Qt Signal Slot Event Loop

• Qt Signals And Slots Without Event Loop
• Qt Signal Slot Event Loop 288

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

9.3.3 Signals and Slots When the main thread of a C program calls qApp-exec , it enters into an event loop, where messages are handled and dispatched. While qApp is executing its event loop, it is possible for QObject s to send messages to one another. I have several signal/slot connections between a worker thread and the GUI thread. Using the default delivery method, which posts an event in the GUI's event queue, one of my connections can occasionally emit signals faster into the event queue than the slot being called. In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. Qt event loop, networking and I/O API. ‒ An event is posted so the slot is called later. ‒ Actual I/O happens in the event loop. Signals notify of. An event loop in a thread makes it possible for the thread to use certain non-GUI Qt classes that require the presence of an event loop (such as QTimer, QTcpSocket, and QProcess). It also makes it possible to connect signals from any threads to slots of a specific thread.

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

Qt Signals And Slots Without Event Loop

GitHub Invitation URL:exercise13

Steps:

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

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 Event Loop 288

You have completed the Exercise.