[Solved]The meaning of the "preview" parameter from the example "declarative-camera"

[A Former User]

@imageCapture {
onImageCaptured: {
photoPreview.source = preview //where are this parameter(preview) come from?
stillControls.previewAvailable = true
cameraUI.state = "PhotoPreview"
}
}@

File "declarative-camera.qml", line 91

These are part of the codes from the Qt5 example ""declarative-camera"
Can't find the source of this parameter, where is it come from?
The preview should be the image the camera captured

[A Former User]

Because of signature is
onImageCaptured(requestId, preview)
so the preview is the parameter of the signal