Qt5:QML:Invalid write to global property QML

本文介绍了解决QML中全局变量赋值错误的方法。当尝试直接为全局变量赋值时,可能会遇到错误。正确的做法是在赋值时通过父ID引用全局变量。文章提供了出错代码示例及修正后的代码。

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描述

QML中给一个全局变量赋值时候提示这样错误

解决方案

全局变量前加上父id再去赋值就可以了

出错代码

TabView {
    id:idFuncSwitchtabView

    property var idKNormalWorkPanel
    property var idKFileManagePanel

    Tab {

        NormalWorkPanel
        {
            id:idNormalWorkPanel

            Component.onCompleted:
            {
                idKNormalWorkPanel=idNormalWorkPanel
            }
        }

    }
    Tab {

        FileManagePanel
        {
            id:idFileManagePanel
            Component.onCompleted:
            {
                idKFileManagePanel=idFileManagePanel
            }
        }

    }
}

正确代码

TabView {
    id:idFuncSwitchtabView

    property var idKNormalWorkPanel
    property var idKFileManagePanel

    Tab {

        NormalWorkPanel
        {
            id:idNormalWorkPanel

            Component.onCompleted:
            {
                idFuncSwitchtabView.idKNormalWorkPanel=idNormalWorkPanel
            }
        }

    }
    Tab {

        FileManagePanel
        {
            id:idFileManagePanel
            Component.onCompleted:
            {
                idFuncSwitchtabView.idKFileManagePanel=idFileManagePanel
            }
        }

    }
}
import QtQuick 2.4import QtQuick.Controls 2.5import QtQuick.Window 2.3ApplicationWindow { visible: true width: 800 height: 600 title: "Drawing Board Example" Item { width: 700 height: 500 property int gridSize: 20 property int scaleFactor: 100 property var rectStartPos: null property var rectEndPos: null Canvas { id: canvas anchors.fill: parent onPaint: { var ctx = getContext("2d"); var width = canvas.width; var height = canvas.height; // 清除画布 ctx.clearRect(0, 0, width, height); ctx.lineWidth = 0.002 * parent.scaleFactor; // 绘制网格线 ctx.strokeStyle = "black"; for (var x = 0; x <= width; x += parent.gridSize) { ctx.beginPath(); ctx.moveTo(x, 0); ctx.lineTo(x, height); ctx.stroke(); } for (var y = 0; y <= height; y += parent.gridSize) { ctx.beginPath(); ctx.moveTo(0, y); ctx.lineTo(width, y); ctx.stroke(); } // 绘制矩形 if (rectStartPos !== null && rectEndPos !== null) { var x = Math.min(rectStartPos.x, rectEndPos.x); var y = Math.min(rectStartPos.y, rectEndPos.y); var width = Math.abs(rectStartPos.x - rectEndPos.x); var height = Math.abs(rectStartPos.y - rectEndPos.y); drawRect(x, y, width, height); } } } MouseArea { anchors.fill: parent onWheel: { parent.scaleFactor += wheel.angleDelta.y / 120; parent.scaleFactor = Math.max(parent.scaleFactor, 10); parent.gridSize = parent.scaleFactor / 5; canvas.width = width * parent.scaleFactor / 100; canvas.height = height * parent.scaleFactor / 100; canvas.requestPaint(); } onPressed: { rectStartPos = mapToItem(canvas, mouse.x, mouse.y); } onReleased: { rectStartPos = null; rectEndPos = null; } onPositionChanged: { if (rectStartPos !== null) { rectEndPos = mapToItem(canvas, mouse.x, mouse.y); canvas.requestPaint(); } } } function drawRect(x, y, width, height) { var ctx = canvas.getContext("2d"); ctx.strokeStyle = "red"; ctx.strokeRect(x, y, width, height); } Button { id: rectButton text: "Draw Rectangle" anchors.bottom: parent.bottom anchors.horizontalCenter: parent.horizontalCenter onClicked: { console.log("Button clicked"); } } }}该代码在运行时出现了ReferenceError: rectStartPos is not defined与Error: Invalid write to global property "rectStartPos"的报错信息,请对上述代码做出修改,使其正常运行
06-09
好的,根据您的问题和报错信息,我认为问题出在`rectStartPos`和`rectEndPos`的定义上。 请尝试将以下代码: ``` property var rectStartPos: null property var rectEndPos: null ``` 修改为: ``` var rectStartPos = null var rectEndPos = null ``` 这样就可以避免全局变量的命名冲突和未定义问题。修改后的完整代码如下: ```qml import QtQuick 2.4 import QtQuick.Controls 2.5 import QtQuick.Window 2.3 ApplicationWindow { visible: true width: 800 height: 600 title: "Drawing Board Example" Item { width: 700 height: 500 property int gridSize: 20 property int scaleFactor: 100 var rectStartPos = null var rectEndPos = null Canvas { id: canvas anchors.fill: parent onPaint: { var ctx = getContext("2d"); var width = canvas.width; var height = canvas.height; // 清除画布 ctx.clearRect(0, 0, width, height); ctx.lineWidth = 0.002 * parent.scaleFactor; // 绘制网格线 ctx.strokeStyle = "black"; for (var x = 0; x <= width; x += parent.gridSize) { ctx.beginPath(); ctx.moveTo(x, 0); ctx.lineTo(x, height); ctx.stroke(); } for (var y = 0; y <= height; y += parent.gridSize) { ctx.beginPath(); ctx.moveTo(0, y); ctx.lineTo(width, y); ctx.stroke(); } // 绘制矩形 if (rectStartPos !== null && rectEndPos !== null) { var x = Math.min(rectStartPos.x, rectEndPos.x); var y = Math.min(rectStartPos.y, rectEndPos.y); var width = Math.abs(rectStartPos.x - rectEndPos.x); var height = Math.abs(rectStartPos.y - rectEndPos.y); drawRect(x, y, width, height); } } } MouseArea { anchors.fill: parent onWheel: { parent.scaleFactor += wheel.angleDelta.y / 120; parent.scaleFactor = Math.max(parent.scaleFactor, 10); parent.gridSize = parent.scaleFactor / 5; canvas.width = width * parent.scaleFactor / 100; canvas.height = height * parent.scaleFactor / 100; canvas.requestPaint(); } onPressed: { rectStartPos = mapToItem(canvas, mouse.x, mouse.y); } onReleased: { rectStartPos = null; rectEndPos = null; } onPositionChanged: { if (rectStartPos !== null) { rectEndPos = mapToItem(canvas, mouse.x, mouse.y); canvas.requestPaint(); } } } function drawRect(x, y, width, height) { var ctx = canvas.getContext("2d"); ctx.strokeStyle = "red"; ctx.strokeRect(x, y, width, height); } Button { id: rectButton text: "Draw Rectangle" anchors.bottom: parent.bottom anchors.horizontalCenter: parent.horizontalCenter onClicked: { console.log("Button clicked"); } } } } ``` 希望这次修改可以帮助您解决问题。
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