Setting and Coding Your Basic QGIS Plugin - Hello World

Setting up and Coding your Plugin –Hello World 

In this tutorial we proceed to tweak the default user interface of the plugin we just created and create button that will then display an Info message of Hello World on the QGIS User Interface.

Prerequisites

In this tutorial we needed the following

Software

QGIS Software required (I use version: 2.12.1 Lyon or any other versions current is QGIS 3.8 Noosa )

Click the link to download the software appropriate to your platform (https://qgis.org/en/site/forusers/download.html) the software is open source

A code Editor

Notepad++ (it’s lightweight and open source) click on the link https://notepad-plus-plus.org/ to download the latest version

IDE

Qt Creator IDE (recommended as I prefer to use it over Notepad++) as they come with GUI (graphical user interface ) toolkit for user interface creation (click https://www.qt.io/download) and use the Open Source Version to download

Platform

 Operating System : In this tutorial Windows 7

Steps on Creating Your Custom Quantum (QGIS) Plugin  

1.     Click on Start Menu ->Click Qt Creator IDE.  We use the Open Source Version for this tutorial

2.     Go to File Menu and go to Open File or Project .Go to the directory where your plugin is placed on the HelloWorld folder and select the QT Ui File  with the suffix dockwidget_base (in this example hello_world_dockwidget_base).

3.     As you see this is the very same plugin displayed in the Part 1 of this tutorial. Now we modify it.

For this tutorial we display a warning or information bar on the top of the QGIS software using Python.

We modify the Label Content first. Click on the Label and change its text Property on the Properties window on the right. Now we have the Hello World label

4.     We then drag a button (Push Button) on the left to the plugin panel and change its text Property on the right to “Display Hello World”

5.Now on the File Menu we click on Open File or Project .We choose the hello_world Python file.

The hello_world module python file contains the logic and code for the plugin. So this is where we will code the Information Display.

6.The IDE code editor then opens. We go to the run module .the run module is where we put the Info “Hello World” because it is executed after the QGIS software has finished initializing its class and its GUI.

7. Now we edit the code just below the self.dockwidget.show()

def run(self):

        """Run method that loads and starts the plugin"""

        if not self.pluginIsActive:

            self.pluginIsActive = True

            #print "** STARTING HelloWorld"

            # dockwidget may not exist if:

            #    first run of plugin

            #    removed on close (see self.onClosePlugin method)

            if self.dockwidget == None:

            # Create the dockwidget (after translation) and keep reference

                self.dockwidget = HelloWorldDockWidget()

            # connect to provide cleanup on closing of dockwidget

            self.dockwidget.closingPlugin.connect(self.onClosePlugin)

            # show the dockwidget

            # TODO: fix to allow choice of dock location

            self.iface.addDockWidget(Qt.LeftDockWidgetArea, self.dockwidget)

            self.dockwidget.show()

 

            self.dockwidget.pushButton.clicked.connect(self.Display)

    def Display(self):

        self.iface.messageBar().pushMessage("Hello World", "Welcome to QGIS Python     Programming",QgsMessageBar.INFO)

So what would be the result be?

After adding the code.

8. Restart QGIS. Click on the Plugins menu and click on the Display Hello World Message Menu Item to activate the plugin.

9. Now as you see the interface looks the same as what we have manipulated in the QT Creator IDE. Now click on the button Display Hello World.

Now as you see. We now have an Information Message “Hello World , Welcome to QGIS Python Programming.” Now there you have it. A basic Plugin . Congratulations on the Part 2 of our Tutorial.

So let us dissect the code that I have written

                self.dockwidget.pushButton.clicked.connect(self.Display)

    def Display(self):

        self.iface.messageBar().pushMessage("Hello World", "Welcome to QGIS Python     Programming",QgsMessageBar.INFO)

(1) self.dockwidget.pushButton.clicked.connect(self.Display)

This portion of code when sets the event handler which is defined by the module Display after it whenever  the user clicks the pushButton located on the dockwidget which is the panel that holds all the controls including the label and the button (pushButton)

(2)def Display(self):

This portion defines the logic by which the MESSAGE BAR will display Hello World

(3)self.iface.messageBar().pushMessage("Hello World", "Welcome to QGIS    

Python     Programming",QgsMessageBar.INFO)

This code snippet displays the Message Bar with the Message Hello World  , Welcome to QGIS Python Programming under INFO type of message. Displayed as Blue Message Bar

All of it is written after the self.dockwidget.show() snippet because it is only executed after the Widget/Plugin shows itself or the plugin has finished loading its component controls.