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[LibreOffice.git] / vcl / README.lifecycle

** Understanding transitional VCL lifecycle **

---------- How it used to look ----------

        All VCL classes were explicitly lifecycle managed; so you would
do:
        Dialog aDialog(...);   // old - on stack allocation
        aDialog.Execute(...);
or:
        Dialog *pDialog = new Dialog(...);  // old - manual heap allocation
        pDialog->Execute(...);
        delete pDialog;
or:
        std::shared_ptr<Dialog> xDialog(new pDialog()); // old
        xDialog->Execute(...);
        // depending who shared the ptr this would be freed sometime

        In several cases this lead to rather unpleasant code, when
various shared_ptr wrappers were used, the lifecycle was far less than
obvious. Where controls were wrapped by other ref-counted classes -
such as UNO interfaces, which were also used by native Window
pointers, the lifecycle became extremely opaque. In addition VCL had
significant issues with re-enterancy and event emission - adding
various means such as DogTags to try to detect destruction of a window
between calls:

        ImplDelData aDogTag( this );    // 'orrible old code
        Show( true, ShowFlags::NoActivate );
        if( !aDogTag.IsDead() )         // did 'this' go invalid yet ?
                Update();

        Unfortunately use of such protection is/was ad-hoc, and far
from uniform, despite the prevelance of such potential problems.

        When a lifecycle problem was hit, typically it would take the
form of accessing memory that had been freed, and contained garbage due
to lingering pointers to freed objects.


---------- Where we are now: ----------

        To fix this situation we now have a VclPtr - which is a smart
        reference-counting pointer (include/vcl/vclptr.hxx) which is
        designed to look and behave -very- much like a normal pointer
        to reduce code-thrash. VclPtr is used to wrap all OutputDevice
        derived classes thus:

        VclPtr<Dialog> pDialog( new Dialog( ... ), SAL_NO_ACQUIRE );
        ...
        pDialog.disposeAndClear();

        However - while the VclPtr reference count controls the
        lifecycle of the Dialog object, it is necessary to be able to
        break reference count cycles. These are extremely common in
        widget hierarchies as each widget holds (smart) pointers to
        its parents and also its children.

        Thus - all previous 'delete' calls are replaced with 'dispose'
        method calls:

** What is dispose ?

        Dispose is defined to be a method that releases all references
        that an object holds - thus allowing their underlying
        resources to be released. However - in this specific case it
        also releases all backing graphical resources. In practical
        terms, all destructor functionality has been moved into
        'dispose' methods, in order to provide a minimal initial
        behavioral change.

        As such a VclPtr can have three states:

        VclPtr<PushButton> pButton;
        ...
        assert (pButton == nullptr || !pButton);    // null
        assert (pButton && !pButton->IsDisposed()); // alive
        assert (pButton &&  pButton->IsDisposed()); // disposed

** ScopedVclPtr - making disposes easier

        While replacing existing code with new, it can be a bit
        tiresome to have to manually add 'disposeAndClear()'
        calls to VclPtr<> instances.

        Luckily it is easy to avoid that with a ScopedVclPtr which
        does this for you when it goes out of scope.

** One extra gotcha - an initial reference-count of 1

        In the normal world of love and sanity, eg. creating UNO
        objects, the objects start with a ref-count of zero. Thus
        the first reference is always taken after construction by
        the surrounding smart pointer.

        Unfortunately, the existing VCL code is somewhat tortured,
        and does a lot of reference and de-reference action on the
        class -during- construction. This forces us to construct with
        a reference of 1 - and to hand that into the initial smart
        pointer with a SAL_NO_ACQUIRE.

        To make this easier, we have 'Instance' template wrappers
        that make this apparently easier, by constructing the
        pointer for you.

