如何用canvas绘制带有箭头的直线

话不多说，直接上代码~~~

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window.onload = function(){
    var cavParam = {};
    cavParam.canvas = document.getElementById("canvasId");
    cavParam.ctx = cavParam.canvas.getContext('2d');
    drawLineArrow(cavParam, 100, 300, 200, 200, "#a3a3a3");
}
/**
 * 绘制带有箭头的直线
 * @param cavParam canvas画布变量
 * @param fromX/fromY 起点坐标
 * @param toX/toY 终点坐标
 * @param color 线与箭头颜色
 **/
function drawLineArrow(cavParam, fromX, fromY, toX, toY, color) {
    var headlen = 10;//自定义箭头线的长度
    var theta = 45;//自定义箭头线与直线的夹角，个人觉得45°刚刚好
    var arrowX, arrowY;//箭头线终点坐标
    // 计算各角度和对应的箭头终点坐标
    var angle = Math.atan2(fromY - toY, fromX - toX) * 180 / Math.PI;
    var angle1 = (angle + theta) * Math.PI / 180;
    var angle2 = (angle - theta) * Math.PI / 180;
    var topX = headlen * Math.cos(angle1);
    var topY = headlen * Math.sin(angle1);
    var botX = headlen * Math.cos(angle2);
    var botY = headlen * Math.sin(angle2);
    cavParam.ctx.beginPath();
    //画直线
    cavParam.ctx.moveTo(fromX, fromY);
    cavParam.ctx.lineTo(toX, toY);

    arrowX = toX + topX;
    arrowY = toY + topY;
    //画上边箭头线
    cavParam.ctx.moveTo(arrowX, arrowY);
    cavParam.ctx.lineTo(toX, toY);

    arrowX = toX + botX;
    arrowY = toY + botY;
    //画下边箭头线
    cavParam.ctx.lineTo(arrowX, arrowY);
    
    cavParam.ctx.strokeStyle = color;
    cavParam.ctx.stroke();
}