jquery - Canvas 在鼠标事件上获取点

Question

我有以下函数来获取鼠标单击位置(坐标)。

$('#myCanvas').on('click', function(e) {
    event = e;
    event = event || window.event;

    var canvas = document.getElementById('myCanvas'),
            x = event.pageX - canvas.offsetLeft,
            y = event.pageY - canvas.offsetTop;
    alert(x + ' ' + y);
});

我需要在单击某个位置时获取鼠标点，并在拖动相同位置后获取第二个鼠标点位置。

即，mousedown 点和 mouseup 点。

Answer 1

尝试一些不同的设置:

var canvas = myCanvas;  //store canvas outside event loop
var isDown = false;     //flag we use to keep track
var x1, y1, x2, y2;     //to store the coords

// when mouse button is clicked and held    
$('#myCanvas').on('mousedown', function(e){
    if (isDown === false) {

        isDown = true;

        var pos = getMousePos(canvas, e);
        x1 = pos.x;
        y1 = pos.y;
    }
});

// when mouse button is released (note: window, not canvas here)
$(window).on('mouseup', function(e){

    if (isDown === true) {

        var pos = getMousePos(canvas, e);
        x2 = pos.x;
        y2 = pos.y;

        isDown = false;

        //we got two sets of coords, process them
        alert(x1 + ',' + y1 + ',' +x2 + ',' +y2);
    }
});

// get mouse pos relative to canvas (yours is fine, this is just different)
function getMousePos(canvas, evt) {
    var rect = canvas.getBoundingClientRect();
    return {
        x: evt.clientX - rect.left,
        y: evt.clientY - rect.top
    };
}

那么我们为什么要监听窗口上的鼠标松开呢？如果您将鼠标悬停在 canvas 之外，然后释放鼠标按钮，则该事件不会在 canvas 中注册。因此我们需要监听一个全局事件，例如window。

由于我们已经在鼠标按下事件中标记了 isDown ，因此我们知道接下来的鼠标松开“属于” Canvas (当我们检查 isDown 标志时)。