Difference between revisions of "Delay action to wait for user interaction"

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(adjust formatting, since <code> now creates a block element)
(fix variable name (typo in previous edit))
 
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== Usecase ==
 
== Usecase ==
  
Have you ever created a text filter that can be used to filter a (big) item view so users can get to their content faster? If you did, you probably simply connected <code>QLineEdit::textChanged ( const QString & text ) </code> to a <code>QSortFilterProxyModel::setFilterXXX </code> method, either via your own slot to add some wildcards or wrap the line edit's string in a regular expression or not. The problem is, now each character typed by the user triggers an update of the whole view, and the proxy model has to go through all the items in your list over and over again. That could get expensive and slow, thereby instead of improving user experience you actually annoy the user…
+
Have you ever created a text filter that can be used to filter a (big) list of items? If you did, you probably simply connected <code>QLineEdit::textChanged ( const QString & text ) </code> to a method like <code>YourClass::filterEntries( const QString & filterText )</code> The problem is, now each character typed by the user triggers an update of the whole view, and the class has to go through all the items in your list over and over again. That could get expensive and slow, thereby instead of improving the user experience, you actually annoy the user.
  
There are many such cases where events can happen in quick succession, and you'd like your application to respond to these changes. On the one hand, it makes sense to respond to as many of those events in one go as possible, but on the other hand you don't want the update to take too much time after the user is done typing his filter string (or whatever it is you are waiting for).
+
There are many such cases where events can happen in quick succession, and you'd like your application to respond to these changes. On the one hand, it makes sense to respond to as many of those events in one go as possible, but on the other hand you don't want the update to take too much time after the user is done typing their filter string (or whatever it is you are waiting for).
  
== Enter DelayedExecutionTimer ==
+
== Solution ==
  
One solution to this problem is to use two timers, one short enough so an update will still appear snappy but long enough that it will probably not trigger before there is more input (for instance, if the user is not done typing his filter), and one longer timer that will trigger a set time after the first event happened, so the update will not be delayed forever as more and more events happen that would otherwise delay the update.
+
The simplest solution to this problem is to use a {{class|QTimer}}. This allows you to, for instance, implement a text filter that waits for the user to stop typing and then run the query. Below is example code that shows how to implement this:
 
 
The <tt>[[#Code|DelayedExecutionTimer]]</tt> class implements this approach, making it very easy to apply in all such cases without mucking about with creating, setting and resetting timers for each case where you need this. <tt>DelayedExecutionTimer</tt> basically provides one slot <tt>trigger()</tt> and one signal <tt>triggered()</tt>. Instead of directly connecting the event to the action (connecting the <tt>textChanged()</tt> to the <tt>setFilter()</tt>, for instance), you connect the <tt>QLineEdit::textChanged()</tt> to <tt>DelayedExecutionTimer::trigger()</tt>, and <tt>DelayedExecutionTimer::triggered()</tt> to <tt>QSortFilterProxyModel::setFilter…()</tt>.
 
 
 
<tt>DelayedExecutionTimer</tt> also provides two more versions of both the <tt>trigger()</tt> slot and the </tt>triggered()</tt> signal for convenience. These allow to pass a {{class|QString}} or an <tt>int</tt> as an argument. The <tt>triggered()</tt> signal will be emitted without argument, and with both a <tt>QString</tt> and with an <tt>int</tt> argument, carrying the last value that was passed in by the corresponding <tt>trigger()</tt> signal (or a default value if no such value was set). As a last convenience feature, you can set pre- and post- strings, that will be added to the string value before the signal is send. This way, connecting the line edit to the <tt>SortFilterProxy</tt> model becomes as simple as this:
 
  
 +
First, add a <tt>{{class|QTimer}} *m_typingTimer</tt>, and a <tt>{{class|QString}} m_filterText</tt> as private member variables for the class.
 +
Initialize the QTimer in the class constructor:
 
<code>
 
<code>
// m_proxy is a QSortFilterProxyModel pointer
+
m_typingTimer = new QTimer( this );
// m_ui->leFilter is a QLineEdit representing the filter string to use
+
m_typingTimer->setSingleShot( true ); // Ensure the timer will fire only once after it was started
 
 
DelayedExecutionTimer* filterDelay = new DelayedExecutionTimer(this);
 
filterDelay->setStringPreFix("'''");
 
filterDelay->setStringPostFix("'''");
 
connect(m_ui->leFilter, SIGNAL (textChanged(QString)), filterDelay, SLOT (trigger(QString)));
 
connect(filterDelay, SIGNAL (triggered(QString)), m_proxy, SLOT (setFilterWildcard(QString)));
 
 
</code>
 
</code>
  
That's all. Of course, you can tweak the timings by either passing in a reasonable minimum and maximum delay in the constructor, or using the setters for these.
+
Then, create a slot where the query will be performed, and connect the timer to it:
 
 
== Code ==
 
 
 
=== delayedexecutiontimer.h ===
 
 
 
 
<code>
 
<code>
/* Copyright © 2011, Andre Somers
+
connect( m_typingTimer, &QTimer::timeout, this, &YourClass::filterEntries );
All rights reserved.
 
