【发布时间】：2018-09-04 18:39:02
【问题描述】：
我正在尝试设置背景图像，在其上我可以拖动不同的形状以突出显示背景图像中的不同区域。 但我只能看到其中的任何一个，无论是形状还是图像。 以下是我的代码：
1）detailground函数负责绘制背景图片。 2) 3 个圆形对象的数组，创建圆形并将其渲染到画布上。 3) 三个鼠标事件及其对应的函数。
我尝试将draw函数传递给detailground，以便可以在图像上绘制圆圈。但是有些方法，我只能看到其中任何一个。 `
//track mouse position on mousemove
var mousePosition;
//track state of mousedown and up
var isMouseDown;
//reference to the canvas element
var c = document.getElementById("mycanvas");
c.width = window.innerWidth * 0.75;
c.height = window.innerHeight * 0.75;
//reference to 2d context
var ctx = c.getContext("2d");
//add listeners
document.addEventListener('mousemove', move, false);
document.addEventListener('mousedown', setDraggable, false);
document.addEventListener('mouseup', setDraggable, false);
//make some circles
var c1 = new Circle(50, 50, 50, "black");
var c2 = new Circle(200, 50, 50, "green", "black");
var c3 = new Circle(350, 50, 50, "blue", "black");
//initialise our circles
var circles = [c1, c2, c3];
drawbackground(c, ctx, draw);
draw(c, ctx);
function drawbackground(c, ctx, onload){
var imagePaper = new Image();
imagePaper.onload = function(){
ctx.drawImage(imagePaper,100, 20, 500,500);
onload(c, ctx);
};
imagePaper.src = "https://upload.wikimedia.org/wikipedia/commons/f/f9/Phoenicopterus_ruber_in_S%C3%A3o_Paulo_Zoo.jpg";
}
//main draw method
function draw(c, ctx) {
//clear canvas
ctx.clearRect(0, 0, c.width, c.height);
drawCircles();
}
//draw circles
function drawCircles() {
for (var i = circles.length - 1; i >= 0; i--) {
circles[i].draw();
}
}
//key track of circle focus and focused index
var focused = {
key: 0,
state: false
}
//circle Object
function Circle(x, y, r, fill, stroke) {
this.startingAngle = 0;
this.endAngle = 2 * Math.PI;
this.x = x;
this.y = y;
this.r = r;
this.fill = fill;
this.stroke = stroke;
this.draw = function () {
ctx.beginPath();
ctx.arc(this.x, this.y, this.r, this.startingAngle, this.endAngle);
// ctx.fillStyle = this.fill;
ctx.lineWidth = 3;
// ctx.fill();
ctx.strokeStyle = this.stroke;
ctx.stroke();
}
}
function move(e) {
if (!isMouseDown) {
return;
}
getMousePosition(e);
//if any circle is focused
if (focused.state) {
circles[focused.key].x = mousePosition.x;
circles[focused.key].y = mousePosition.y;
draw(c, ctx);
return;
}
//no circle currently focused check if circle is hovered
for (var i = 0; i < circles.length; i++) {
if (intersects(circles[i])) {
circles.move(i, 0);
focused.state = true;
break;
}
}
draw(c, ctx);
}
//set mousedown state
function setDraggable(e) {
var t = e.type;
if (t === "mousedown") {
isMouseDown = true;
} else if (t === "mouseup") {
isMouseDown = false;
releaseFocus();
}
}
function releaseFocus() {
focused.state = false;
}
function getMousePosition(e) {
var rect = c.getBoundingClientRect();
mousePosition = {
x: Math.round(e.x - rect.left),
y: Math.round(e.y - rect.top)
}
}
//detects whether the mouse cursor is between x and y relative to the radius specified
function intersects(circle) {
// subtract the x, y coordinates from the mouse position to get coordinates
// for the hotspot location and check against the area of the radius
var areaX = mousePosition.x - circle.x;
var areaY = mousePosition.y - circle.y;
//return true if x^2 + y^2 <= radius squared.
return areaX * areaX + areaY * areaY <= circle.r * circle.r;
}
Array.prototype.move = function (old_index, new_index) {
if (new_index >= this.length) {
var k = new_index - this.length;
while ((k--) + 1) {
this.push(undefined);
}
}
this.splice(new_index, 0, this.splice(old_index, 1)[0]);
};
draw(c, ctx);<canvas id="mycanvas" style="border:1px solid red;" ></canvas>`
这里是小提琴的链接： https://jsfiddle.net/y9fr1q7n/101/
【问题讨论】：