5-WebAPI/code/91-动画返回顶部效果.html

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
        header,
        nav,
        main,
        footer,
        .banner {
            width: 80%;
            margin: 0 auto;
        }
        
        header {
            height: 200px;
            background-color: pink;
        }
        
        nav {
            height: 100px;
            background-color: #ccc;
        }
        
        .banner {
            height: 300px;
            background-color: rgb(185, 240, 185);
        }
        
        main {
            height: 1200px;
            background-color: #8cc9fc;
        }
        
        footer {
            height: 300px;
            background-color: #999;
        }
        
        .right {
            width: 50px;
            height: 120px;
            position: absolute;
            right: 50px;
            top: 500px;
            background: pink;
        }
        
        .backTop {
            display: none;
        }
    </style>
    <script>
        window.addEventListener("load", function() {
            var right = document.querySelector(".right");
            var banner_top = document.querySelector(".banner").offsetTop;
            var slider_top = right.offsetTop - banner_top;
            // 回到头部按钮
            var backTop = document.querySelector(".backTop");
            window.addEventListener("scroll", function() {
                // 页面被卷去的头部:window.pageYOffset
                if (window.pageYOffset >= banner_top) {
                    right.style.position = "fixed";
                    right.style.top = slider_top + "px";
                    // console.log(window.pageYOffset);
                } else {
                    right.style.position = "absolute";
                    right.style.top = "500px";
                }
                if (window.pageYOffset >= 930) {
                    backTop.style.display = "block";
                } else {
                    backTop.style.display = "none";
                }
            });

            // -------- 1. 回到顶部点击事件
            backTop.addEventListener("click", function() {
                animate(window, 0);
            });

            function animate(obj, target, callback) {
                // console.log(callback);  callback = function() {}  调用的时候 callback()

                // 先清除以前的定时器，只保留当前的一个定时器执行
                clearInterval(obj.timer);
                obj.timer = setInterval(function() {
                    // 步长值写到定时器的里面
                    // 把我们步长值改为整数 不要出现小数的问题
                    // var step = Math.ceil((target - obj.offsetLeft) / 10);
                    var step = (target - window.pageYOffset) / 10;
                    step = step > 0 ? Math.ceil(step) : Math.floor(step);
                    if (window.pageYOffset == target) {
                        // 停止动画 本质是停止定时器
                        clearInterval(obj.timer);
                        // 回调函数写到定时器结束里面
                        // if (callback) {
                        //     // 调用函数
                        //     callback();
                        // }
                        callback && callback();
                    }
                    // 把每次加1 这个步长值改为一个慢慢变小的值  步长公式：(目标值 - 现在的位置) / 10
                    // window.scroll就是窗口滚动方法
                    window.scroll(0, window.pageYOffset + step);
                }, 15);
            }
        });
    </script>
</head>

<body>
    <header>头部</header>
    <nav>导航栏</nav>
    <div class="banner">banner</div>
    <main>主体部分</main>
    <footer>底部部分</footer>
    <div class="right">
        <button class="backTop">回到顶部</button>
    </div>
</body>

</html>