我需要你上述的完整代码

好的，以下是增加了button按钮并实现长按绘制矩形功能的完整代码：

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 (mouseArea.rectStartPos !== null && mouseArea.rectEndPos !== null) {
                    var x = Math.min(mouseArea.rectStartPos.x, mouseArea.rectEndPos.x);
                    var y = Math.min(mouseArea.rectStartPos.y, mouseArea.rectEndPos.y);
                    var width = Math.abs(mouseArea.rectStartPos.x - mouseArea.rectEndPos.x);
                    var height = Math.abs(mouseArea.rectStartPos.y - mouseArea.rectEndPos.y);
                    drawRect(x, y, width, height);
                }
            }
        }

        MouseArea {
            anchors.fill: parent
            property int gridSize: parent.gridSize
            property int scaleFactor: parent.scaleFactor

            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();
            }

            property var rectStartPos: null
            property var rectEndPos: null
            onLongPressed: {
                rectStartPos = mapToItem(canvas, mouse.x, mouse.y);
            }
            onLongPressCanceled: {
                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");
            }
        }
    }
}