun slime-repl-send-string (string &optional command-string) 
  (let ((string (if slime-send-through-qapplication 
                    (qt-appify-form string) 
                    string))) 
    (funcall slime-repl-send-string-fn string command-string))) 

(defun slime-eval-defun-for-qt () 
  (interactive) 
  (let ((slime-send-through-qapplication t)) 
    (slime-eval-defun))) 

(defun slime-repl-closing-return-for-qt () 
  (interactive) 
  (let ((slime-send-through-qapplication t)) 
    (slime-repl-closing-return))) 

(defun slime-repl-return-for-qt (&optional end-of-input) 
  (interactive) 
  (let ((slime-send-through-qapplication t)) 
    (slime-repl-return end-of-input))) 

Christo