DisplaySettings: Lazily load wallpapers

Load the wallpaper in a background action instead of on the main thread.

This reduces the time to first paint, and makes the UI feel more
responsive when clicking on wallpaper thumbnails.

The behavior of the method is changed slightly to return true if it
succesfully "loads" the empty path. This makes the API a little more
consistent, where "true" means "I made changes" and "false" means "I did
not make changes". No call sites currently use the return value, so no
changes are needed to those.

Userland/Applications/DisplaySettings/MonitorWidget.cpp

@@ -10,6 +10,7 @@
 #include <LibGUI/Painter.h>
 #include <LibGfx/Bitmap.h>
 #include <LibGfx/Font.h>
+#include <LibThreading/BackgroundAction.h>
 
 REGISTER_WIDGET(DisplaySettings, MonitorWidget)
 
@@ -26,23 +27,35 @@ MonitorWidget::MonitorWidget()
 
 bool MonitorWidget::set_wallpaper(String path)
 {
-    if (path == m_desktop_wallpaper_path)
+    if (!is_different_to_current_wallpaper_path(path))
         return false;
 
-    if (path.is_empty()) {
-        m_wallpaper_bitmap = nullptr;
+    Threading::BackgroundAction<RefPtr<Gfx::Bitmap>>::create(
+        [path](auto&) {
+            RefPtr<Gfx::Bitmap> bmp;
+            if (!path.is_empty())
+                bmp = Gfx::Bitmap::try_load_from_file(path);
+            return bmp;
+        },
+
+        [this, path](RefPtr<Gfx::Bitmap> bitmap) {
+            // If we've been requested to change while we were loading the bitmap, don't bother spending the cost to
+            // move and render the now stale bitmap.
+            if (is_different_to_current_wallpaper_path(path))
+                return;
+            if (!bitmap.is_null())
+                m_wallpaper_bitmap = move(bitmap);
+            else
+                m_wallpaper_bitmap = nullptr;
+            m_desktop_dirty = true;
+            update();
+        });
+
+    if (path.is_empty())
         m_desktop_wallpaper_path = nullptr;
-        m_desktop_dirty = true;
-        update();
-        return false;
-    }
+    else
+        m_desktop_wallpaper_path = move(path);
 
-    auto bitmap = Gfx::Bitmap::try_load_from_file(path);
-    if (bitmap)
-        m_wallpaper_bitmap = move(bitmap);
-    m_desktop_wallpaper_path = move(path);
-    m_desktop_dirty = true;
-    update();
     return true;
 }
 

Userland/Applications/DisplaySettings/MonitorWidget.h

@@ -47,6 +47,11 @@ private:
     Gfx::IntSize m_desktop_resolution;
     int m_desktop_scale_factor { 1 };
     Gfx::Color m_desktop_color;
+
+    bool is_different_to_current_wallpaper_path(String const& path)
+    {
+        return (!path.is_empty() && path != m_desktop_wallpaper_path) || (path.is_empty() && m_desktop_wallpaper_path != nullptr);
+    }
 };
 
 }