add get-default-percentages to panel:dragable and then add a dragable panel

to the debugger

original commit: 951de8cc51b446b6041e64bc37356392266a3532

collects/framework/private/panel.rkt

@@ -1,6 +1,7 @@
 #lang racket/unit
 
-  (require mzlib/class
+  (require racket/class
+           racket/list
            "sig.rkt"
            mred/mred-sig)
 
@@ -169,7 +170,8 @@
       after-percentage-change
       set-percentages
       get-percentages
-      get-vertical?))
+      get-vertical?
+      get-default-percentages))
   
   (define vertical-dragable<%>
     (interface (dragable<%>)))
@@ -244,12 +246,23 @@
           (unless (= len-children (length percentages))
             (cond
               [(zero? len-children)
-               '()]
+               (set! percentages '())]
               [else
-               (let ([rat (/ 1 len-children)])
-                 (set! percentages (build-list len-children (λ (i) (make-percentage rat)))))])
+               (set! percentages (map make-percentage (get-default-percentages len-children)))])
             (after-percentage-change))))
       
+      (define/pubment (get-default-percentages i) 
+        (define res (inner (if (zero? i) '() (make-list i (/ i)))
+                           get-default-percentages i))
+        (unless (and (list? res)
+                     (andmap (λ (x) (and (real? x) (<= 0 x 1))) res)
+                     (= 1 (apply + res))
+                     (= (length res) i))
+          (error 'get-default-percentages 
+                 "expected inner call to return a list of real numbers that sum to 1 and has length ~a"
+                 i))
+        res)
+      
       (define/override (after-new-child child)
         (update-percentages))
       

collects/scribblings/framework/panel.scrbl

@@ -45,7 +45,12 @@
 }
 
 @defmixin[panel:single-window-mixin (panel:single<%> window<%>) (panel:single-window<%>)]{
-  @defmethod*[#:mode override (((container-size (info (listof (list/c exact-integer? exact-integer? boolean? boolean?)))) (values exact-integer? exact-integer?)))]{
+  @defmethod*[#:mode override 
+                     (((container-size (info (listof (list/c exact-integer? 
+                                                             exact-integer? 
+                                                             boolean? 
+                                                             boolean?))))
+                       (values exact-integer? exact-integer?)))]{
     Factors the border width into the size calculation.
   }
 }
@@ -66,6 +71,15 @@
     Use @method[panel:dragable<%> get-percentages] to find the current
     percentages.
   }
+  
+  @defmethod[(get-default-percentages [subwindow-count exact-positive-integer?])
+             (listof (and/c real? (between/c 0 1)))]{
+     Called when the number of children in the panel changes;
+     the result is used as the initial percentages for each of the new
+     windows. 
+     
+     The numbers in the result list must sum to @racket[1].
+  }
 
   @defmethod*[(((set-percentages (new-percentages (listof number?))) void?))]{
     Call this method to set the percentages that each window takes up of the