Add a method to create a Pixbuf with the preview in the metadata

This code was moved from sugar expandedentry.py to be available to use
in other places. Some activities like Portfolio or JournalShare
have this code copied.

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>

src/sugar3/graphics/objectchooser.py

@@ -20,6 +20,8 @@ STABLE.
 """
 
 import logging
+import StringIO
+import cairo
 
 from gi.repository import GObject
 from gi.repository import Gtk
@@ -27,6 +29,7 @@ from gi.repository import Gdk
 import dbus
 
 from sugar3.datastore import datastore
+from sugar3.activity.activity import PREVIEW_SIZE
 
 
 J_DBUS_SERVICE = 'org.laptop.Journal'
@@ -38,6 +41,63 @@ FILTER_TYPE_GENERIC_MIME = 'generic_mime'
 FILTER_TYPE_ACTIVITY = 'activity'
 
 
+def get_preview_pixbuf(preview_data, width=-1, height=-1):
+    """Retrive a pixbuf with the content of the preview field
+
+    Keyword arguments:
+    metadata -- the metadata dictionary.
+                Can't be None, use metadata.get('preview', '')
+    width -- the pixbuf width, if is not set, the default width will be used
+    height -- the pixbuf width, if is not set, the default height will be used
+
+    Return: a Pixbuf or None if couldn't create it
+
+    """
+    if width == -1:
+        width = PREVIEW_SIZE[0]
+
+    if height == -1:
+        height = PREVIEW_SIZE[1]
+
+    pixbuf = None
+
+    if len(preview_data) > 4:
+        if preview_data[1:4] != 'PNG':
+            # TODO: We are close to be able to drop this.
+            import base64
+            preview_data = base64.b64decode(preview_data)
+
+        png_file = StringIO.StringIO(preview_data)
+        try:
+            # Load image and scale to dimensions
+            surface = cairo.ImageSurface.create_from_png(png_file)
+            png_width = surface.get_width()
+            png_height = surface.get_height()
+
+            preview_surface = cairo.ImageSurface(cairo.FORMAT_ARGB32,
+                                                 width, height)
+            cr = cairo.Context(preview_surface)
+
+            scale_w = width * 1.0 / png_width
+            scale_h = height * 1.0 / png_height
+            scale = min(scale_w, scale_h)
+
+            cr.scale(scale, scale)
+
+            cr.set_source_rgba(1, 1, 1, 0)
+            cr.set_operator(cairo.OPERATOR_SOURCE)
+            cr.paint()
+            cr.set_source_surface(surface)
+            cr.paint()
+
+            pixbuf = Gdk.pixbuf_get_from_surface(preview_surface, 0, 0,
+                                                 width, height)
+        except Exception:
+            logging.exception('Error while loading the preview')
+
+    return pixbuf
+
+
 class ObjectChooser(object):
 
     def __init__(self, parent=None, what_filter=None, filter_type=None):