Set the wallpaper more intelligently

Usually users want a wallpaper on all their screens. With the current code, this
resulted in a loop of upload-wallpaper, {download-wallpaper, add-new-part,
upload-wallpaper}*.

Fix this by being more intelligent: Instead of setting the wallpaper
immediately, this now uses gears.timer.delayed_call() to set the wallpaper. All
following modifications which come in before the delayed call runs will still be
part of the current update. This should mean that during startup, there is just
a single upload of a wallpaper.

(The above is what happens if there is no wallpaper yet. If there is already
one, we use :create_similar() and thus should only upload the part of the
wallpaper that changed, but this doesn't really make a difference.)

As a side-effect, the new code no longer draws to the old wallpaper to modify
it, but always creates a copy of it. This means that:

Fixes https://github.com/awesomeWM/awesome/issues/288.
Closes https://github.com/awesomeWM/awesome/pull/530.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/gears/wallpaper.lua

@@ -8,6 +8,7 @@
 local cairo = require("lgi").cairo
 local color = require("gears.color")
 local surface = require("gears.surface")
+local timer = require("gears.timer")
 
 local wallpaper = { mt = {} }
 
@@ -27,29 +28,56 @@ do
     end
 end
 
---- Prepare the needed state for setting a wallpaper
+-- A cairo surface that we still want to set as the wallpaper
+local pending_wallpaper = nil
+
+--- Prepare the needed state for setting a wallpaper.
+-- This function returns a cairo context through which a wallpaper can be drawn.
+-- The context is only valid for a short time and should not be saved in a
+-- global variable.
 -- @param s The screen to set the wallpaper on or nil for all screens
--- @return The available geometry (table with entries width and height), a
---   that should be used for setting the wallpaper and a cairo context for
---   drawing to this surface
-local function prepare_wallpaper(s)
+-- @return[1] The available geometry (table with entries width and height)
+-- @return[1] A cairo context that the wallpaper should be drawn to
+function wallpaper.prepare_context(s)
     local geom = s and screen[s].geometry or root_geom
-    local img = surface(root.wallpaper())
+    local cr
 
-    if not img then
-        -- No wallpaper yet, create an image surface for just the part we need
-        img = cairo.ImageSurface(cairo.Format.RGB24, geom.width, geom.height)
-        img:set_device_offset(-geom.x, -geom.y)
+    if not pending_wallpaper then
+        -- Prepare a pending wallpaper
+        local wp = surface(root.wallpaper())
+
+        if not wp then
+            -- No wallpaper yet
+            wp = cairo.ImageSurface(cairo.Format.RGB24, 0, 0)
+        end
+
+        pending_wallpaper = wp:create_similar(cairo.Content.COLOR, root_geom.width, root_geom.height)
+
+        -- Copy the old wallpaper to the new one
+        cr = cairo.Context(pending_wallpaper)
+        cr:save()
+        cr.operator = cairo.Operator.SOURCE
+        cr:set_source_surface(wp, 0, 0)
+        cr:paint()
+        cr:restore()
+
+        -- Set the wallpaper (delayed)
+        timer.delayed_call(function()
+            local wp = pending_wallpaper
+            pending_wallpaper = nil
+            wallpaper.set(wp)
+        end)
+    else
+        -- Draw to the already-pending wallpaper
+        cr = cairo.Context(pending_wallpaper)
     end
 
-    local cr = cairo.Context(img)
-
     -- Only draw to the selected area
     cr:translate(geom.x, geom.y)
     cr:rectangle(0, 0, geom.width, geom.height)
     cr:clip()
 
-    return geom, img, cr
+    return geom, cr
 end
 
 --- Set the current wallpaper.
@@ -75,7 +103,7 @@ end
 -- @param background The background color that should be used. Gets handled via
 --   gears.color. The default is black.
 function wallpaper.centered(surf, s, background)
-    local geom, img, cr = prepare_wallpaper(s)
+    local geom, cr = wallpaper.prepare_context(s)
     local surf = surface(surf)
     local background = color(background)
 
@@ -91,8 +119,6 @@ function wallpaper.centered(surf, s, background)
     cr:clip()
     cr:set_source_surface(surf, 0, 0)
     cr:paint()
-
-    wallpaper.set(img)
 end
 
 --- Set a tiled wallpaper.
@@ -101,7 +127,7 @@ end
 --   all screens are set.
 -- @param offset This can be set to a table with entries x and y.
 function wallpaper.tiled(surf, s, offset)
-    local geom, img, cr = prepare_wallpaper(s)
+    local geom, cr = wallpaper.prepare_context(s)
 
     if offset then
         cr:translate(offset.x, offset.y)
@@ -112,8 +138,6 @@ function wallpaper.tiled(surf, s, offset)
     cr.source = pattern
     cr.operator = cairo.Operator.SOURCE
     cr:paint()
-
-    wallpaper.set(img)
 end
 
 --- Set a maximized wallpaper.
@@ -124,7 +148,7 @@ end
 --   The default is to honor the aspect ratio.
 -- @param offset This can be set to a table with entries x and y.
 function wallpaper.maximized(surf, s, ignore_aspect, offset)
-    local geom, img, cr = prepare_wallpaper(s)
+    local geom, cr = wallpaper.prepare_context(s)
     local surf = surface(surf)
     local w, h = surface.get_size(surf)
     local aspect_w = geom.width / w
@@ -147,8 +171,6 @@ function wallpaper.maximized(surf, s, ignore_aspect, offset)
     cr:set_source_surface(surf, 0, 0)
     cr.operator = cairo.Operator.SOURCE
     cr:paint()
-
-    wallpaper.set(img)
 end
 
 --- Set a fitting wallpaper.
@@ -158,7 +180,7 @@ end
 -- @param background The background color that should be used. Gets handled via
 --   gears.color. The default is black.
 function wallpaper.fit(surf, s, background)
-    local geom, img, cr = prepare_wallpaper(s)
+    local geom, cr = wallpaper.prepare_context(s)
     local surf = surface(surf)
     local background = color(background)
 
@@ -179,8 +201,6 @@ function wallpaper.fit(surf, s, background)
     cr:scale(scale, scale)
     cr:set_source_surface(surf, 0, 0)
     cr:paint()
-
-    wallpaper.set(img)
 end
 
 return wallpaper