我想使用PyQGIS创建一个自定义处理算法，它能够接受矢量图层作为输入（在这种情况下是点类型），然后对其要素进行处理。只要选择整个图层，它就能正常工作。但是如果我尝试仅处理选定的要素，它就无法正常工作。
我使用`QgsProcessingParameterFeatureSource`来允许仅处理选定的要素。选项显示出来，我可以启用复选框。但是当我执行算法时，`parameterAsVectorLayer`返回`NoneType`。
以下是一个最小工作示例，用于复现这个问题：
```python
from qgis.PyQt.QtCore import QCoreApplication
from qgis.core import (
    QgsProcessing,
    QgsProcessingAlgorithm,
    QgsProcessingParameterFeatureSource
)
name = "selectedonly"
display_name = "仅选定要素示例"
group = "测试"
group_id = "test"
short_help_string = "用于展示我的问题的最小工作示例代码。"
class ExampleProcessingAlgorithm(QgsProcessingAlgorithm):
    def tr(self, string):
        return QCoreApplication.translate('Processing', string)
    def createInstance(self):
        return ExampleProcessingAlgorithm()
    def name(self):
        return name
    def displayName(self):
        return self.tr(display_name)
    def group(self):
        return self.tr(group)
    def groupId(self):
        return group_id
    def shortHelpString(self):
        return self.tr(short_help_string)
    def initAlgorithm(self, config=None):
        self.addParameter(
            QgsProcessingParameterFeatureSource(
                'INPUT',
                self.tr('某些点矢量图层。'),
                types=[QgsProcessing.TypeVectorPoint]
            )
        )
    def processAlgorithm(self, parameters, context, feedback):
        layer = self.parameterAsVectorLayer(
            parameters,
            'INPUT',
            context
        )
        return {"OUTPUT": layer}

<details>
<summary>英文:</summary>
I want to create a custom processing algorithm with PyQGIS, which is able to take a vector layer as input (in this case of type point) and then do something with it&#39;s features. It&#39;s working well as long as I just choose the whole layer. But it doesn&#39;t work if I&#39;m trying to work on selected features only.
I&#39;m using `QgsProcessingParameterFeatureSource` to be able to work on selected features only. The option is shown and I can enable the checkbox. But when I&#39;m executing the algorithm, I get `NoneType` as return of `parameterAsVectorLayer`.
Below you&#39;ll find a minimal working example to reproduce the problem:

def createInstance(self):
    return ExampleProcessingAlgorithm()
def name(self):
    return name
def displayName(self):
    return self.tr(display_name)
def group(self):
    return self.tr(group)
def groupId(self):
    return group_id
def shortHelpString(self):
    return self.tr(short_help_string)
def initAlgorithm(self, config=None):
    self.addParameter(
        QgsProcessingParameterFeatureSource(
            'INPUT',
            self.tr('Some point vector layer.'),
            types=[QgsProcessing.TypeVectorPoint]
        )
    )
def processAlgorithm(self, parameters, context, feedback):
    layer = self.parameterAsVectorLayer(
        parameters,
        'INPUT',
        context
    )
    return {"OUTPUT": layer}

If I'm working on the whole layer, the output is {'OUTPUT': \<QgsVectorLayer: 'Neuer Temporärlayer' (memory)\>}, which is what I would expect.
If I'm working on selected features only, my output is {'OUTPUT': None}, which doesn't makes sense to me. I selected some of the features before executing of course.
I'm using QGIS-version 3.22 LTR, if it's relevant.
Can anybody tell me what I'm doing wrong?
</details>
# 答案1
**得分**: 0
我建议您尝试在"processAlgorithm"方法中使用"parameterAsSource"方法。
layer = self.parameterAsSource(
        parameters,
        'INPUT',
        context
    )
<details>
<summary>英文:</summary>
I would suggest you trying to use the method &#39;parameterAsSource&#39; in the &#39;processAlgorithm&#39; method.
    layer = self.parameterAsSource(
            parameters,
            &#39;INPUT&#39;,
            context
        )
</details>