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: 400 height: 400 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); // 绘制网格线 ctx.strokeStyle = "black"; ctx.lineWidth = 0.2; // 将线宽改小 for (var x = -width/2; x <= width/2; x += parent.gridSize) { ctx.beginPath(); ctx.moveTo(x, -height/2); ctx.lineTo(x, height/2); ctx.stroke(); } for (var y = -height/2; y <= height/2; y += parent.gridSize) { ctx.beginPath(); ctx.moveTo(-width/2, y); ctx.lineTo(width/2, y); ctx.stroke(); } } } MouseArea { anchors.fill: parent property int gridSize: parent.gridSize property int scaleFactor: parent.scaleFactor onWheel: { // 根据滚轮事件的delta属性，计算缩放比例 parent.scaleFactor += wheel.angleDelta.y / 120; parent.scaleFactor = Math.max(parent.scaleFactor, 10); // 最小值为10% // 根据缩放比例重新计算gridSize和canvas的宽度和高度 var newGridSize = parent.scaleFactor / 5; var newWidth = width * parent.scaleFactor / 100; var newHeight = height * parent.scaleFactor / 100; var xDiff = (newWidth - width) / 2; // 计算横向偏移量 var yDiff = (newHeight - height) / 2; // 计算纵向偏移量 parent.gridSize = newGridSize; canvas.width = newWidth; canvas.height = newHeight; canvas.x -= xDiff; // 调整canvas的位置 canvas.y -= yDiff; canvas.requestPaint(); } } } }这段代码的画布大小相对而言变得太小了，请找出问题所在，并改进

问题所在是在缩放比例计算与应用时，没有正确计算新的画布宽高和偏移量，导致画布缩放后被裁剪。正确的代码应该是：

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: 400
        height: 400

        property int gridSize: 20
        property real scaleFactor: 1.0

        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);

                // 绘制网格线
                ctx.strokeStyle = "black";
                ctx.lineWidth = 0.2 * parent.scaleFactor;

                for (var x = -width/2; x <= width/2; x += parent.gridSize) {
                    ctx.beginPath();
                    ctx.moveTo(x, -height/2);
                    ctx.lineTo(x, height/2);
                    ctx.stroke();
                }

                for (var y = -height/2; y <= height/2; y += parent.gridSize) {
                    ctx.beginPath();
                    ctx.moveTo(-width/2, y);
                    ctx.lineTo(width/2, y);
                    ctx.stroke();
                }
            }
        }

        MouseArea {
            anchors.fill: parent

            property int gridSize: parent.gridSize
            property real scaleFactor: parent.scaleFactor

            onWheel: {
                // 根据滚轮事件的delta属性，计算缩放比例
                parent.scaleFactor += wheel.angleDelta.y / 120;
                parent.scaleFactor = Math.max(parent.scaleFactor, 0.1); // 最小值为10%

                // 根据缩放比例重新计算gridSize和canvas的宽度和高度
                var newGridSize = parent.gridSize * parent.scaleFactor;
                var newWidth = width * parent.scaleFactor;
                var newHeight = height * parent.scaleFactor;

                // 计算横向和纵向偏移量
                var xDiff = (newWidth - width) / 2;
                var yDiff = (newHeight - height) / 2;

                parent.gridSize = newGridSize;
                canvas.width = newWidth;
                canvas.height = newHeight;
                canvas.x -= xDiff;
                canvas.y -= yDiff;

                canvas.requestPaint();
            }
        }
    }
}

主要改动是：

- 将scaleFactor从int类型改为real类型，以便支持小数点以下的缩放比例。
- 在绘制网格线时，将线宽乘以缩放比例。
- 在计算新的宽高和偏移量时，将缩放比例乘以原始的gridSize，而不是除以5。
- 在计算偏移量时，将横向和纵向偏移分别计算，避免画布只发生横向或纵向偏移的情况。