Delay action to wait for user interaction: Difference between revisions

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=Delay action to wait for more user input=
{{LangSwitch}}
== Usecase ==


==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 <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.


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 &amp; 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…
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).


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).
== Solution ==


==Enter DelayedExecutionTimer==
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:


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


The <code>DelayedExecutionTimer</code> 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. <code>DelayedExecutionTimer</code> basically provides one slot <code>trigger()</code> and one signal <code>triggered()</code>. Instead of directly connecting the event to the action (connecting the <code>textChanged()</code> to the <code>setFilter()</code>, for instance), you connect the <code>QLineEdit::textChanged()</code> to <code>DelayedExecutionTimer::trigger()</code>, and <code>DelayedExecutionTimer::triggered()</code> to <code>QSortFilterProxyModel::setFilter…()</code>.
Then, create a slot where the query will be performed, and connect the timer to it:
<code>
connect( m_typingTimer, &QTimer::timeout, this, &YourClass::filterEntries );


<code>DelayedExecutionTimer</code> also provides two more versions of both the trigger() slot and the triggered() signal for convenience. These allow to pass a QString or an int as an argument. The triggered() signal will be emitted without argument, and with both a QString and with an int argument, carrying the last value that was passed in by the corresponding trigger() 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 <code>SortFilterProxy</code> model becomes as simple as this:
void YourClass::filterEntries()
{
    // The actual filtering code goes here, using the string stored in the m_filterText class variable
}
</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.
Finally, create a <tt>YourClass::onTextEdited</tt> slot that stores the newly edited filter text and starts the countdown to call the filtering method:
<code>
connect( m_textEdit, QLineEdit::textChanged, this, &YourClass::onTextEdited );


==Code==
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
}
</code>


===delayedexecutiontimer.h===
:''Note: the code above is adapted from a [http://stackoverflow.com/a/21945529/266309 StackOverflow answer] by [http://stackoverflow.com/users/2502409/nazar554 Nazar554].''
 
And here’s the <span class="caps">CPP</span> code:
 
===delayedexecutiontimer.cpp===
 
===Categories:===
 
* [[:Category:snippets|snippets]]

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.
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