overflow与underflow

是新近的firefox浏览器中支持overflow, underflow这两个事件,当某一元素的大小超出父元素的显示范围就会触发overflow事件,如果从超出显示再变回不超出的状态则触发underflow事件.

<!DOCTYPE HTML>
<html>
    <head>
        <meta charset="UTF-8">
        <title>测试用例 by 司徒正美</title>
    </head>
    <body >
        <div id="wrapper">
            <div id="child"></div>
        </div>
        <br/>
        <label><input type="checkbox" id="toggle" checked/> Overflow</label>
 
        <style>
            #wrapper {
                width: 300px;
                height: 300px;
                background: blue;
                overflow: hidden;
            }
 
            #child {
                width: 200px;
                height: 200px;
             
                background: yellow;
            }
        </style>
 
        <script>
            var wrapper = document.getElementById("wrapper"),
                    child = document.getElementById("child"),
                    toggle = document.getElementById("toggle");
 
            wrapper.addEventListener("overflow", function(event) {
                console.log(event);
            }, false);
 
            wrapper.addEventListener("underflow", function(event) {
                console.log(event);
            }, false);
 
 
 
            toggle.addEventListener("change", function(event) {
                if (event.target.checked) {
                    child.style.width = "400px";
                    child.style.height = "400px";
                } else {
                    child.style.width = "200px";
                    child.style.height = "200px";
                }
 
            }, false);
        </script>
    </body>
</html>

<!DOCTYPE HTML>
<html>
    <head>
        <meta charset="UTF-8">
        <title>测试用例 by 司徒正美</title>
    </head>
    <body >
        <div id="wrapper">
            <div id="child"></div>
        </div>
        <br/>
        <label><input type="checkbox" id="toggle" checked/> Overflow</label>

#child {
                width: 200px;
                height: 200px;
            
                background: yellow;
            }
        </style>

wrapper.addEventListener("overflow", function(event) {
                console.log(event);
            }, false);

wrapper.addEventListener("underflow", function(event) {
                console.log(event);
            }, false);

toggle.addEventListener("change", function(event) {
                if (event.target.checked) {
                    child.style.width = "400px";
                    child.style.height = "400px";
                } else {
                    child.style.width = "200px";
                    child.style.height = "200px";
                }

}, false);
        </script>
    </body>
</html>

如果是webkit系统的浏览器,则用overflowchanged这个事件代替

#child {
                width: 200px;
                height: 200px;
            
                background: yellow;
            }
        </style>

wrapper.addEventListener("overflowchanged", function(event) {
                var overflow = event.verticalOverflow || event.horizontalOverflow
                var type = overflow ? "overflow" : "underflow"
                delete event.type
                event.type = type
                console.log(event)
            }, false);

}, false);
        </script>
    </body>
</html>

对于不支持的浏览器,那只能轮询判定是否存在滚动条了,可以看这里