1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
|
/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** No Commercial Usage
** This file contains pre-release code and may not be distributed.
** You may use this file in accordance with the terms and conditions
** contained in the Technology Preview License Agreement accompanying
** this package.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Nokia gives you certain additional
** rights. These rights are described in the Nokia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
**
** If you have questions regarding the use of this file, please contact
** Nokia at qt-info@nokia.com.
**
**
**
**
**
**
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page gestures-overview.html
\startpage index.html Qt Reference Documentation
\title Gestures Programming
\ingroup howto
\brief An overview of the Qt support for Gesture programming.
Qt includes a framework for gesture programming that gives has the ability
to form gestures from a series of events, independently of the input methods
used. A gesture could be a particular movement of a mouse, a touch screen
action, or a series of events from some other source. The nature of the input,
the interpretation of the gesture and the action taken are the choice of the
developer.
\tableofcontents
\section1 Overview
QGesture is the central class in Qt's gesture framework, providing
the API used by classes that represent specific gestures, such as
QPanGesture, QPinchGesture, and QSwipeGesture. These standard
classes are ready to use, and each exposes functions and
properties that give gesture-specific information about the user's
input. This is described in the section \l{Using Standard Gestures
With Widgets}.
QGesture is also designed to be subclassed and extended so that
support for new gestures can be implemented by developers. Adding
support for a new gesture involves implementing code to recognize
the gesture from incoming events. This is described in the section
\l{Creating Your Own Gesture Recognizer}.
\section1 Using Standard Gestures With Widgets
Gesture objects are applied directly to widgets and other controls that accept
user input \mdash these are the \e{target objects}. When a gesture object is
constructed, the target object is typically passed to the constructor, though
it can also be passed as the argument to the \l{QGesture::}{setGestureTarget()}
function.
\snippet examples/gestures/imageviewer/imagewidget.cpp construct swipe gesture
In the above code, the gesture is set up in the constructor of the target object
itself, so the argument to the QSwipeGesture constructor is \e this.
When the user performs a gesture, various signals may be emitted by the
gesture object. To monitor the user's actions, you need to connect signals
from the gesture object to slots in your code.
\snippet examples/gestures/imageviewer/imagewidget.cpp connect swipe gesture
Here, the \l{QGesture::} {triggered()} signal is used to inform
the application that a gesture was used. More precise monitoring
of a gesture can be implemented by connecting its \l{QGesture::}
{started()}, \l{QGesture::} {canceled()} and \l{QGesture::}
{finished()} signals to slots.
Responding to a signal is simply a matter of obtaining the gesture that sent
it and examining the information it contains.
\snippet examples/gestures/imageviewer/imagewidget.cpp swipe slot start
\snippet examples/gestures/imageviewer/imagewidget.cpp swipe slot finish
Here, we examine the direction in which the user swiped the widget and modify
its contents accordingly.
\section1 Using Standard Gestures with Graphics Items
The approach used for applying gestures to widgets can also be used with
graphics items. However, instead of passing a target object to the
gesture object's constructor, you set a target graphics item with the
\l{QGesture::}{setGraphicsItem()} function.
\section1 Creating Your Own Gesture Recognizer
QGesture is a base class for a user defined gesture recognizer class. In
order to implement the recognizer you will need to subclass the
QGesture class and implement the pure virtual function
\l{QGesture::}{filterEvent()} to filter out events that are not relevant
to your gesture.
Once you have implemented the \l{QGesture::}{filterEvent()} function to
make your own recognizer you can process events. A sequence of events may,
according to your own rules, represent a gesture. The events can be singly
passed to the recognizer via the \l{QGesture::}{filterEvent()} function
or as a stream of events by specifying a parent source of events. The events
can be from any source and could result in any action as defined by the user.
The source and action need not be graphical, though that would be the most
likely scenario. To find how to connect a source of events to automatically
feed into the recognizer see the QGesture documentation.
