QQuickWindow::grabWindows() can't get image.

android device
I can get a normal image by call QQuickWindow::grabWindows() at most devices,but only at a few device I gotted a black image.
why? i think android device cause this problem, but the greater reason is Qt.

a demo in attachment. you can check it.

void ALGrabber::grabPicture(QQuickWindow *w)
{
// emit sigSharePicture(w->grabWindow());
QImage image = w->grabWindow();
QString savePath = "/sdcard/temppp.png";
QDir dir = QFileInfo(savePath).absoluteDir();
if(!dir.exists())

image.save(savePath);
}