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 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); } } } property var rectStartPos: null property var rectEndPos: null 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"); } } } }}这段代码运行时出现了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

在Canvas的onPaint函数中，对rectStartPos和rectEndPos进行操作，需要在Item中声明这两个属性，所以需要在Item中添加如下代码：

property var rectStartPos: null
property var rectEndPos: null

修改后的完整代码如下：

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
        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");
            }
        }
    }
}