淡入淡出 javascript

Question

我是 JavaScript 的新手，我正在尝试编写一个代码，用我熟悉的命令淡入图片。我在这里看到了几个例子，但它们没有用。这是我尝试做的：

 function myFunction() {
   for (i = 1; i < 20; i++) {
     setTimeout(function() {
       o = 1 - 1 / i
     }, 200); //this should increase the opacity
     document.getElementById("dog").style.opacity = o
   }
 }

img {
  opacity: 0;
  filter: alpha(opacity=40);
}

<center>
  <img id="dog" src="dog.jpg" draggable="true" ondragstart="drag(event)" width="500" height="500">
</center>

<button onclick="myFunction()">Lets Rock</button>

当我运行它时，它不会淡入。它从空白屏幕开始（它应该如此），但不是在我单击按钮后淡入，而是弹出 (不淡入）点击几下后。

非常感谢您给予的帮助
爱丽儿

Answer 1

您可能会发现使用 CSS 更容易控制不透明度。它对浏览器的工作较少，只需要很少的 javascript 来切换 class.

var img = document.getElementById("dog"),
    btn = document.getElementById("button");

btn.addEventListener("click", function( event ) {
    img.className = (img.className==="in")?"out":"in";
}, false);

#dog {
  opacity: 0;
  -webkit-transition: opacity 2s ease;
  -moz-transition: opacity 2s ease;
  -ms-transition: opacity 2s ease;
  -o-transition: opacity 2s ease;
  transition: opacity 2s ease;
}
#dog.in {
    opacity: 1;
}

<button id="button">Lets Rock</button><br />
<img id="dog" src="http://lorempixel.com/500/500/cats/" class="off" draggable="true" ondragstart="drag(event)" width="500" height="500">

Answer 2

试试这个：http://jsfiddle.net/kishoresahas/4wg8zcsg/

function fadeIn(el) {
    el.style.opacity = 0;
    var tick = function () {
        el.style.opacity = +el.style.opacity + 0.01;
        if (+el.style.opacity < 1) {
            (window.requestAnimationFrame && requestAnimationFrame(tick)) || setTimeout(tick, 16)
        }
    };
    tick();
}

function myFunction() {
    var el = document.getElementById("dog");
  console.log(el);
    fadeIn(el);
}

img {
    opacity: 0;
    filter: alpha(opacity=40);
}

<button onclick="myFunction()">Lets Rock</button>

<center>
    <img id="dog" src="https://i.stack.imgur.com/NHlV8.jpg" draggable="true" ondragstart="drag(event)" width="500" height="500">
</center>

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