Recognizers based on QGesture can emit any of the following signals:
\snippet doc/src/snippets/gestures/qgesture.h qgesture-signals
These signals are emitted when the state changes with the call to
\l{QGesture::}{updateState()}, more than one signal may
be emitted when a change of state occurs. There are four GestureStates
\table
\header \o New State \o Description \o QGesture Actions on Entering this State
\row \o Qt::NoGesture \o Initial value \o emit \l {QGesture::}{canceled()}
\row \o Qt::GestureStarted \o A continuous gesture has started \o emit \l{QGesture::}{started()} and emit \l{QGesture::}{triggered()}
\row \o Qt::GestureUpdated \o A gesture continues \o emit \l{QGesture::}{triggered()}
\row \o Qt::GestureFinished \o A gesture has finished. \o emit \l{QGesture::}{finished()}
\endtable
\note \l{QGesture::started()}{started()} can be emitted if entering any
state greater than NoGesture if NoGesture was the previous state. This
means that your state machine does not need to explicitly use the
Qt::GestureStarted state, you can simply proceed from NoGesture to
Qt::GestureUpdated to emit a \l{QGesture::started()}{started()} signal
and a \l{QGesture::triggered()}{triggered()} signal.
You may use some or all of these states when implementing the pure
virtual function \l{QGesture::filterEvent()}{filterEvent()}.
\l{QGesture::filterEvent()}{filterEvent()} will usually implement a
state machine using the GestureState enums, but the details of which
states are used is up to the developer.
You may also need to reimplement the virtual function \l{QGesture::reset()}{reset()}
if internal data or objects need to be re-initialized. The function must
conclude with a call to \l{QGesture::updateState()}{updateState()} to
change the current state to Qt::NoGesture.
\section1 The ImageViewer Example
To illustrate how to use QGesture we will look at the ImageViewer
example. This example uses QPanGesture, a standard gesture, and an
implementation of TapAndHoldGesture. Note that TapAndHoldGesture is
platform-dependent.
\snippet doc/src/snippets/gestures/imageviewer/tapandholdgesture.cpp tapandhold-reset
In ImageViewer we see that the ImageWidget class uses two gestures:
\l QPanGesture and TapAndHoldGesture. The
QPanGesture is a standard gesture which is part of Qt.
TapAndHoldGesture is defined and implemented as part of the example.
The ImageWidget listens for signals from the gestures, but is not
interested in the \l{QGesture::started()}{started()} signal.
\snippet doc/src/snippets/gestures/imageviewer/imagewidget.h imagewidget-slots
TapAndHoldGesture uses QTouchEvent events and mouse events to detect
start, update and end events that can be mapped onto the GestureState
changes. The implementation in this case uses a timer as well. If the
timeout event occurs a given number of times after the start of the gesture
then the gesture is considered to have finished whether or not the
appropriate touch or mouse event has occurred. Also if a large jump in
the position of the event occurs, as calculated by the \l {QPoint::manhattanLength()}{manhattanLength()}
call, then the gesture is canceled by calling \l{QGesture::reset()}{reset()}
which tidies up and uses \l{QGesture::updateState()}{updateState()} to
change state to NoGesture which will result in the \l{QGesture::canceled()}{canceled()}
signal being emitted by the recognizer.
ImageWidget handles the signals by connecting the slots to the signals,
although \c canceled() is not connected here.
\snippet doc/src/snippets/gestures/imageviewer/imagewidget.cpp imagewidget-connect
These functions in turn will have to be aware of which gesture
object was the source of the signal since we have more than one source
per slot. This is easily done by using the QObject::sender() function
as shown here
\snippet doc/src/snippets/gestures/imageviewer/imagewidget.cpp imagewidget-triggered-1
As \l{QGesture::triggered()}{triggered()} signals are handled by
gestureTriggered() there may be position updates invoking calls to,
for example, goNextImage(), this will cause a change in the image
handling logic of ImageWidget and a call to updateImage() to display
the changed state.
Following the logic of how the QEvent is processed we can summmarize
it as follows:
\list
\o filterEvent() becomes the event filter of the parent ImageWidget object
for a QPanGesture object and a TapAndHoldGesture object.
\o filterEvent() then calls updateState() to change states
\o updateState() emits the appropriate signal(s) for the state change.
\o The signals are caught by the defined slots in ImageWidget
\o The widget logic changes and an update() results in a paint event.
\endlist
*/
|
