投射 QObject 子类实例 [英] Casting a QObject subclass instance

问题描述

我有一个带有附加类方法 make 的 QPixmap 子类:

I have a QPixmap subclass with additional class method make:

class Screenshot(QtGui.QPixmap):
    @classmethod
    def make(cls):
        desktop_widget = QtGui.QApplication.desktop()
        image = cls.grabWindow(
            desktop_widget.winId(), rect.x(), rect.y(), rect.width(), rect.height())
        import ipdb; ipdb.set_trace() 
        image.save()
        return image

当我调用 Screenshot.make() 时，传递了正确的类 cls，但通过 cls.grabWindow 创建的实例不是截图:

When I call Screenshot.make() the correct class cls is passed, but the instance created via cls.grabWindow is not a Screenshot:

ipdb> ...py(30)make()
     29         import ipdb; ipdb.set_trace()
---> 30         image.save()
     31         return image

ipdb> cls
<class 'viewshow.screenshot.Screenshot'>
ipdb> image
<PyQt4.QtGui.QPixmap object at 0x7f0f8c4a9668>

更短:

ipdb> Screenshot.grabWindow(desktop_widget.winId())
<PyQt4.QtGui.QPixmap object at 0x7f0f8154c438>

如何获取Screenshot 实例?

推荐答案

Screenshot 继承自 QPixmap 的所有方法都会返回一个 QPixmap，所以你需要显式地创建并返回一个 Screenshot 的实例.

All the methods Screenshot inherits from QPixmap will return a QPixmap, so you need to explicitly create and return an instance of Screenshot instead.

唯一真正的问题是避免低效复制.然而，QPixmap 提供了一个非常快速的复制构造函数来实现这一点，所以你所需要的只是这样:

The only real issue is to avoid inefficient copying. However, QPixmap provides a very fast copy-constructor for doing just that, so all you need is something like this:

class Screenshot(QtGui.QPixmap):
    @classmethod
    def make(cls):
        ...
        image = cls.grabWindow(...)
        return cls(image)

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