javascript - 事件处理程序没有响应

Question

我从 JavaScript 和 D3 开始，尝试编写一个简单的代码，允许用户在 Canvas 上绘制曲线。第一步是能够检测鼠标按钮何时按下，这样如果鼠标在按钮按下时移动，数据就会被记录。

我尝试将“.on”更改为“.addEventListener”，但结果相同。另外，我正在使用 Firefox。

当阅读下面的代码时，我知道每次移动鼠标时让标志 clickState 递增没有什么意义；我这样做只是为了调试目的，看看函数 mousemovefx 是否在鼠标移动时被调用。

当我打开此网页时，输出 div 中的文本立即更改为“向下”，并且我所做的任何操作(例如在 Canvas 中单击和移动)都不会更改它。其他函数似乎也没有被执行。

我既不明白(a)为什么在这些事件上没有调用处理程序，也不明白(b)为什么当我不仅没有点击而且我的鼠标甚至不在 Canvas 上时立即调用 mousedown 处理程序.

    document.body.style.backgroundColor = "black";

    canvw = 500;
    canvh = 500;
    border = 6;

    clickState = 0;

    var canvas = d3.select("body")
                    .append("svg")      //Add an SVG to the body
                    .attr("width",canvw+2*border)
                    .attr("height",canvh+2*border)

    var canvbg = canvas.append("rect")
                    .attr("x",border)
                    .attr("y",border)
                    .attr("width",canvw)
                    .attr("height",canvh)
                    .attr("fill","white")
                    .attr("stroke","rgb(200,200,200)")
                    .attr("stroke-width",5)

    var mydiv = d3.select("body")
                    .append("div")
                    .attr("id","outputdiv")
                    .style("color","orange")

    canvas.on("mousedown", mousedownfx());

    canvas.on("mouseup", mouseupfx());

    canvas.on("mousemove",mousemovefx());

    canvas.on("mouseout",mouseoutfx());

    function mousedownfx() {
        clickState = 1;
        mydiv.text("Down")
    }

    function mouseupfx() {
        clickState = 0;
        mydiv.text("Up")
    }

    function mousemovefx() {
        mydiv.text(clickState)
        clickState = clickState + 1;
    }

    function mouseoutfx() {
        clickstate = 0;
    }

Answer 1

document.body.style.backgroundColor = "black";

    canvw = 500;
    canvh = 500;
    border = 6;

    clickState = 0;

    var canvas = d3.select("body")
                    .append("svg")      //Add an SVG to the body
                    .attr("width",canvw+2*border)
                    .attr("height",canvh+2*border)

    var canvbg = canvas.append("rect")
                    .attr("x",border)
                    .attr("y",border)
                    .attr("width",canvw)
                    .attr("height",canvh)
                    .attr("fill","white")
                    .attr("stroke","rgb(200,200,200)")
                    .attr("stroke-width",5)

    var mydiv = d3.select("body")
                    .append("div")
                    .attr("id","outputdiv")
                    .style("color","orange")

    canvas.on("mousedown", mousedownfx);

    canvas.on("mouseup", mouseupfx);

    canvas.on("mousemove",mousemovefx);

    canvas.on("mouseout",mouseoutfx);

    function mousedownfx() {
        clickState = 1;
        mydiv.text("Down")
    }

    function mouseupfx() {
        clickState = 0;
        mydiv.text("Up")
    }

    function mousemovefx() {
        mydiv.text(clickState)
        clickState = clickState + 1;
    }

    function mouseoutfx() {
        clickstate = 0;
    }

<script src="https://d3js.org/d3.v5.min.js"></script>

当您设置监听器时，您正在调用函数更改:

canvas.on("mousedown", mousedownfx());
canvas.on("mouseup", mouseupfx());
canvas.on("mousemove",mousemovefx());
canvas.on("mouseout",mouseoutfx());

致:

canvas.on("mousedown", mousedownfx);
canvas.on("mouseup", mouseupfx);
canvas.on("mousemove",mousemovefx);
canvas.on("mouseout",mouseoutfx);

这将传递该函数，然后单击按钮时将调用该函数。