Transcript Touch Gestures

CHAPTER 7
TouchGestures
Chapter objectives:
• To code the detection of and response to touch
gestures.
• Patterns of common touches to create touch gestures
that can be interpreted by an Android device.
• The use of MotionEvents
• The differences between touch events and motion
events
• How to build applications using multi-touch gestures
7.1 Touchscreens
• Most Android devices are equipped with a
capacitive touchscreen
• They rely on the electrical properties of the
human body to detect when and where on a
display the user is touching
• Capacitive technology provides numerous
design opportunities for Android developers
7.2 Touch Gestures
• Gestures are the primary way in which users
interact with most Android devices
• Touch gestures represent a fundamental form
of communication with an Android device
• A touch gesture is an action, typically a
movement of a user’s finger on a touchscreen
7.3 The Basics of Touch Events
• The MotionEvent class provides a
collection of methods to report on the
properties of a given touch gesture
• Motion events describe movements in
terms of an action code and a set of axis
values
• Each action code specifies a state change
produced by a touch occurrence, such as a
pointer going down or up
• The axis values describe the position and
movement properties.
• three basic MotionEvents can be
combined to create touch gestures
• ACTION_DOWN
• ACTION_MOVE
• ACTION_UP
7.4 Gesture Detector
• The easiest approach is to use the GestureDetector class for
detecting specific touch gestures.
• This employs the OnGestureListener to signal when a
gesture occurs and then pass the triggered motion event to
the GestureDetector’s onTouchEvent() method
• The onTouchEvent() method will determine exactly what
action patterns are occurring on the screen
• The scroll events produced by a fling gesture can be used to
provide information about the velocity of a moving finger
and the distance it has traveled on the screen
• A completed fling gesture is performed by the execution of
the following callback methods: onDown(), onScroll(), and
onFling().
• GestureDetector has limited usage for Android
application because it is not able to handle all
types of gestures
• The onDown() method is automatically called
when a tap occurs with the down MotionEvent
that triggered it
• The onLongPress() is called when a long press
occurs with the initial onDown MotionEvent
that triggered it
• The onSingleTapUp() callback will occur when
a tap gesture takes place with an up motion
that triggered it
• Similar to the onSingleTapUp() callback,
onSingleTapConfirmed() will occur when a
detected tap gesture is confirmed by the
system as a single tap and not part of a
double tap gesture
7.5 The MotionEvent Class
• GestureDetector class allows basic detections for
common gestures
• This type of gesture detection is suitable for
applications that require simple gestures
• The GestureDetector class is not designed for
handling complicated gestures.
• A more sophisticated form of gesture detection is
to register an OnTouchListener event handler to a
specific View, such as a graphic object on stage
that can be dragged
• To provide touch event notification, the
onTouchEvent() method can be overridden for an
Activity or touchable View
• The MotionEvent class provides a collection of
methods to query the position and other properties
of fingers used in a gesture
• getX(): Returns the x axis coordinate value at the
finger’s location on the screen
• getY(): Returns the y axis coordinate value at the
finger’s location on the screen
• getDownTime(): Returns the time when the user
initially pressed down to begin a series of events
• getPrecisionX(): Returns the precision of the X
coordinate
• getAction(): Returns the type of action being
performed by the user, such as ACTION_DOWN
7.6 The Drag and Drop Gesture
• drag-and-drop is the action of tapping on a
virtual object and dragging it to a different
location
• Drag-and-drop is a gesture that is most often
associated with methods of data transfer
• This gesture assumes a drag source and a
drop target exist
• The Android framework for drag-and-drop
includes a drag event class, drag listeners,
and helper methods and classes
• Source and target containers must be
created to hold the View elements that will
be dragged and eventually dropped
• As Figure 7- 13 shows, a basic drag-anddrop design relies on at least two Views
• All elements that are the intended moveable
objects in a drag-and-drop process must be
registered with an appropriate listener event
• A touch listener, setOnTouchListener(), must
be attached to each draggable View
• This will register a callback to be invoked when
an explicit touch event is sent to this draggable
View object
• A view container that functions as a source or
target container must be registered with an
explicit “on drag” listener event.
• During a drag-and-drop operation, the
system provides a separate image that the
user drags
• For data movement, this image represents a
copy of the object being dragged
• This mechanism makes it clear to the user
that an object is in the process of being
dragged and has not yet been placed in its
final target location
• This dragged image is called a drag shadow
because it is a shadow version of itself
7.7 Fling Gesture
• A fling is a core touchscreen gesture that is also
known as a swipe
• A fling is a quick swiping movement of a finger
across a touchscreen
• As with other gestures, the MotionEvent object
can be used to report a fling event
• The motion events will describe the fling
movement in terms of an action code and a set of
axis values describing movement properties
• For common gestures, such as onFling,
Android provides the GestureDetector class
that can be used in tandem with the
onTouchEvent callback method
• An application Activity can implement the
GestureDetector.OnGestureListener
interface to identify when a specific touch
event has occurred
• Once these events are received they can be
handed off to the overriden onTouchEvent
callback
7.8 Fling Velocity
• Tracking the movement in a fling gesture
requires the start and end positions of the
finger, and the velocity of the movement
across the touchscreen
• The direction of a fling can be determined by
the x and y coordinates captured by the
ACTION_DOWN and ACTION_UP
MotionEvents and the resulting velocities
• VelocityTracker is an Android helper class
for tracking the velocity of touch events,
including the implementation of a fling
• In applications such as games, a fling is a
movment-based gesture that produces
behavior based on the distance and
direction a finger travels with a gesture
• The VelocityTracker class is designed to
simplify velocity calculations in movementbased gestures
7.9 Multi-Touch Gestures
• A pinch gesture involves two fingers placed on
the screen
• The finger positions, and the distance
between them, are recorded
• When the fingers are lifted from the screen,
the distance separating them is recorded
• If the second recorded distance is less than
the first distance, the gesture is recognized as
a pinch
• A spread gesture is similar to a pinch
gesture, in that it also records the start and
end distance between the fingers on the
touchscreen
• If the second recorded distance is greater
than the first, it is a spread gesture.