Fix pango API misuse, add pie-chart cache

widgets/piechart.lua

@@ -13,7 +13,7 @@ local function draw_label(cr,angle,radius,center_x,center_y,text)
   cr:line_to(center_x+(1.5*radius)*math.cos(angle),center_y+(1.5*radius)*math.sin(angle))
   local x,y = cr:get_current_point()
   cr:line_to(x+(x>center_x and radius/2 or -radius/2),y)
-  cr:move_to(x+(x>center_x and radius/2 + 5 or (-radius/2 - cr:text_extents(text).width - 5)),y+(cr:font_extents().height/4))
+  cr:move_to(x+(x>center_x and radius/2 + 5 or (-radius/2 - cr:text_extents(text).width - 5)),y+(cr.font_extents.height/4))
   cr:show_text(text)
   cr:stroke()
   cr:arc(center_x+(radius/2)*math.cos(angle),center_y+(radius/2)*math.sin(angle),2,0,2*math.pi)
@@ -40,17 +40,35 @@ end
 
 local function draw(self, w, cr, width, height)
   if not self._data then return end
+
+  -- Load from cache
+  local cached = self._cache[width*123+height*89]
+  if cached then
+    cr:set_source_surface(cached)
+    cr:paint()
+    return
+  end
+
+  local img = cairo.ImageSurface.create(cairo.Format.ARGB32, width, height)
+  local cr2 = cairo.Context(img)
+
   local radius = (height > width and width or height) / 4
   local sum,start,count = compute_sum(self._data),0,0
   for k,v in pairs(self._data) do
     local end_angle = start + 2*math.pi*(v/sum)
-    draw_pie(cr,start,end_angle,radius,colors[math.mod(count,4)+1],width/2,height/2)
-    draw_label(cr,start+(end_angle-start)/2,radius,width/2,height/2,k)
+    draw_pie(cr2,start,end_angle,radius,colors[math.mod(count,4)+1],width/2,height/2)
+    draw_label(cr2,start+(end_angle-start)/2,radius,width/2,height/2,k)
     start,count = end_angle,count+1
   end
+  self._cache[width*123+height*89] = img
+
+  --Paint on the drawable
+  cr:set_source_surface(img)
+  cr:paint()
 end
 
 local function set_data(self,data)
+  self._cached = {}
   self._data = data
   self:emit_signal("widget::updated")
 end
@@ -59,6 +77,7 @@ local function new(data)
   if not colors then colors = {color(beautiful.fg_normal),color(beautiful.bg_alternate),color(beautiful.fg_focus),color(beautiful.bg_highlight)} end
   local im = wibox.widget.imagebox()
   im._data,im.draw,im.set_data = dummy_dataset,draw,set_data
+  im._cache = {}
   return im
 end