Use new macro debounce-reduce to make mouse scaling of images more responsive

* lisp/emacs-lisp/timer.el (debounce, debounce-reduce): New macros.

* lisp/image.el (image-increase-size, image-decrease-size):
Use funcall to call image--change-size-function.
(image--change-size-function): Move code from defun of
image--change-size to defvar that has the value of lambda
returned from debounce-reduce.  (Bug#38187)

1 files changed, 44 insertions, 0 deletions

diff --git a/lisp/emacs-lisp/timer.el b/lisp/emacs-lisp/timer.el
index 561cc70078f..5fdf9a426a7 100644
--- a/lisp/emacs-lisp/timer.el
+++ b/lisp/emacs-lisp/timer.el
@@ -488,6 +488,50 @@ The argument should be a value previously returned by `with-timeout-suspend'."
 If the user does not answer after SECONDS seconds, return DEFAULT-VALUE."
   (with-timeout (seconds default-value)
     (y-or-n-p prompt)))
+
+(defmacro debounce (secs function)
+  "Call FUNCTION after SECS seconds have elapsed.
+Postpone FUNCTION call until after SECS seconds have elapsed since the
+last time it was invoked.  On consecutive calls within the interval of
+SECS seconds, cancel all previous calls that occur rapidly in quick succession,
+and execute only the last call.  This improves performance of event processing."
+  (declare (indent 1) (debug t))
+  (let ((timer-sym (make-symbol "timer")))
+    `(let (,timer-sym)
+       (lambda (&rest args)
+         (when (timerp ,timer-sym)
+           (cancel-timer ,timer-sym))
+         (setq ,timer-sym
+               (run-with-timer
+                ,secs nil (lambda ()
+                            (apply ,function args))))))))
+
+(defmacro debounce-reduce (secs initial-state state-function function)
+  "Call FUNCTION after SECS seconds have elapsed.
+Postpone FUNCTION call until after SECS seconds have elapsed since the
+last time it was invoked.  On consecutive calls within the interval of
+SECS seconds, cancel all previous calls that occur rapidly in quick succession,
+and execute only the last call.  This improves performance of event processing.
+
+STATE-FUNCTION can be used to accumulate the state on consecutive calls
+starting with the value of INITIAL-STATE, and then execute the last call
+with the collected state value."
+  (declare (indent 1) (debug t))
+  (let ((timer-sym (make-symbol "timer"))
+        (state-sym (make-symbol "state")))
+    `(let (,timer-sym (,state-sym ,initial-state))
+       (lambda (&rest args)
+         (setq ,state-sym (apply ,state-function ,state-sym args))
+         (when (timerp ,timer-sym)
+           (cancel-timer ,timer-sym))
+         (setq ,timer-sym
+               (run-with-timer
+                ,secs nil (lambda ()
+                            (apply ,function (if (listp ,state-sym)
+                                                 ,state-sym
+                                               (list ,state-sym)))
+                            (setq ,state-sym ,initial-state))))))))
+
 
 (defconst timer-duration-words
   (list (cons "microsec" 0.000001)