list-view: avoid more work when keys don't change (#58)

* list-view: avoid more work when keys don't change

According to existing discussion:
- The removal loop does not trigger the inner conditional when the keys
  haven't changed anyway, because (member old-k new-keys) should always
  be true.
- The call to change-children is redundant when the keys haven't changed
  because the list of children should also be the same.

Thus we perform some work (looping, conditionals, and importantly a call
to change-children) that is unnecessary: for performance, skip it.

Close #57

Best-viewed-with: --ignore-space-change

* list: avoid computing keys twice

---------

Co-authored-by: Bogdan Popa <bogdan@defn.io>

gui-easy-lib/gui/easy/private/view/list.rkt

@@ -96,12 +96,15 @@
              [stretchable-height h-s?]))
       (define keys-to-children
         (get-keys-to-children the-panel))
+      (define entries (peek @entries))
+      (define keys (map key-proc entries))
+      (send the-panel set-context 'current-keys keys)
       (begin0 the-panel
         (with-container-sequence the-panel
-          (for ([e (in-list (peek @entries))])
-            (define k (key-proc e))
+          (for ([e (in-list entries)]
+                [k (in-list keys)])
             (define v (make-view k (make-keyed-obs k e)))
-            (define w (send v create the-panel))
+            (define w (send v create the-panel)) ;; noqa
             (add-child-handlers! the-panel v)
             (add-child the-panel v w)
             (hash-set! keys-to-children k v)))))
@@ -122,8 +125,10 @@
                    (add-child-handlers! v child-v)
                    (add-child v child-v child-w)
                    (hash-set! keys-to-children k child-v)))))
-           (for ([(old-k old-v) (in-hash keys-to-children)])
-             (unless (member old-k new-keys)
+           (unless (equal? new-keys (send v get-context 'current-keys null))
+             (send v set-context 'current-keys new-keys)
+             (for ([(old-k old-v) (in-hash keys-to-children)]
+                   #:unless (member old-k new-keys))
                (define old-w (get-child v old-v))
                (define focused? (send old-w has-focus?))
                (send old-v destroy old-w)
@@ -135,11 +140,11 @@
                  (define children (send v get-children))
                  (cond
                    [(null? children) (send v focus)]
-                   [else (send (last children) focus)]))))
-           (send v change-children
-                 (λ (_)
-                   (for/list ([k (in-list new-keys)])
-                     (get-child v (hash-ref keys-to-children k))))))]
+                   [else (send (last children) focus)])))
+             (send v change-children
+                   (λ (_)
+                     (for/list ([k (in-list new-keys)])
+                       (get-child v (hash-ref keys-to-children k)))))))]
         [@alignment
          (send/apply v set-alignment val)]
         [@enabled?