记得以前做网站时，曾经需要实现一个图片上传到服务器前，先预览的功能。当时用html的<input type="file"/>标签一直实现不了，最后舍弃了这个标签，使用了其他方式来实现了这个功能。
今天无意发现了一个知识点，用html的file标签就能实现图片上传前预览，感觉很棒，记录一下！就是通过file标签和js的FileReader接口，把选择的图片文件调用readAsDataURL方法，把图片数据转成base64字符串形式显示在页面上。
1、闲话少说，测试一下，图片上传前预览(选择图片)：
实现代码：
<div style="border:2px dashed red;">
<p>
图片上传前预览：<input type="file" id="xdaTanFileImg" onchange="xmTanUploadImg(this)" accept="image/*"/>
<input type="button" value="隐藏图片" onclick="document.getElementById('xmTanImg').style.display = 'none';"/>
<input type="button" value="显示图片" onclick="document.getElementById('xmTanImg').style.display = 'block';"/>
</p>
<img id="xmTanImg"/>
<div id="xmTanDiv"></div>
</div>
<hr />
<script type="text/javascript">
//判断浏览器是否支持FileReader接口
if (typeof FileReader == 'undefined') {
document.getElementById("xmTanDiv").InnerHTML = "<h1>当前浏览器不支持FileReader接口</h1>";
//使选择控件不可操作
document.getElementById("xdaTanFileImg").setAttribute("disabled", "disabled");
} //选择图片，马上预览
function xmTanUploadImg(obj) {
var file = obj.files[0]; console.log(obj);console.log(file);
console.log("file.size = " + file.size); //file.size 单位为byte var reader = new FileReader(); //读取文件过程方法
reader.onloadstart = function (e) {
console.log("开始读取....");
}
reader.onprogress = function (e) {
console.log("正在读取中....");
}
reader.onabort = function (e) {
console.log("中断读取....");
}
reader.onerror = function (e) {
console.log("读取异常....");
}
reader.onload = function (e) {
console.log("成功读取...."); var img = document.getElementById("xmTanImg");
img.src = e.target.result;
//或者 img.src = this.result; //e.target == this
} reader.readAsDataURL(file)
}
</script>
------------------------------- end -----------------------------
2、另外 FileReader除了有函数readAsDataURL，另外还有另外两个函数readAsBinaryString 和 readAsText，分别可以将选择的文件读取成二进制和文本格式
测试一下，选择文本(txt、cs、html、js、css、xml)，读取成二进制或者文本：
选择文件：
using System; using System.Collections.Generic; using System.IO; using System.Linq; using System.Security.Cryptography; using System.Text; using System.Web; /// /// å
å«MD5å å¯ï¼DESå å¯è§£å¯ /// public class Secret { //DES8ä½ç§é¥ private const string desKey = "xlkxlk33"; #region DESå å¯ è§£å¯ /// /// DESå å¯å符串 /// ///å¾
å å¯çå符串 ///å å¯å¯é¥,è¦æ±ä¸º8ä½ ///å å¯æåè¿åå å¯åçå符串ï¼å¤±è´¥è¿åæºä¸² public static string EncryptDES(string encryptString) { try { byte[] rgbKey = Encoding.UTF8.GetBytes(desKey); byte[] rgbIV = { 0x12, 0x34, 0x56, 0x78, 0x90, 0xAB, 0xCD, 0xEF }; byte[] inputByteArray = Encoding.UTF8.GetBytes(encryptString); DESCryptoServiceProvider dCSP = new DESCryptoServiceProvider(); MemoryStream mStream = new MemoryStream(); CryptoStream cStream = new CryptoStream(mStream, dCSP.CreateEncryptor(rgbKey, rgbIV), CryptoStreamMode.Write); cStream.Write(inputByteArray, 0, inputByteArray.Length); cStream.FlushFinalBlock(); return Convert.ToBase64String(mStream.ToArray()); } catch { return encryptString; } } /// /// DES解å¯å符串 /// ///å¾
