如何检测 JSON 对象内的变化？

Question

import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.0

Window {
    visible: true
    width: 640
    height: 480

    property var json: {
        "name": "old"
    }

    Component.onCompleted: json.name = "new"

    onJsonChanged: console.log("Changed to : " + json.name)

    Button {
        onClicked: console.log("Button clicked: " + json.name)
    }
}

qml: Changed to : old

But I'd expect an output of:

qml: Changed to : old
qml: Changed to : new

QML has no problem recognizing the onJsonChanged: signal, but the signal doesn't actually seem to activate. I know json.name is successfully changing, because I've debugged during runtime and see that it has the new value. So why doesn't the signal activate, and/or how can I detect the change in the json object?

Answer 1

The signal associated with the X property is emitted when the object changes, in your case changing some internal attribute does not change the object (the memory space) only modifies internal properties. To verify this you can modify your example to:

import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.0

Window {
    visible: true
    width: 640
    height: 480

    property var json: {
        "name": "bar"
    }

    Component.onCompleted: json = {"name3": "foo"}

    onJsonChanged: console.log("Changed to : " + json)

    Button {
        onClicked: console.log("Button clicked: " + json)
    }
}

The following is obtained:

qml: Changed to : [object Object]
qml: Changed to : [object Object]