我想在点击开始按钮时,小球开始移动 ,但为什么一加载完成小球就开始移动了呢??onclick事件处理程序在那呢 怎么没用???求教
<canvas id="draw" width="500px" height="200px"></canvas>
<button id ="start">开始</button> var start = document.getElementById("start");
var draw = document.getElementById("draw");
var context = draw.getContext("2d");
var y = 50;
var i = 1;
function ball() {
context.clearRect(0,0,500,200);
context.fillStyle = 'red';
context.beginPath();
context.arc(100,y,20,0,Math.PI*2,false);
context.fill();
y = y + i;
if(y==20||y==180) {
i=i*-1;
}
}
start.onclick = setInterval(ball,5);
<canvas id="draw" width="500px" height="200px"></canvas>
<button id ="start">开始</button> var start = document.getElementById("start");
var draw = document.getElementById("draw");
var context = draw.getContext("2d");
var y = 50;
var i = 1;
function ball() {
context.clearRect(0,0,500,200);
context.fillStyle = 'red';
context.beginPath();
context.arc(100,y,20,0,Math.PI*2,false);
context.fill();
y = y + i;
if(y==20||y==180) {
i=i*-1;
}
}
start.onclick = setInterval(ball,5);
改成这样试试
这句 start.onclick = setInterval(ball,5);
setInterval(ball,5)这是调用函数的写法,也就是说此时函数已经执行了,onclick成员获得了该函数的返回值(该定时器对象)
明白了么,执行到这一句时,定时器已经产生了,被你赋值给onclick成员了
当然楼上的写法是不合理的,每次点击都生成一个新的定时器,很不好start.onclick = function(){setInterval(ball,5);start.onclick=null;}或者
var timer;
start.onclick = function(){if(!timer) timer=setInterval(ball,5);}