0dac91cffc
MetaGravity is an enum, where the values match the X11 macros used for gravity, with the exception that `ForgetGravity` was renamed `META_GRAVITY_NONE` to have less of a obscure name. The motivation for this is to rely less on libX11 data types and macros in generic code. https://gitlab.gnome.org/GNOME/mutter/merge_requests/705
284 lines
14 KiB
Plaintext
284 lines
14 KiB
Plaintext
File contents:
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Basic Ideas
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Important points to remember
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Explanation of fields in the ConstraintInfo struct
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Gory details of resize_gravity vs. fixed_directions
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IMPORTANT NOTE: There's a big comment at the top of constraints.c
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explaining how to add extra constraints or tweak others. Read it. I put
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that information there because it may be enough information by itself for
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people to hack on constraints.c. I won't duplicate that information in
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this file; this file is for deeper details.
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---------------------------------------------------------------------------
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Basic Ideas
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---------------------------------------------------------------------------
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There are a couple basic ideas behind how this constraints.c code works and
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why it works that way:
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1) Split the low-level error-prone operations into a special file
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2) Add robustness by prioritizing constraints
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3) Make use of a minimal spanning set of rectangles for the
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"onscreen region" (screen minus struts).
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4) Constraints can be user-action vs app-action oriented
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5) Avoid over-complification ;-)
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Some more details explaining these basic ideas:
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1) Split tedious operations out
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boxes.[ch] have been added which contain many common, tedious, and
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error-prone operations. I find that this separation helps a lot for
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managing the complexity and ensuring that things work correctly.
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Also, note that testboxes.c thoroughly tests all functionality in
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boxes.[ch] and a testboxes program is automatically compiled.
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Note that functions have also been added to this file to handle some
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of the tedium necessary for edge resistance as well.
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2) Prioritize constraints
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In the old code, if each and every constraint could not be
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simultaneously satisfied, then it would result in some
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difficult-to-predict set of constraints being violated. This was
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because constraints were applied in order, with the possibility for
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each making changes that violated previous constraints, with no
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checking done at the end.
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Now, all constraints have an associated priority, defined in the
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ConstraintPriority enum near the top of constraints.c. The
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constraints are all applied, and then are all checked; if not all are
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satisfied then the least important constraints are dropped and the
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process is repeated. This ensures that the most important constraints
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are satisfied.
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A special note to make here is that if any one given constraint is
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impossible to satisfy even individually (e.g. if minimum size hints
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specify a larger window than the screen size, making the
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fully-onscreen constraint impossible to satisfy) then we treat the
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constraint as being satisfied. This sounds counter-intuitive, but the
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idea is that we want to satisfy as many constraints as possible and if
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we treat it as a violation then all constraints with a lesser priority
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also get dropped along with the impossible to satisfy one.
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3) Using maximal/spanning rectangles
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The constraints rely heavily on something I call spanning rectangles
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(which Soeren referred to as maximal rectangles, a name which I think
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I like better but I don't want to go change all the code now). These
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spanning rectangles have the property that a window will fit on the
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screen if and only if it fits within at least one of the rectangles.
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Soeren had an alternative way of describing these rectangles, namely
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that they were rectangles with the property that if you made any of
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them larger in any direction, they would overlap with struts or be
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offscreen (with the implicit assumption that there are enough of these
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rectangles that combined they cover all relevant parts of the screen).
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Note that, by necessity, these spanning/maximal rectangles will often
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overlap each other.
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Such a list makes it relatively easy to define operations like
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window-is-onscreen or clamp-window-to-region or
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shove-window-into-region. Since we have a on-single-xinerama
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constraint in addition to the onscreen constraint(s), we cache
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number_xineramas + 1 of these lists in the workspace. These lists
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then only need to be updated whenever the workarea is (e.g. when strut
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list change or screen or xinerama size changes).