 
 
Redistribution and use in source and binary forms, with or without
 
modification, are permitted provided that the following conditions are met:
 
* Redistributions of source code must retain the above copyright
 
notice, this list of conditions and the following disclaimer.
 
* Redistributions in binary form must reproduce the above copyright
 
notice, this list of conditions and the following disclaimer in the
 
documentation and/or other materials provided with the distribution.
 
* Neither the name of the Rathenau Instituut, Andre Somers nor the
 
names of its contributors may be used to endorse or promote products
 
derived from this software without specific prior written permission.
 
 
 
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 
DISCLAIMED. IN NO EVENT SHALL ANDRE SOMERS AND/OR RATHENAU INSTITUTE BE LIABLE FOR ANY
 
DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
*/
 
 
 
#ifndef DELAYEDEXECUTIONTIMER_H
 
#define DELAYEDEXECUTIONTIMER_H
 
 
 
#include <QObject>
 
 
 
class QTimer;
 
  
// Class to delay execution an action in response to events that may come in bursts
+
void YourClass::filterEntries()
class DelayedExecutionTimer : public QObject
 
 
{
 
{
     Q_OBJECT
+
     // The actual filtering code goes here, using the string stored in the m_filterText class variable
public:
+
}
    DelayedExecutionTimer(int maximumDelay = 1000, int minimumDelay = 250, QObject* parent = 0);
 
    DelayedExecutionTimer(QObject* parent);
 
 
 
/*
 
The minimum delay is the time the class will wait after being triggered before
 
emitting the triggered() signals.
 
*/
 
void setMinimumDelay(int delay) {m_minimumDelay = delay;}
 
int minimumDelay() const {return m_minimumDelay;}
 
/*
 
The maximum delay is the maximum time that will pass before a call to the trigger() slot
 
leads to a triggered() signal.
 
*/
 
void setMaximumDelay(int delay) {m_maximumDelay = delay;}
 
int maximumDelay() const {return m_maximumDelay;}
 
 
 
/*
 
Sets a string to be attached to the end of the last string set using trigger(QString)
 
in the triggered(QString) signal. This is useful for, for instance, appending wildcard
 
characters to a filter string.
 
*/
 
void setStringPostfix(const QString& postfix) {m_postfix = postfix;}
 
/*
 
Sets a string to be prepended to the beginning of the last string set using trigger(QString)
 
in the triggered(QString) signal. This is useful for, for instance, appending wildcard
 
characters to a filter string.
 
*/
 
void setStringPrefix(const QString& prefix) {m_prefix = prefix;}
 
 
 
signals:
 
    void triggered();
 
    void triggered(QString);
 
    void triggered(int);
 
 
 
public slots:
 
    void trigger();
 
    void trigger(QString);
 
    void trigger(int);
 
 
 
private slots:
 
    void timeout();
 
 
 
private:
 
    int m_minimumDelay;
 
    int m_maximumDelay;
 
 
 
    QTimer* m_minimumTimer;
 
    QTimer* m_maximumTimer;
 
 
 
    QString m_lastString;
 
    int m_lastInt;
 
 
 
    QString m_prefix;
 
    QString m_postfix;
 
};
 
 
 
#endif // DELAYEDEXECUTIONTIMER_H
 
 
</code>
 
</code>
  
And here's the CPP code:
+
Finally, create a <tt>YourClass::onTextEdited</tt> slot that stores the newly edited filter text and starts the countdown to call the filtering method:
 
 
=== delayedexecutiontimer.cpp ===
 
 
 
 
<code>
 
<code>
/* Copyright © 2011, Andre Somers
+
connect( m_textEdit, QLineEdit::textChanged, this, &YourClass::onTextEdited );
* All rights reserved.
 
* File licence not repeated here for space reasons. See file "delayedexecutiontimer.h" for details of licence.
 