解å¯çå符串 ///解å¯å¯é¥ï¼è¦æ±8ä½ /// public static string DecryptDES(string decryptString) { try { //é»è®¤å¯é¥åé byte[] Keys = { 0x12, 0x34, 0x56, 0x78, 0x90, 0xAB, 0xCD, 0xEF }; byte[] rgbKey = Encoding.UTF8.GetBytes(desKey); byte[] rgbIV = Keys; byte[] inputByteArray = Convert.FromBase64String(decryptString); DESCryptoServiceProvider DCSP = new DESCryptoServiceProvider(); MemoryStream mStream = new MemoryStream(); CryptoStream cStream = new CryptoStream(mStream, DCSP.CreateDecryptor(rgbKey, rgbIV), CryptoStreamMode.Write); cStream.Write(inputByteArray, 0, inputByteArray.Length); cStream.FlushFinalBlock(); return Encoding.UTF8.GetString(mStream.ToArray()); } catch { return decryptString; } } #endregion #region MD5å å¯ /// /// MD5å å¯ /// ////// public static string MD5(string s) { var md5 = new System.Security.Cryptography.MD5CryptoServiceProvider(); var result = ""; if (!string.IsNullOrWhiteSpace(s)) { result = BitConverter.ToString(md5.ComputeHash(UnicodeEncoding.UTF8.GetBytes(s.Trim()))); } return result; } #endregion }
实现代码：
<script type="text/javascript">
//判断浏览器是否支持FileReader接口
if (typeof FileReader == 'undefined') {
document.getElementById("xmTanContentDiv").InnerHTML = "<p>当前浏览器不支持FileReader接口！</p>";
document.getElementById("xmTanFile").setAttribute("disabled", "disabled");
} //选择文件
function xmTanUploadFile(obj){
if (obj.files.length < 1) return; var file = obj.files[0]; if (file.size > 1024 * 1024) {
alert("文件大于1M， 太大了，小点吧！");
obj.value = "";
return;
}
} //读取文件为二进制
function readAsBinaryString() {
var obj = document.getElementById("xmTanFile");
if (obj.files.length < 1) return; var file = obj.files[0];
var reader = new FileReader(); //将文件以二进制形式读入页面
reader.readAsBinaryString(file);
reader.onload = function (f) {
document.getElementById("xmTanContentDiv").innerHTML = this.result;
}
} //读取文件为文本
function readAsText() {
var obj = document.getElementById("xmTanFile");
if (obj.files.length < 1) return; var file = obj.files[0];
var reader = new FileReader(); //将文件以文本形式读入页面
reader.readAsText(file);
reader.onload = function (f) {
document.getElementById("xmTanContentDiv").innerHTML = this.result;
}
}
</script>
<div style="border: 2px dashed red; padding: 20px 0px;">
<label>选择文件：</label>
<input type="file" id="xmTanFile" accept=".html,.js,.css,.txt,.cs,.xml" onchange="xmTanUploadFile(this)"/>
<input type="button" value="读取成二进制数据" onclick="readAsBinaryString()" />
<input type="button" value="读取成文本数据" onclick="readAsText()" />
<input type="button" value="隐藏读取内容" onclick="document.getElementById('xmTanContentDiv').style.display = 'none';"/>
<input type="button" value="显示读取内容" onclick="document.getElementById('xmTanContentDiv').style.display = 'block';"/>
<div id="xmTanContentDiv"></div>
</div>
---------------------------
3、----------- a标签之download属性 -------------
设置a标签href为图片链接，再设置download属性，点此链接可以直接下载图片
点此下载
实现代码：
<div style="text-align:center; padding: 5px 20px;width: 70%;">
<img id="xmTanShowImg" src=""/>
<h1><a href="javascript:void()" download="girl.jpg" id="xmTanDownload">点此下载</a></h1>
</div>
<script type="text/javascript">
//图片转成base64位字符串数据
var imgData = "data:image/png;base64,.........";
//或直接设置图片链接： var imgData = "images/picture.png"; document.getElementById("xmTanShowImg").setAttribute("src", imgData); //给图片标签设置src
document.getElementById("xmTanDownload").setAttribute("href", imgData); //给a标签设置href
</script>