canvas addeventlistener不会停止处理

Question

按钮事件触发下面的processLine JS函数。当第一个警报关闭时，该过程将立即跳至起始行功能。当关闭起始行功能时，该过程立即跳转到结束行功能。当此警报关闭时，将执行context.rectangle。如何使它在第一个事件侦听器处停止，然后等到我将鼠标悬停在画布上才能继续执行startline函数>

function processLine(evt)  {
    alert("PROCESS "+canvas+"  "+evt)
context.fillStyle = "#FF0000";
context.fillRect(0, 0, 150, 75);
    toolSelection = "line";
    canvas.addEventListener("mousedown",startLine(event));
}

function getMousePos(canvas, evt) {
    var rect = canvas.getBoundingClientRect();
    return {
      x: evt.clientX - rect.left,
      y: evt.clientY - rect.top
    };
}

function startLine(evt)  {
    alert("START "+evt.currentTarget)
    var pos = getMousePos(canvas, evt);
    var posx = pos.x;
    var posy = pos.y;               
    canvas.onmouseup = endLine(event);
}

function endLine(evt)  {
    alert("END")
    var pos = getMousePos(canvas, evt);
    var posx = pos.x;
    var posy = pos.y;   
}