*/
 
#include "delayedexecutiontimer.h"
 
#include <QTimer>
 
#include <QStringBuilder>
 
  
DelayedExecutionTimer::DelayedExecutionTimer(int maximumDelay, int minimumDelay, QObject* parent):
+
void YourClass::onTextEdited( const QString & newText )
QObject(parent),
 
m_minimumDelay(minimumDelay),
 
m_maximumDelay(maximumDelay),
 
m_minimumTimer(new QTimer(this)),
 
m_maximumTimer(new QTimer(this)),
 
m_lastInt(0)
 
 
{
 
{
connect(m_minimumTimer, SIGNAL (timeout()), SLOT (timeout()));
+
    m_filterText = newText;
connect(m_maximumTimer, SIGNAL (timeout()), SLOT (timeout()));
+
    m_typingTimer->start( 100 ); // This will fire filterEntries after 100 ms.
}
+
    // If the user types something before it fires, the timer restarts counting
 
 
DelayedExecutionTimer::DelayedExecutionTimer(QObject* parent):
 
QObject(parent),
 
m_minimumDelay(250),
 
m_maximumDelay(1000),
 
m_minimumTimer(new QTimer(this)),
 
m_maximumTimer(new QTimer(this)),
 
m_lastInt(0)
 
{
 
connect(m_minimumTimer, SIGNAL (timeout()), SLOT (timeout()));
 
connect(m_maximumTimer, SIGNAL (timeout()), SLOT (timeout()));
 
}
 
 
 
void DelayedExecutionTimer::timeout()
 
{
 
m_minimumTimer->stop();
 
m_maximumTimer->stop();
 
emit triggered();
 
emit triggered(m_prefix % m_lastString % m_postfix);
 
emit triggered(m_lastInt);
 
}
 
 
 
void DelayedExecutionTimer::trigger()
 
{
 
if (!m_maximumTimer->isActive()) {
 
m_maximumTimer->start(m_maximumDelay);
 
}
 
m_minimumTimer->stop();
 
m_minimumTimer->start(m_minimumDelay);
 
}
 
 
 
void DelayedExecutionTimer::trigger(QString string)
 
{
 
m_lastString = string;
 
trigger();
 
 
}
 
}
 +
</code>
  
void DelayedExecutionTimer::trigger(int i)
+
:''Note: the code above is adapted from a [http://stackoverflow.com/a/21945529/266309 StackOverflow answer] by [http://stackoverflow.com/users/2502409/nazar554 Nazar554].''
{
 
m_lastInt = i;
 
trigger();
 
}
 
</code>
 

Latest revision as of 11:04, 25 July 2016

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Usecase

Have you ever created a text filter that can be used to filter a (big) list of items? If you did, you probably simply connected
QLineEdit::textChanged ( const QString & text )
to a method like
YourClass::filterEntries( const QString & filterText )
The problem is, now each character typed by the user triggers an update of the whole view, and the class has to go through all the items in your list over and over again. That could get expensive and slow, thereby instead of improving the user experience, you actually annoy the user.

There are many such cases where events can happen in quick succession, and you'd like your application to respond to these changes. On the one hand, it makes sense to respond to as many of those events in one go as possible, but on the other hand you don't want the update to take too much time after the user is done typing their filter string (or whatever it is you are waiting for).

Solution

The simplest solution to this problem is to use a QTimer. This allows you to, for instance, implement a text filter that waits for the user to stop typing and then run the query. Below is example code that shows how to implement this:

First, add a QTimer *m_typingTimer, and a QString m_filterText as private member variables for the class. Initialize the QTimer in the class constructor:

m_typingTimer = new QTimer( this );
m_typingTimer->setSingleShot( true ); // Ensure the timer will fire only once after it was started

Then, create a slot where the query will be performed, and connect the timer to it:

connect( m_typingTimer, &QTimer::timeout, this, &YourClass::filterEntries );

void YourClass::filterEntries()
{
    // The actual filtering code goes here, using the string stored in the m_filterText class variable
}

Finally, create a YourClass::onTextEdited slot that stores the newly edited filter text and starts the countdown to call the filtering method:

connect( m_textEdit, QLineEdit::textChanged, this, &YourClass::onTextEdited );

void YourClass::onTextEdited( const QString & newText )
{
    m_filterText = newText;
    m_typingTimer->start( 100 ); // This will fire filterEntries after 100 ms. 
    // If the user types something before it fires, the timer restarts counting
}
Note: the code above is adapted from a StackOverflow answer by Nazar554.