github.com/cybriq/giocore@v0.0.7-0.20210703034601-cfb9cb5f3900/io/pointer/doc.go (about)

     1  // SPDX-License-Identifier: Unlicense OR MIT
     2  
     3  /*
     4  Package pointer implements pointer events and operations.
     5  A pointer is either a mouse controlled cursor or a touch
     6  object such as a finger.
     7  
     8  The InputOp operation is used to declare a handler ready for pointer
     9  events. Use an event.Queue to receive events.
    10  
    11  Types
    12  
    13  Only events that match a specified list of types are delivered to a handler.
    14  
    15  For example, to receive Press, Drag, and Release events (but not Move, Enter,
    16  Leave, or Scroll):
    17  
    18  	var ops op.Ops
    19  	var h *Handler = ...
    20  
    21  	pointer.InputOp{
    22  		Tag:   h,
    23  		Types: pointer.Press | pointer.Drag | pointer.Release,
    24  	}.Add(ops)
    25  
    26  Cancel events are always delivered.
    27  
    28  Areas
    29  
    30  The area operations are used for specifying the area where
    31  subsequent InputOp are active.
    32  
    33  For example, to set up a rectangular hit area:
    34  
    35  	r := image.Rectangle{...}
    36  	pointer.Rect(r).Add(ops)
    37  	pointer.InputOp{Tag: h}.Add(ops)
    38  
    39  Note that areas compound: the effective area of multiple area
    40  operations is the intersection of the areas.
    41  
    42  Matching events
    43  
    44  StackOp operations and input handlers form an implicit tree.
    45  Each stack operation is a node, and each input handler is associated
    46  with the most recent node.
    47  
    48  For example:
    49  
    50  	ops := new(op.Ops)
    51  	var stack op.StackOp
    52  	var h1, h2 *Handler
    53  
    54  	state := op.Save(ops)
    55  	pointer.InputOp{Tag: h1}.Add(Ops)
    56  	state.Load()
    57  
    58  	state = op.Save(ops)
    59  	pointer.InputOp{Tag: h2}.Add(ops)
    60  	state.Load()
    61  
    62  implies a tree of two inner nodes, each with one pointer handler.
    63  
    64  When determining which handlers match an Event, only handlers whose
    65  areas contain the event position are considered. The matching
    66  proceeds as follows.
    67  
    68  First, the foremost matching handler is included. If the handler
    69  has pass-through enabled, this step is repeated.
    70  
    71  Then, all matching handlers from the current node and all parent
    72  nodes are included.
    73  
    74  In the example above, all events will go to h2 only even though both
    75  handlers have the same area (the entire screen).
    76  
    77  Pass-through
    78  
    79  The PassOp operations controls the pass-through setting. A handler's
    80  pass-through setting is recorded along with the InputOp.
    81  
    82  Pass-through handlers are useful for overlay widgets such as a hidden
    83  side drawer. When the user touches the side, both the (transparent)
    84  drawer handle and the interface below should receive pointer events.
    85  
    86  Disambiguation
    87  
    88  When more than one handler matches a pointer event, the event queue
    89  follows a set of rules for distributing the event.
    90  
    91  As long as the pointer has not received a Press event, all
    92  matching handlers receive all events.
    93  
    94  When a pointer is pressed, the set of matching handlers is
    95  recorded. The set is not updated according to the pointer position
    96  and hit areas. Rather, handlers stay in the matching set until they
    97  no longer appear in a InputOp or when another handler in the set
    98  grabs the pointer.
    99  
   100  A handler can exclude all other handler from its matching sets
   101  by setting the Grab flag in its InputOp. The Grab flag is sticky
   102  and stays in effect until the handler no longer appears in any
   103  matching sets.
   104  
   105  The losing handlers are notified by a Cancel event.
   106  
   107  For multiple grabbing handlers, the foremost handler wins.
   108  
   109  Priorities
   110  
   111  Handlers know their position in a matching set of a pointer through
   112  event priorities. The Shared priority is for matching sets with
   113  multiple handlers; the Grabbed priority indicate exclusive access.
   114  
   115  Priorities are useful for deferred gesture matching.
   116  
   117  Consider a scrollable list of clickable elements. When the user touches an
   118  element, it is unknown whether the gesture is a click on the element
   119  or a drag (scroll) of the list. While the click handler might light up
   120  the element in anticipation of a click, the scrolling handler does not
   121  scroll on finger movements with lower than Grabbed priority.
   122  
   123  Should the user release the finger, the click handler registers a click.
   124  
   125  However, if the finger moves beyond a threshold, the scrolling handler
   126  determines that the gesture is a drag and sets its Grab flag. The
   127  click handler receives a Cancel (removing the highlight) and further
   128  movements for the scroll handler has priority Grabbed, scrolling the
   129  list.
   130  */
   131  package pointer