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Pointer Events
==============
Scope
-----
The following input devices are supported by sky:
- fingers on multitouch screens
- mice, including mouse wheels
- styluses on screens
- other devices that emulate mice (track pads, track balls)
- [Keyboard](keyboard.md)
The following input devices are not supported natively by sky, but can
be used by connecting directly to the mojo application servicing the
relevant device:
- joysticks
- track balls that move focus (or raw data from track balls)
- raw data from track pads (e.g. multitouch gestures)
- raw data from styluses that have their own absolute pads
- raw data from mice (e.g. to handle mouse capture in 3D games)
The following interactions are intended to be easy to handle:
- one finger starts panning, another finger is placed on the surface
(and ignored), the first finger is lifted, and the second finger
continues panning (without the scroll position jumping when the
first finger is lifted)
- right-clicking doesn't trigger buttons by default
- fingers after the first within a surface don't trigger buttons by
default
- if there are two independent surfaces, they capture fingers
unrelated to each other
Frameworks are responsible for converting pointer events described
below into widget-specific events such as the following:
- a click/tap/activation, as distinct from a short drag
- a context menu request (e.g. right-click, long-press)
- a drag (moving an item)
- a pan (scroll)
- a zoom/rotation (whether using two finger gestures, or one finger
with the double-tap-and-hold gesture)
- a double-tap autozoom
In particular, this means distinguishing whether a finger tap consists
of a tap, a drag, or a long-press; it also means distinguishing
whether a drag, once established as such, should be treated as a pan
or a drag, and deciding whether a secondary touch should begin a
zoom/rotation or not.
This is done using the [gesture recogniser API](gestures.md)
Pointers
--------
Each touch or pointer is tracked individually.
New touches and pointers can appear and disappear over time.
Each pointer has a list of current targets.
When a new one enters the system, a non-bubbling 'pointer-added' event
is fired at the application's document, and the pointer's current
targets list is initialised to just that Document object.
When it is removed, a non-bubbling 'pointer-removed' event is fired at
the application's document and at any other objects in the pointer's
current targets list. Currently, at the time of a pointer-removed, the
list will always consist of only the document.
A pointer can be "up" or "down". Initially all pointers are "up".
A pointer switches from "up" to "down" when it is a touch or stylus
that is in contact with the display surface, or when it is a mouse
that is being clicked, and from "down" back to "up" when this ends.
(Note that clicking a button on a stylus doesn't change it from up to
down. A stylus can have a button pressed while "up".) In the case of a
mouse with multiple buttons, the pointer switches back to "up" only
when all the buttons have been released.
When a pointer switches from "up" to "down", the following algorithm
is run:
1. Hit test the position of the pointer, let 'node' be the result.
2. Fire a bubbling pointer-down event at the layoutManager for
'node', with an empty array as the default return value. Let
'result1' be the returned value.
3. If result1 is not an array of EventTarget objects, set it to the
empty array and (if this is debug mode) report the issue.
4. Fire a bubbling pointer-down event at the Element for 'node', with
an empty array as the default return value. Let 'result2' be the
returned value.
5. If result2 is not an array of EventTarget objects, set it to the
empty array and (if this is debug mode) report the issue.
6. Let result be the concatenation of result1's contents, result2's
contents, and the application document.
7. Let 'result' be this pointer's current targets.
When an object is one of the current targets of a pointer and no other
pointers have that object as a current target so far, and either there
are no buttons (touch, stylus) or only the primary button is active
(mouse) and this is not an inverted stylus, then that pointer is
considered the "primary" pointer for that object. The pointer remains
the primary pointer for that object until the corresponding pointer-up
event (even if the buttons change).
When a pointer moves, a non-bubbling 'pointer-move' event is fired at
each of the pointer's current targets in turn (maintaining the order
they had in the 'pointer-down' event, if there's more than one). If
the return value of a 'pointer-moved' event is 'cancel', and the
pointer is currently down, then the pointer is canceled (see below).