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4) Constraints can be user-action vs app-action oriented
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Such differentiation requires special care for the constraints to be
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consistent; e.g. if the user does something and one constraint
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applies, then the app does something you have to be careful that the
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constraint on the app action doesn't result in some jarring motion.
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In particular, the constraints currently allow offscreen movement or
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resizing for user actions only. The way consistency is handled is
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that at the end of the constraints, update_onscreen_requirements()
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checks to see if the window is offscreen or split across xineramas and
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updates window->require_fully_onscreen and
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window->require_on_single_xinerama appropriately.
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5) Avoid over-complification
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The previous code tried to reform the constraints into terms of a
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single variable. This made the code rather difficult to
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understand. ("This is a rather complicated fix for an obscure bug
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that happened when resizing a window and encountering a constraint
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such as the top edge of the screen.") It also failed, even on the
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very example for which it used as justification for the complexity
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(bug 312104 -- when keyboard resizing the top of the window,
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Metacity extends the bottom once the titlebar hits the top panel),
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though the reason why it failed is somewhat mysterious as it should
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have worked. Further, it didn't really reform the constraints in
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terms of a single variable -- there was both an x_move_delta and an
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x_resize_delta, and the existence of both caused bug 109553
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(gravity with simultaneous move and resize doesn't work)
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---------------------------------------------------------------------------
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Important points to remember
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---------------------------------------------------------------------------
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- Inner vs Outer window
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Note that because of how configure requests work and
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meta_window_move_resize_internal() and friends are set up, that the
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rectangles passed to meta_window_constrain() are with respect to inner
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window positions instead of outer window positions (meaning that window
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manager decorations are not included in the position/size). For the
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constraints that need to be enforced with respect to outer window
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positions, you'll need to make use of the extend_by_frame() and
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unextend_by_frame() functions.
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- meta_window_move_resize_internal() accepts a really hairy set of
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inputs. See the huge comment at the beginning of that function.
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constraints gets screwed up if that function can't sanitize the input,
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so be very careful about that. It used to be pretty busted.
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---------------------------------------------------------------------------
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Explanation of fields in the ConstraintInfo strut
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---------------------------------------------------------------------------
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As of the time of this writing, ConstraintInfo had the following fields:
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orig
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current
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fgeom
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action_type
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is_user_action
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resize_gravity
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fixed_directions
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work_area_xinerama
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entire_xinerama
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usable_screen_region
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usable_xinerama_region
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A brief description of each and/or pointers to more information are found
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below:
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orig
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The previous position and size of the window, ignoring any window
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decorations
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current
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The requested position and size of the window, ignoring any window
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decorations. This rectangle gets modified by the various constraints
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to specify the allowed position closest to the requested position.
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fgeom
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The geometry of the window frame (i.e. "decorations"), if it exists.
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Otherwise, it's a dummy 0-size frame for convenience (i.e. this pointer
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is guaranteed to be non-NULL so you don't have to do the stupid check).
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action_type
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Whether the action being constrained is a move, resize, or a combined
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move and resize. Some constraints can run faster with this information
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(e.g. constraining size increment hints or min size hints don't need to
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do anything for pure move operations). This may also be used for
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providing slightly different behavior (e.g. clip-to-region instead of
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shove-into-region for resize vs. moving operations), but doesn't
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currently have a lot of use for this.
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is_user_action
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Used to determine whether the action being constrained is a user
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action. If so, certain parts of the constraint may be relaxed. Note
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that this requires care to get right; see item 4 of the basic ideas
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section for more details.
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resize_gravity
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The gravity used in the resize operation, used in order to make sure
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windows are resized correctly if constraints specify that their size
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must be modified. Explained further in the resize_gravity
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vs. fixed_directions section.
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fixed_directions
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There may be multiple solutions to shoving a window back onscreen.
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Typically, the shortest distance used is the solution picked, but if
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e.g. an application only moved its window in a single direction, it's
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more desirable that the window is shoved back in that direction than in
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a different one. fixed_directions facilitates that. Explained further
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in the resize_gravity vs. fixed_directions section.
