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
|
/****************************************************************************
**
** Copyright (C) 2019 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\example webenginequick/lifecycle
\title WebEngine Lifecycle Example
\ingroup webengine-examples
\brief Freezes and discards background tabs to reduce CPU and memory usage.
\image lifecycle.png
\e {WebEngine Lifecycle Example} demonstrates how the \l
{WebEngineView::}{lifecycleState} and \l {WebEngineView::}{recommendedState}
properties of the \l {WebEngineView} can be used to reduce the CPU and
memory usage of background tabs in a tabbed browser.
For an overview of the lifecycle feature, see \l {Page Lifecycle API}.
\include examples-run.qdocinc
\section1 UI Elements of the Example
The example uses \l {Qt Quick Controls 2} to implement a traditional tabbed
browser in the \l {Material Style} (dark variant). The main application
window (\c {WebBrowser.qml}) is divided into a header bar at the top and a
main viewing area filling the rest of the window. The header contains the
tab bar (\c {WebTabBar.qml}) with one button per tab (\c
{WebTabButton.qml}). The main area consists of a stack of tabs (\c
{WebTabStack.qml} and \c {WebTab.qml}). Each tab in turn has a tool bar at
the top and a \l {WebEngineView} for displaying web pages. Finally, the main
window also has a \l {Drawer} for changing settings. The drawer can be
opened by clicking the "⋮" button on the tool bar.
\section1 Lifecycle States in the Example
The example implements two ways of changing the lifecycle state: manual and
automatic. The manual way uses the \l {WebEngineView::}{lifecycleState}
property directly to change the web view lifecycle state, while the
automatic way is timer-based and also takes into account the \l
{WebEngineView::}{recommendedState}.
The tab titles in the tab bar are color coded with frozen tabs shown in blue
and discarded in red.
\section2 Manual Lifecycle Control
\image lifecycle-manual.png
Manual control is provided by context menus on the tab bar buttons (\c
{WebTabButton.qml}). The menu has three radio buttons, one for each
lifecycle state, with the current state checked. Some buttons may be
disabled, either because they represent illegal state transitions (for
example, a \c {Discarded} view cannot directly transition to the \c {Frozen}
state), or because other preconditions are not fulfilled (for example, a
visible view can only be in the \c {Active} state).
\section2 Automatic Lifecycle Control
\image lifecycle-automatic.png
Automatic control is implemented with a \l {Timer} in the \c {WebTab}
component (\c {WebTab.qml}). The timer is started whenever the \l
{WebEngineView::}{lifecycleState} of the web view does not match it's \l
{WebEngineView::}{recommendedState}. Once the timer fires, the view's
lifecycle state is set to the recommended state.
The time delay is used to avoid changing the lifecycle state too quickly
when the user is switching between tabs. The freezing and discarding delays
can be changed in the settings drawer accessed through the "⋮" button on the
tool bar.
This is a rather simple algorithm for automatic lifecycle control, however
more sophisticated algorithms could also be conceived and implemented on the
basis of the \l {WebEngineView::}{lifecycleState} property. For example, the
Chromium browser experimentally uses a pretrained deep neural network to
predict the next tab activation time by the user, essentially ranking tabs
based on how interesting they are to the user. Implementing such an
algorithm is left as an exercise to the reader for now.
*/
|
