[Bugs] #967 UNSP: Loading screens are impossible

[SugarLabs Bugs]

#967: Loading screens are impossible
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    Reporter:  bemasc                     |          Owner:  tomeu                      
        Type:  defect                     |         Status:  new                        
    Priority:  Unspecified by Maintainer  |      Milestone:  Unspecified by Release Team
   Component:  sugar                      |        Version:  0.82.x                     
    Severity:  Unspecified                |       Keywords:                             
Distribution:  Unspecified                |   Status_field:  Unconfirmed                
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 If an activity author knows that read_file is going to take a long time,
 it would be great to be able to show a specific loading screen, rather
 than waiting at the pulsing activity icon. Unfortunately, this is
 impossible (at least in 0.82).  The problem is that set_canvas() causes
 read_file to be invoked, if the activity is being resumed.  read_file() is
 invoked before the screen is drawn, so there is no way to use set_canvas()
 to show a loading screen.

 One reason this might be is: in activity.py, set_canvas() connects 'map'
 to __canvas_map_cb().  __canvas_map_cb() then calls read_file if there is
 a file to read.  However, disabling this callback entirely does not solve
 the problem.  There must be something more complicated going on that
 prevents the activity from showing a GUI until after read_file.

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Ticket URL: <http://dev.sugarlabs.org/ticket/967>
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