** How does my familiar code change ?

        Lets tweak the exemplary code above to fit the new model:

-       Dialog aDialog(... dialog params ... );
-       aDialog.Execute(...);
+       ScopedVclPtrInstance<Dialog> pDialog(... dialog params ... );
+       pDialog->Execute(...); // VclPtr behaves much like a pointer

or:
-       Dialog *pDialog = new Dialog(... dialog params ...);
+       VclPtrInstance<Dialog> pDialog(... dialog params ...);
        pDialog->Execute(...);
-       delete pDialog;
+       pDialog.disposeAndClear(); // done manually - replaces a delete
or:
-       std::shared_ptr<Dialog> xDialog(new Dialog(...));
+       ScopedVclPtrInstance<Dialog> xDialog(...);
        xDialog->Execute(...);
+       // depending how shared_ptr was shared perhaps
+       // someone else gets a VclPtr to xDialog
or:
-       VirtualDevice aDev;
+       ScopedVclPtrInstance<VirtualDevice> pDev;

        Other things that are changed are these:

-       pButton = new PushButton(NULL);
+       pButton = VclPtr<PushButton>::Create(nullptr);
...
-       vcl::Window *pWindow = new PushButton(NULL);
+       VclPtr<vcl::Window> pWindow;
+       pWindow.reset(VclPtr<PushButton>::Create(nullptr));

** Why are these 'disposeOnce' calls in destructors ?

        This is an interim measure while we are migrating, such that
        it is possible to delete an object conventionally and ensure
        that its dispose method gets called. In the 'end' we would
        instead assert that a Window has been disposed in its
        destructor, and elide these calls.

        As the object's vtable is altered as we go down the
        destruction process, and we want to call the correct dispose
        methods we need this disposeOnce(); call for the interim in
        every destructor. This is enforced by a clang plugin.

        The plus side of disposeOnce is that the mechanics behind it
        ensure that a dispose() method is only called a single time,
        simplifying their implementation.


---------- Who owns & disposes what ? ----------

        Window sub-classes tend to create their widgets in one of two
ways and often both.

        1. Derive from VclBuilderContainer. The VclBuilder then owns
           many of the sub-windows, which are fetched by a 'get'
           method into local variables often in constructors eg.

           VclPtr<PushButton> mpButton;  // in the class
           , get(mpButton, "buttonName") // in the constructor
           mpButton.clear();             // in dispose.

           We only clear, not disposeAndClear() in our dispose method
           for this case, since the VclBuilder / Container truly owns
           this Window, and needs to dispose its hierarchy in the
           right order - first children then parents.

         2. Explicitly allocated Windows. These are often created and
            managed by custom widgets:

            VclPtr<ComplexWidget> mpComplex;                     // in the class
            , mpComplex( VclPtr<ComplexWidget>::Create( this ) ) // constructor
            mpComplex.disposeAndClear();                         // in dispose

            ie. an owner has to dispose things they explicitly allocate.

          In order to ensure that the VclBuilderConstructor
          sub-classes have their Windows disposed at the correct time
          there is a disposeBuilder(); method - that should be added
          -only- to the class immediately deriving from
          VclBuilderContainer's dispose.

---------- What remains to be done ? ----------

        * Cleanup DogTags and LazyDelete.

        * Expand the VclPtr pattern to many other less
          than safe VCL types.

        * create factory functions for VclPtr<> types and privatize
          their constructors.

        * Pass 'const VclPtr<> &' instead of pointers everywhere
                + add 'explicit' keywords to VclPtr constructors to
                  accelerate compilation etc.

        * Cleanup common existing methods such that they continue to
          work post-dispose.

        * Dispose functions should be audited to:
                + not leave dangling pointsr
                + shrink them - some work should incrementally
                  migrate back to destructors.

        * VclBuilder
                + ideally should keep a reference to pointers assigned
                  in 'get()' calls - to avoid needing explicit 'clear'
                  code in destructors.

        * VclBuilder 'makeFoo' methods
                + these should return VclPtr<> types and have their
                  signatures adjusted en-masse.
                + currently we use a VclPtr<> constructor with
                  SAL_NO_ACQUIRE inside the builder.

