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[Michael Zanetti <michael.zanetti@xxxxxxxxxxxxx>]

Should probably be added to this Qt api:

http://doc.qt.io/qt-5/qml-qtqml-qt.html#application-prop

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https://bugs.launchpad.net/bugs/1475678

Title:
  Unity8 not informing clients that they are not visible and thus can
  stop rendering

Status in QtMir:
  In Progress
Status in qtubuntu:
  In Progress
Status in unity8 package in Ubuntu:
  In Progress

Bug description:
  Currently on the phone, apps stop rendering when shell hides them,
  because shell also lifecycle stops the app. App isn't actually told it
  should stop rendering, we just SIGSTOP its process.

  But some apps are not lifecycle stopped, e.g. music. So we should have
  unity8 tell the app to stop rendering.

  
  This will be a multi-step process:
  1. Qtubuntu
  Mir has a attribute to set on surfaces to tell them they are visible/occluded: (see mir/include/common/mir_toolkit/common.h)
  mir_surface_attrib_visibility - part of the MirSurfaceAttrib enum. 

  This indicates these possible values:
  typedef enum MirSurfaceVisibility
  {
      mir_surface_visibility_occluded = 0,
      mir_surface_visibility_exposed
  } MirSurfaceVisibility;

  On the client side (qtubuntu), you need to listen for this attribute change, and connect it to QWindowSystemInterface::handleExposeEvent() which tells Qt how much of a QWindow is being drawn.
  Note there is also a QWindowSystemInterface::handleWindowStateChanged() handler, which use use to explicitly set QWindow show/hide, but Qt tends to release its GL context on hide, which we may not desire. It may not hurt, worth a look.

  2. QtMir
  There are methods on mir::scene::Surface to set and get the MirSurfaceAttrib attributes. I think it would make sense to use the pre-existing MirSurfaceItem::visible property, so that if a surface is marked visible=false, then that will dispatch the mir_surface_visibility_occluded event to the client, so it will stop drawing.

  3. Unity8
  Correctly set visible=false when an application surface is occluded, and true when visible. QML has no automatic way of doing that (i.e. occlusion detection)

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