When a pointer's button state changes but this doesn't impact whether
it is "up" or "down", e.g. when a mouse with a button down gets a
second button down, or when a stylus' buttons change state, but the
pointer doesn't simultaneously move, then a 'pointer-moved' event is
fired anyway, as described above, but with dx=dy=0.
When a pointer switches from "down" to "up", a non-bubbling
'pointer-up' event is fired at each of the pointer's current targets
in turn (maintaining the order they had in the 'pointer-down' event,
if there's more than one), and then the pointer's current target list
is emptied except for the application's document. The buttons exposed
on the 'pointer-up' event are those that were down immediately prior
to the buttons being released.
At the time of a 'pointer-up' event, for each object that is a current
target of the pointer, and for which the pointer is considered the
"primary" pointer for that object, if there is another pointer that is
already down, which is of the same kind, which also has that object as
a current target, and that has either no buttons or only its primary
button active, then that pointer becomes the new "primary" pointer for
that object before the 'pointer-up' event is sent. Otherwise, the
"primary" pointer stops being "primary" just _after_ the 'pointer-up'
event. (This matters for whether the 'primary' field is set.)
When a pointer is canceled, if it is "down", pretend that the pointer
moved to "up", sending 'pointer-up' as described above, and entirely
empty its current targets list. AFter the pointer actually switches
from "down" to "up", replace the current targets list with an object
that only contains the application's document.
Nothing special happens when a pointer's current target moves in the
DOM.
The x and y position of an -up or -down event always match those of
the previous -moved or -added event, so their dx and dy are always 0.
Positions are floating point numbers; they can have subpixel values.
For each pointer, only a single pointer-added or pointer-removed event
is fired per frame. If a pointer would have been added and removed in
the same frame, the pointer is ignored, and no events are fired for
that pointer.
For each pointer, only a single pointer-down or pointer-up event is
fired per frame, representing the change in state from the last frame,
if any. Exactly when the event is fired is up to the implementation
and may depend on the hardware.
For each pointer, at most two pointer-move events are fired per frame,
one before the pointer-down or pointer-up event, if any, and one
after. If the pointer didn't change "down" state, then only one
pointer-move event is fired. All the actual moves that the pointer
experienced are coallesced into the event.
Example:
If a mouse experiences the following events:
- move +1, down, move +2, up, move +4, down, move +8
...the events might be:
- move +7, down, move +8
...or:
- move +1, down, move +14
TODO(ianh): expose the unfiltered uncoalesced stream of events for
programs that want more precision (e.g. drawing apps)
These data of all these events is an object with the following fields:
pointer: an integer assigned to this touch or pointer when it
enters the system, never reused, increasing
monotonically every time a new value is assigned,
starting from 1 (if the system gets a new tap every
microsecond, this will cause a problem after 285
years)
kind: one of 'touch', 'mouse', 'stylus', 'inverted-stylus'
x: x-position relative to the top-left corner of the
surface of the node on which the event was fired
y: y-position relative to the top-left corner of the
surface of the node on which the event was fired
dx: difference in x-position since last pointer-moved
event
dy: difference in y-position since last pointer-moved
event
buttons: a bitfield of the buttons pressed, from the following
list:
1: primary mouse button (not available on stylus)
2: secondary mouse button, primary stylus button
3: middle mouse button, secondary stylus button
4: back button
5: forward button
additional buttons can be represented by numbers
greater than six:
n: (n-2)th mouse button, ignoring any buttons that
are explicitly back or forward buttons
(n-4)th stylus button, again ignoring any
explictly back or forward buttons
note that stylus buttons can be pressed even when the
pointer is not "down"
e.g. if the left mouse button and the right mouse
button are pressed at the same time, the value will
be 3 (bits 1 and 2); if the right mouse button and
the back button are pressed at the same time, the
value will be 10 (bits 2 and 4)
down: true if the pointer is down (in pointer-down event or
subsequent pointer-move events); false otherwise (in
pointer-added, pointer-up, and pointer-removed
events, and in pointer-move events that aren't
between pointer-down and pointer-up events)
primary: true if this is a primary pointer/touch (see above)
can only be set for pointer-moved and pointer-up
obscured: true if the system was rendering another view on top
of the sky application at the time of the event (this
is intended to enable click-jacking protections)
When down is true:
pressure: the pressure of the touch as a number ranging from
0.0, indicating a touch with no discernible pressure,
to 1.0, indicating a touch with "normal" pressure,
and possibly beyond, indicating a stronger touch; for
devices that do not detect pressure (e.g. mice),
returns 1.0
pressure-min: the minimum value that pressure can return for this
pointer
pressure-max: the maximum value that pressure can return for this
pointer
When kind is 'touch', 'stylus', or 'stylus-inverted':
distance: distance of detected object from surface (e.g.