---------- FAQ / debugging hints ----------

** Compile with dbgutil

        This is by far the best way to turn on debugging and
        assertions that help you find problems. In particular
        there are a few that are really helpful:

        vcl/source/window/window.cxx (Window::dispose)
                "Window ( N4sfx27sidebar20SidebarDockingWindowE (Properties))
                          ^^^ class name                 window title ^^^
                 with live children destroyed:  N4sfx27sidebar6TabBarE ()
                 N4sfx27sidebar4DeckE () 10FixedImage ()"

        You can de-mangle these names if you can't read them thus:

        $ c++filt -t N4sfx27sidebar20SidebarDockingWindowE
        sfx2::sidebar::SidebarDockingWindow

        In the above case - it is clear that the children have not been
        disposed before their parents. As an aside, having a dispose chain
        separate from destructors allows us to emit real type names for
        parents here.

        To fix this, we will need to get the dispose ordering right,
        occasionally in the conversion we re-ordered destruction, or
        omitted a disposeAndClear() in a ::dispose() method.

        => If you see this, check the order of disposeAndClear() in
           the sfx2::Sidebar::SidebarDockingWindow::dispose() method

        => also worth git grepping for 'new sfx::sidebar::TabBar' to
           see where those children were added.

** Check what it used to do

        While a ton of effort has been put into ensuring that the new
        lifecycle code is the functional equivalent of the old code,
        the code was created by humans. If you identify an area where
        something asserts or crashes here are a few helpful heuristics:

        * Read the git log -u -- path/to/file.cxx

        => Is the order of destruction different ?

           in the past many things were destructed (in reverse order of
           declaration in the class) without explicit code. Some of these
           may be important to do explicitly at the end of the destructor.

           eg. having a 'Idle' or 'Timer' as a member, may now need an
               explicit .Stop() and/or protection from running on a
               disposed Window in its callback.

        => Is it 'clear' not 'disposeAndClear' ?

           sometimes we get this wrong. If the code previously used to
           use 'delete pFoo;' it should now read pFoo->disposeAndClear();
           Conversely if it didn't delete it, it should be 'clear()' it
           is by far the best to leave disposing to the VclBuilder where
           possible.

           In simple cases, if we allocate the widget with VclPtrInstance
           or VclPtr<Foo>::Create - then we need to disposeAndClear it too.

** Event / focus / notification ordering

        In the old world, a large amount of work was done in the
        ~Window destructor that is now done in Window::dispose.

        Since those Windows were in the process of being destroyed
        themselves, their vtables were adjusted to only invoke Window
        methods. In the new world, sub-classed methods such as
        PreNotify, GetFocus, LoseFocus and others are invoked all down
        the inheritance chain from children to parent, during dispose.

        The easiest way to fix these is to just ensure that these
        cleanup methods, especially LoseFocus continue to work even
        on disposed Window sub-class instances.

** It crashes with some invalid memory ...

        Assuming that the invalid memory is a Window sub-class itself,
        then almost certainly there is some cockup in the
        reference-counting; eg. if you hit an OutputDevice::release
        assert on mnRefCount - then almost certainly you have a
        Window that has already been destroyed. This can easily
        happen via this sort of pattern:

        ModelessDialog *pDlg = VclPtr<ModelessDialog>(nullptr /* parent */);
        // by here the pDlg quite probably points to free'd memory
        ...

        It is necessary in these cases to ensure that the *pDlg is
        a VclPtr<ModelessDialog> instead.

** It crashes with some invalid memory #2 ...

        Often a ::dispose method will free some pImpl member, but
        not NULL it; and (cf. above) we can now get various virtual
        methods called post-dispose; so:

        a) delete pImpl; pImpl = NULL; // in the destructor
        b) if (pImpl && ...)           // in the subsequently called method