Creating plugin in QGIS

Question

I have a problem with my first QGIS 3 plugin. I want to calculate lenght each segment in my polyline and next creating buffer equal 1/2 length of line.

The first 13 lines code have to do calculating the length of the selected layer and it works. But I have a problem with the buffer. I tried many ways that I found on the internet and it still doesn't work. Now I have no errors, but the plugin does not return anything.

Can you solve this problem?

if result:
    for layer in QgsProject.instance().mapLayers().values():
        features = layer.getFeatures()
        coluna = QgsField('Length', QVariant.Double)
        res = layer.dataProvider().addAttributes([coluna])
        leng = layer.dataProvider().fieldNameIndex
        layer.updateFields()
        fieldIndex = layer.dataProvider().fieldNameIndex( "Length" )
        layer.startEditing()
        for f in features:
            geom = f.geometry()
            leng = geom.length()
            layer.changeAttributeValue(f.id(),fieldIndex, leng)
        result = processing.run("native:buffer",
            {'INPUT': layer, 
            'DISTANCE' : 100,
            'SEGMENTS' : 10,
            'DISSOLVE': True,
            'END_CAP_STYLE' : 1,
            'JOIN_STYLE': 0,
            'MITER_LIMIT': 10,
            'OUTPUT': 'memory:buffer'})
        layer.commitChanges()
        layer.updateFields()

Answer 1

After erasing some repeated lines in your code and changing 'run' processing method by 'runAndLoadResults' (also modifying 'DISTANCE' : 100 with 'DISTANCE' : leng/2), I tried your code out and it worked.

for layer in QgsProject.instance().mapLayers().values():
    features = layer.getFeatures()
    coluna = QgsField('Length', QVariant.Double)
    res = layer.dataProvider().addAttributes([coluna])
    layer.updateFields()
    fieldIndex = layer.dataProvider().fieldNameIndex( "Length" )
    layer.startEditing()
    for f in features:
        geom = f.geometry()
        leng = geom.length()
        layer.changeAttributeValue(f.id(),fieldIndex, leng)
    result = processing.runAndLoadResults("native:buffer",
                                          {'INPUT': layer, 
                                           'DISTANCE' : leng/2,
                                           'SEGMENTS' : 10,
                                           'DISSOLVE': True,
                                           'END_CAP_STYLE' : 1,
                                           'JOIN_STYLE': 0,
                                           'MITER_LIMIT': 10,
                                           'OUTPUT': 'memory:buffer'})
    layer.commitChanges()

Result can be observed in following image with a only one feature line layer:

However, in a plugin you need to trigger an action in some button (e.g. OK button). So, I created a test plugin capable to recreate this behavior with 'creating_buffer' function. For working in a plugin, these were the modifications in following code snippet:

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from PyQt5.QtCore import QSettings, QTranslator, qVersion, QCoreApplication, QVariant
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from qgis.core import QgsProject, QgsField
import processing
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    def add_action(
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        okBtn = self.dlg.okButton
        okBtn.clicked.connect(self.creating_buffer)

        return action
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    def creating_buffer(self):

        for layer in QgsProject.instance().mapLayers().values():
            features = layer.getFeatures()
            coluna = QgsField('Length', QVariant.Double)
            res = layer.dataProvider().addAttributes([coluna])
            layer.updateFields()
            fieldIndex = layer.dataProvider().fieldNameIndex( "Length" )
            layer.startEditing()
            for f in features:
                geom = f.geometry()
                leng = geom.length()
                layer.changeAttributeValue(f.id(),fieldIndex, leng)
            result = processing.runAndLoadResults("native:buffer",
                                                  {'INPUT': layer, 
                                                   'DISTANCE' : leng/2,
                                                   'SEGMENTS' : 10,
                                                   'DISSOLVE': True,
                                                   'END_CAP_STYLE' : 1,
                                                   'JOIN_STYLE': 0,
                                                   'MITER_LIMIT': 10,
                                                   'OUTPUT': 'memory:buffer'})
            layer.commitChanges()

    def run(self):
        """Run method that performs all the real work"""
        # show the dialog
        self.dlg.show()
        # Run the dialog event loop
        result = self.dlg.exec_()
        # See if OK was pressed
        if result:
            # Do something useful here - delete the line containing pass and
            # substitute with your code.
            pass

After clicking in OK button plugin, result was identical to above image.