Qt Qtimer Signal Slot

I have a QTimer in MainWindow class but the update slot doesn't get called. I have no idea what it would be. Connect return true and I get neither warning from messages window in QT.

In this paper, a QTimer is defined, and the slot function onTimeOut is defined as the slot function corresponding to the timeout signal. The code in the constructor of the window class is as follows. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. The QTimerclass provides a high-level programming interface for timers. To use it, create a QTimer, connect its timeout signal to the appropriate slots, and call start. From then on, it will emit the timeout signal at constant intervals. Example for a one second (1000 millisecond) timer (from the Analog Clockexample).

The QTimer class provides repetitive and single-shot timers.More...

Inherits QObject.

Methods

• int interval (self)
• bool isSingleShot (self)
• setSingleShot (self, bool asingleShot)
• start (self)
• timerEvent (self, QTimerEvent)

Qt Signal Slot Performance

Static Methods

• singleShot (int msec, QObject receiver, SLOT()SLOT() member)

Qt Signals

• void timeout ()

Detailed Description

The QTimer class provides repetitive and single-shot timers.

The QTimer class provides a high-level programming interface fortimers. To use it, create a QTimer, connect its timeout() signal to the appropriateslots, and call start(). From thenon it will emit the timeout()signal at constant intervals.

Example for a one second (1000 millisecond) timer (from theAnalog Clock example):

From then on, the update() slot is called everysecond.

You can set a timer to time out only once by callingsetSingleShot(true). You can also use the static QTimer.singleShot() function to calla slot after a specified interval:

In multithreaded applications, you can use QTimer in any threadthat has an event loop. To start an event loop from a non-GUIthread, use QThread.exec(). Qtuses the timer's thread affinityto determine which thread will emit the timeout() signal. Because of this, youmust start and stop the timer in its thread; it is not possible tostart a timer from another thread.

As a special case, a QTimer with a timeout of 0 will time out assoon as all the events in the window system's event queue have beenprocessed. This can be used to do heavy work while providing asnappy user interface:

processOneThing() will from then on be calledrepeatedly. It should be written in such a way that it alwaysreturns quickly (typically after processing one data item) so thatQt can deliver events to widgets and stop the timer as soon as ithas done all its work. This is the traditional way of implementingheavy work in GUI applications; multithreading is now becomingavailable on more and more platforms, and we expect thatzero-millisecond QTimers will gradually be replaced by QThreads.

Accuracy and Timer Resolution

Timers will never time out earlier than the specified timeoutvalue and they are not guaranteed to time out at the exact valuespecified. In many situations, they may time out late by a periodof time that depends on the accuracy of the system timers.

The accuracy of timers depends on the underlying operatingsystem and hardware. Most platforms support a resolution of 1millisecond, though the accuracy of the timer will not equal thisresolution in many real-world situations.

If Qt is unable to deliver the requested number of timer clicks,it will silently discard some.

Alternatives to QTimer

An alternative to using QTimer is to call QObject.startTimer() for your objectand reimplement the QObject.timerEvent() event handlerin your class (which must inherit QObject). The disadvantage is that timerEvent() does not support suchhigh-level features as single-shot timers or signals.

Another alternative to using QTimer is to use QBasicTimer. It is typically less cumbersomethan using QObject.startTimer() directly. SeeTimers for an overview of all threeapproaches.

Some operating systems limit the number of timers that may beused; Qt tries to work around these limitations.

Method Documentation

Qt Signal Slot Parameter

QTimer.__init__ (self, QObjectparent = None)

The parent argument, if not None, causes self to be owned by Qt instead of PyQt.

Constructs a timer with the given parent.

int QTimer.interval (self)

bool QTimer.isActive (self)

bool QTimer.isSingleShot (self)

QTimer.setInterval (self, int msec)

QTimer.setSingleShot (self, bool asingleShot)

QTimer.singleShot (int msec, QObjectreceiver, SLOT()SLOT() member)

This static function calls a slot after a given timeinterval.

It is very convenient to use this function because you do notneed to bother with a timerEvent or create a local QTimer object.

Example:

This sample program automatically terminates after 10 minutes(600,000 milliseconds).

The receiver is the receiving object and themember is the slot. The time interval is msecmilliseconds.

Note: This function is reentrant.

See alsosetSingleShot() and start().

QTimer.singleShot (int msec, callable receiver)

QTimer.start (self, int msec)

If the timer is already running, it will be stopped and restarted.

If singleShot is true,the timer will be activated only once.

QTimer.start (self)

This function overloads start().

Starts or restarts the timer with the timeout specified ininterval.

If the timer is already running, it will be stopped and restarted.

If singleShot is true,the timer will be activated only once.

QTimer.stop (self)

See alsostart().

QTimer.timerEvent (self, QTimerEvent)

Reimplemented from QObject.timerEvent().

int QTimer.timerId (self)

Qt Signal Documentation

void timeout ()

See alsointerval, start(), and stop().

Qt Qtimer Signal Slot Machine

Today we will learn about a variation of the Observer design patternthat is used prominently within Qt, called signals and slots.

• Observer and Publish/Subscribe Pattern
• Observers as callback functions
• Observers using signals
• Qt signals
• Examples
• Exercise

GitHub Invitation URL:exercise13

Steps:

Qt Qtimer Signal Slot Casino Game

1. Clone the assignment for today after accepting the GitHubinvitation at the link above. The repository contains four files:

• traffic_light.h defines a Qt widget that uses three radio buttons tosimulate a traffic light.
• traffic_light.cpp is the implementation of the traffic light
• main.ccp sets up the application and starts the event loop
• CMakeLists.txt is the build configuration for the example

Qt Qtimer Signal Slot Machine

2. Add code to the TrafficLight class to add a slot called toggle tochange the light currently lit. The sequence should gored->green->yellow->red repeating after that. You will need to addsome internal members to the TrafficLight class to accomplish this.
3. Read the documentation forQTimer and, in the mainfunction of main.cpp, add code to setup and start a timer thatgoes off every one second, resulting in the traffic light beingtoggled. This will require connecting a signal from the timer to theslot implemented in step 2.
4. Build and run your application. Does the light change in the correctsequence?
5. Now, use git to commit the source files changed to the localrepository.
6. Use git push to synchronize the repository with that onGitHub.
7. Finally, post the screenshot of output of your program to Canvas.

Qt Signal Slot Thread

You have completed the Exercise.