distance of stylus or finger from screen), if
supported and down is not true, otherwise 0.0.
distance-min: the minimum value that distance can return for this
pointer (always 0.0)
distance-max: the maximum value that distance can return for this
pointer (0.0 if not supported)
When kind is 'touch', 'stylus', or 'stylus-inverted' and down is true:
radius-major: the radius of the contact ellipse along the major
axis, in pixels
radius-minor: the radius of the contact ellipse along the major
axis, in pixels
radius-min: the minimum value that could be reported for
radius-major or radius-minor for this pointer
radius-max: the maximum value that could be reported for
radius-major or radius-minor for this pointer
When kind is 'touch' and down is true:
orientation: the angle of the contact ellipse, in radians in the
range
-pi/2 < orientation <= pi/2
...giving the angle of the major axis of the ellipse
with the y-axis (negative angles indicating an
orientation along the top-left / bottom-right
diagonal, positive angles indicating an orientation
along the top-right / bottom-left diagonal, and zero
indicating an orientation parallel with the y-axis)
When kind is 'stylus' or 'stylus-inverted':
orientation: the angle of the stylus, in radians in the range
-pi < orientation <= pi
...giving the angle of the axis of the stylus
projected onto the screen, relative to the positive
y-axis of the screen (thus 0 indicates the stylus, if
projected onto the screen, would go from the contact
point vertically up in the positive y-axis direction,
pi would indicate that the stylus would go down in
the negative y-axis direction; pi/4 would indicate
that the stylus goes up and to the right, -pi/2 would
indicate that the stylus goes to the left, etc)
tilt: the angle of the stylus, in radians in the range
0 <= tilt <= pi/2
...giving the angle of the axis of the stylus,
relative to the axis perpendicular to the screen
(thus 0 indicates the stylus is orthogonal to the
plane of the screen, while pi/2 indicates that the
stylus is flat on the screen)
TODO(ianh): add an API that exposes the currently existing pointers,
so that you can determine e.g. if you have a mouse.
Wheel events
------------
When a wheel input device is turned, a 'wheel' event that bubbles is
fired at the application's document, with the following fields:
wheel: an integer assigned to this wheel by the system. The
same wheel on the same system must always be given
the same ID. The primary wheel (e.g. the vertical
wheel on a mouse) must be given ID 1.
delta: an floating point number representing the fraction of
the wheel that was turned, with positive numbers
representing a downward movement on vertical wheels,
rightward movement on horizontal wheels, and a
clockwise movement on wheels with a user-facing side.
Additionally, if the wheel is associated with a pointer (e.g. a mouse
wheel), the following fields must be present also:
pointer: the integer assigned to the pointer in its
'pointer-add' event (see above).
x: x-position relative to the top-left corner of the
display, in global layout coordinates
y: x-position relative to the top-left corner of the
display, in global layout coordinates
Note: The only wheels that are supported are mouse wheels and physical
dials. Track balls are not reported as mouse wheels.