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work_area_xinerama
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This region is defined in the workspace and just cached here for
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convenience. It is basically the area obtained by taking the current
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xinerama, treating all partial struts as full struts, and then
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subtracting all struts from the current xinerama region. Useful
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e.g. for enforcing maximization constraints.
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entire_xinerama
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Just a cache of the rectangle corresponding to the entire current
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xinerama, including struts. Useful e.g. for enforcing fullscreen
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constraints.
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usable_screen_region
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The set of maximal/spanning rectangles for the entire screen; this
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region doesn't overlap with any struts and helps to enforce
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e.g. onscreen constraints.
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usable_xinerama_region
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The set of maximal/spanning rectangles for the current xinerama; this
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region doesn't overlap with any struts on the xinerama and helps to
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enforce e.g. the on-single-xinerama constraint.
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---------------------------------------------------------------------------
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Gory details of resize_gravity vs. fixed_directions
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---------------------------------------------------------------------------
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Note that although resize_gravity and fixed_directions look similar, they
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are used for different purposes:
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- resize_gravity is only for resize operations and is used for
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constraints unrelated to keeping a window within a certain region
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- fixed_directions is for both move and resize operations and is
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specifically for keeping a window within a specified region.
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Examples of where each are used:
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- If a window is simultaneously moved and resized to the southeast corner
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with META_GRAVITY_SOUTH_EAST, but it turns out that the window was sized to
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something smaller than the minimum size hint, then the size_hints
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constraint should resize the window using the resize_gravity to ensure
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that the southeast corner doesn't move.
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- If an application resizes itself so that it grows downward only (which
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I note could be using any of three different gravities, most likely
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NorthWest), and happens to put the southeast part of the window under a
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partial strut, then the window needs to be forced back on screen.
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(Yes, shoved onscreen and not clipped; see bug 136307). It may be the
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case that moving the window to the left results in less movement of the
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window than moving the window up, which, in the absence of fixed
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directions would cause us to chose moving to the left. But since the
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user knows that only the height of the window is changing, they would
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find moving to the left weird (especially if this were a dialog that
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had been centered on its parent). It'd be better to shove the window
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upwards so we make sure to keep the left and right sides fixed in this
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case. Note that moving the window upwards (or leftwards) is probably
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totally against the gravity in this case; but that's okay because
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gravity typically assumes there's more than enough onscreen space for
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the resize and we only override the gravity when that assumption is
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wrong.
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For the paranoid, a fixed directions might give an impossible to fulfill
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constraint (I don't think that's true currently in the code, but I haven't
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thought it through in a while). If this ever becomes a problem, it should
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be relatively simple to throw out the fixed directions when this happens
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and rerun the constraint. Of course, it might be better to rethink things
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to just avoid such a problem.
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The nitty gritty of what gets fixed:
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User move:
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in x direction - y direction fixed
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in y direction - x direction fixed
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in both dirs. - neither direction fixed
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User resize: (note that for clipping, only 1 side ever changed)
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in x direction - y direction fixed (technically opposite x side fixed too)
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in y direction - x direction fixed (technically opposite y side fixed too)
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in both dirs. - neither direction fixed
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App move:
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in x direction - y direction fixed
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in y direction - x direction fixed
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in both dirs. - neither direction fixed
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App resize
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in x direction - y direction fixed
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in y direction - x direction fixed
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in 2 parallel directions (center side gravity) - other dir. fixed
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in 2 orthogonal directions (corner gravity) - neither dir. fixed
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in 3 or 4 directions (a center-like gravity) - neither dir. fixed
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Move & resize
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Treat like resize case though this will usually mean all four sides
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change and result in neither direction being fixed
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Note that in all cases, if neither direction moves it is likely do to a
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change in struts and thus neither direction should be fixed despite the
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lack of movement.
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