在畫布元素html5中創建多個可拖動矩形

Question

$(document).ready(function () {

    var canvas = document.getElementById('canvas'),
    ctx = canvas.getContext('2d'),
    rect = {},
    drag = false;

    function init() {
        canvas.addEventListener('mousedown', mouseDown, false);
        canvas.addEventListener('mouseup', mouseUp, false);
        canvas.addEventListener('mousemove', mouseMove, false);
    }

    function mouseDown(e) {
        rect.startX = e.pageX - this.offsetLeft;
        rect.startY = e.pageY - this.offsetTop;
        drag = true;
    }
    function mouseUp() {
        drag = false;
    }

    function mouseMove(e) {
        if (drag) {
            rect.w = (e.pageX - this.offsetLeft) - rect.startX;
            rect.h = (e.pageY - this.offsetTop) - rect.startY;
            ctx.clearRect(0, 0, canvas.width, canvas.height);
            draw();
        }
    }
    function draw() {
        ctx.fillRect(rect.startX, rect.startY, rect.w, rect.h);
    }
    init();


});

到目前為止，這就是我所擁有的，但是當我創建另一個矩形時，最后一個將被自動擦除。 我需要創建多個矩形，並使它們能夠在畫布內拖動。

Answer 1

使用drawOldShapes（）方法，可以保留舊矩形。

var canvas = document.getElementById('canvas'),
ctx = canvas.getContext('2d'),
// ctx.globalAlpha = 0.5;
rect = {},
drag = false;
var rectStartXArray = new Array() ;
var rectStartYArray = new Array() ;
var rectWArray = new Array() ;
var rectHArray = new Array() ;

function init() {
    canvas.addEventListener('mousedown', mouseDown, false);
    canvas.addEventListener('mouseup', mouseUp, false);
    canvas.addEventListener('mousemove', mouseMove, false);
}

function mouseDown(e) {
    rect.startX = e.pageX - this.offsetLeft;
    rect.startY = e.pageY - this.offsetTop;
    drag = true;
}
function mouseUp() {
    rectStartXArray[rectStartXArray.length] = rect.startX;
    rectStartYArray[rectStartYArray.length] = rect.startY;
    rectWArray[rectWArray.length] = rect.w;
    rectHArray[rectHArray.length] = rect.h;
    drag = false;
}

function mouseMove(e) {
  if (drag) {
        rect.w = (e.pageX - this.offsetLeft) - rect.startX;
        rect.h = (e.pageY - this.offsetTop) - rect.startY;
        ctx.clearRect(0, 0, canvas.width, canvas.height);
        draw();
    }

    drawOldShapes();
}
function draw() {
    ctx.beginPath();
    ctx.rect(rect.startX, rect.startY, rect.w, rect.h);
    ctx.stroke();
}
function drawOldShapes(){
    for(var i=0;i<rectStartXArray.length;i++)
    {
        if(rectStartXArray[i]!= rect.startX && rectStartYArray[i] != rect.startY && rectWArray[i] != rect.w && rectHArray[i] != rect.h)
        {
            ctx.beginPath();
            ctx.rect(rectStartXArray[i], rectStartYArray[i], rectWArray[i], rectHArray[i]);
            ctx.stroke();
        }
    }
}
init();

Answer 2

我想您需要將每個矩形存儲在數組中，並且每次調用draw()都需要繪制所有矩形。

就像是：

rects = [];
rect = null;

//onmouse down:

rect = { x1 : <value from Mouse>,y1: <value from Mouse>,x2 : 0 ,y2 : 0 };

//onmouse up

rect.x2 = <value from Mouse>;
rect.y2 = <value from Mouse>;

rects.push( rect );
rect = null;

//draw

 for( var i = 0; i < rects.length; i++ ) {
     //drawing each rectangle
 }

這只是一種建議。 在坐標旁邊，您還可以存儲有關填充顏色，筆划寬度和-style等的信息。

祝好運！