Communicating with the user

This section shows some methods and elements that should be used to communicate with the user, in order to keep consistency in the User Interface.

Showing messages. The QgsMessageBar class

Using message boxes can be a bad idea from a user experience point of view. For showing a small info line or a warning/error messages, the QGIS message bar is usually a better option.

Using the reference to the QGIS interface object, you can show a message in the message bar with the following code

from qgis.core import Qgis
iface.messageBar().pushMessage("Error", "I'm sorry Dave, I'm afraid I can't do that", level=Qgis.Critical)

QGIS Message bar

You can set a duration to show it for a limited time

iface.messageBar().pushMessage("Ooops", "The plugin is not working as it should", level=Qgis.Critical, duration=3)

QGIS Message bar with timer

The examples above show an error bar, but the level parameter can be used to creating warning messages or info messages, using the Qgis.MessageLevel enumeration. You can use up to 4 different levels:

0. Info

1. Warning

2. Critical

3. Success

QGIS Message bar (info)

Widgets can be added to the message bar, like for instance a button to show more info

def showError():
    pass

widget = iface.messageBar().createMessage("Missing Layers", "Show Me")
button = QPushButton(widget)
button.setText("Show Me")
button.pressed.connect(showError)
widget.layout().addWidget(button)
iface.messageBar().pushWidget(widget, Qgis.Warning)

QGIS Message bar with a button

You can even use a message bar in your own dialog so you don't have to show a message box, or if it doesn't make sense to show it in the main QGIS window

class MyDialog(QDialog):
    def __init__(self):
        QDialog.__init__(self)
        self.bar = QgsMessageBar()
        self.bar.setSizePolicy( QSizePolicy.Minimum, QSizePolicy.Fixed )
        self.setLayout(QGridLayout())
        self.layout().setContentsMargins(0, 0, 0, 0)
        self.buttonbox = QDialogButtonBox(QDialogButtonBox.Ok)
        self.buttonbox.accepted.connect(self.run)
        self.layout().addWidget(self.buttonbox, 0, 0, 2, 1)
        self.layout().addWidget(self.bar, 0, 0, 1, 1)
    def run(self):
        self.bar.pushMessage("Hello", "World", level=Qgis.Info)

myDlg = MyDialog()
myDlg.show()

QGIS Message bar in custom dialog

Showing progress

Progress bars can also be put in the QGIS message bar, since, as we have seen, it accepts widgets. Here is an example that you can try in the console.

import time
from qgis.PyQt.QtWidgets import QProgressBar
from qgis.PyQt.QtCore import *
progressMessageBar = iface.messageBar().createMessage("Doing something boring...")
progress = QProgressBar()
progress.setMaximum(10)
progress.setAlignment(Qt.AlignLeft|Qt.AlignVCenter)
progressMessageBar.layout().addWidget(progress)
iface.messageBar().pushWidget(progressMessageBar, Qgis.Info)

for i in range(10):
    time.sleep(1)
    progress.setValue(i + 1)

iface.messageBar().clearWidgets()

Also, you can use the built-in status bar to report progress, as in the next example:

vlayer = QgsProject.instance().mapLayersByName("countries")[0]

count = vlayer.featureCount()
features = vlayer.getFeatures()

for i, feature in enumerate(features):
    # do something time-consuming here
    print('') # printing should give enough time to present the progress

    percent = i / float(count) * 100
    # iface.mainWindow().statusBar().showMessage("Processed {} %".format(int(percent)))
    iface.statusBarIface().showMessage("Processed {} %".format(int(percent)))

iface.statusBarIface().clearMessage()

Logging

You can use the QGIS logging system to log all the information that you want to save about the execution of your code.

# You can optionally pass a 'tag' and a 'level' parameters
QgsMessageLog.logMessage("Your plugin code has been executed correctly", 'MyPlugin', level=Qgis.Info)
QgsMessageLog.logMessage("Your plugin code might have some problems", level=Qgis.Warning)
QgsMessageLog.logMessage("Your plugin code has crashed!", level=Qgis.Critical)

警告

Use of the Python print statement is unsafe to do in any code which may be multithreaded. This includes expression functions, renderers, symbol layers and Processing algorithms (amongst others). In these cases you should always use thread safe classes (QgsLogger or QgsMessageLog) instead.

注解

You can see the output of the QgsMessageLog in the Log Messages Panel

注解

• QgsLogger is for messages for debugging / developers (i.e. you suspect they are triggered by some broken code)

• QgsMessageLog is for messages to investigate issues by sysadmins (e.g. to help a sysadmin to fix configurations)