pointer

Overview ¶

Package pointer implements pointer events and operations. A pointer is either a mouse controlled cursor or a touch object such as a finger.

The InputOp operation is used to declare a handler ready for pointer events. Use an event.Queue to receive events.

Areas ¶

The area operations are used for specifying the area where subsequent InputOp are active.

For example, to set up a rectangular hit area:

var ops op.Ops
var h *Handler = ...

r := image.Rectangle{...}
pointer.Rect().Add(ops)
pointer.InputOp{Key: h}.Add(ops)

Note that areas compound: the effective area of multiple area operations is the intersection of the areas.

Matching events ¶

StackOp operations and input handlers form an implicit tree. Each stack operation is a node, and each input handler is associated with the most recent node.

For example:

ops := new(op.Ops)
var stack op.StackOp
var h1, h2 *Handler

stack.Push(ops)
pointer.InputOp{Key: h1}.Add(Ops)
stack.Pop()

stack.Push(ops)
pointer.InputOp{Key: h2}.Add(ops)
stack.Pop()

implies a tree of two inner nodes, each with one pointer handler.

When determining which handlers match an Event, only handlers whose areas contain the event position are considered. The matching proceeds as follows.

First, the foremost matching handler is included. If the handler has pass-through enabled, this step is repeated.

Then, all matching handlers from the current node and all parent nodes are included.

In the example above, all events will go to h2 only even though both handlers have the same area (the entire screen).

Pass-through ¶

The PassOp operations controls the pass-through setting. A handler's pass-through setting is recorded along with the InputOp.

Pass-through handlers are useful for overlay widgets such as a hidden side drawer. When the user touches the side, both the (transparent) drawer handle and the interface below should receive pointer events.

Disambiguation ¶

When more than one handler matches a pointer event, the event queue follows a set of rules for distributing the event.

As long as the pointer has not received a Press event, all matching handlers receive all events.

When a pointer is pressed, the set of matching handlers is recorded. The set is not updated according to the pointer position and hit areas. Rather, handlers stay in the matching set until they no longer appear in a InputOp or when another handler in the set grabs the pointer.

A handler can exclude all other handler from its matching sets by setting the Grab flag in its InputOp. The Grab flag is sticky and stays in effect until the handler no longer appears in any matching sets.

The losing handlers are notified by a Cancel event.

For multiple grabbing handlers, the foremost handler wins.

Priorities ¶

Handlers know their position in a matching set of a pointer through event priorities. The Shared priority is for matching sets with multiple handlers; the Grabbed priority indicate exclusive access.

Priorities are useful for deferred gesture matching.

Consider a scrollable list of clickable elements. When the user touches an element, it is unknown whether the gesture is a click on the element or a drag (scroll) of the list. While the click handler might light up the element in anticipation of a click, the scrolling handler does not scroll on finger movements with lower than Grabbed priority.

Should the user release the finger, the click handler registers a click.

However, if the finger moves beyond a threshold, the scrolling handler determines that the gesture is a drag and sets its Grab flag. The click handler receives a Cancel (removing the highlight) and further movements for the scroll handler has priority Grabbed, scrolling the list.