HTML5之FileReader的使用

客官°小女子只卖身不卖艺 2021-09-23 00:20 249阅读 0赞

HTML5定义了FileReader作为文件API的重要成员用于读取文件，根据W3C的定义，FileReader接口提供了读取文件的方法和包含读取结果的事件模型。

FileReader的使用方式非常简单，可以按照如下步骤创建FileReader对象并调用其方法：

1.检测浏览器对FileReader的支持

if(window.FileReader) {
        var fr = new FileReader();
        // add your code here
    }
    else {
        alert("Not supported by your browser!");
    }

2. 调用FileReader对象的方法

FileReader 的实例拥有 4 个方法，其中 3 个用以读取文件，另一个用来中断读取。下面的表格列出了这些方法以及他们的参数和功能，需要注意的是 ，无论读取成功或失败，方法并不会返回读取结果，这一结果存储在 **result**属性中。

<table style="margin:0px 0px 30px; padding:0px; border:0px; font-size:14px; font-family:'Microsoft Yahei',Arial; vertical-align:middle; border-spacing:0px; width:658px; border-collapse:collapse; color:rgb(8,45,63)"> 
 <thead style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
  <tr style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
   <th style="margin:0px; padding:10px 8px; border-width:0px 0px 2px; border-bottom-style:solid; border-bottom-color:rgb(164,198,23); font-weight:normal; font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 方法名</th> 
   <th style="margin:0px; padding:10px 8px; border-width:0px 0px 2px; border-bottom-style:solid; border-bottom-color:rgb(164,198,23); font-weight:normal; font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 参数</th> 
   <th style="margin:0px; padding:10px 8px; border-width:0px 0px 2px; border-bottom-style:solid; border-bottom-color:rgb(164,198,23); font-weight:normal; font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 描述</th> 
  </tr> 
 </thead> 
 <tbody style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
  <tr style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> abort</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> none</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 中断读取</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> readAsBinaryString</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> file</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 将文件读取为二进制码</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> readAsDataURL</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> file</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 将文件读取为 DataURL</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> readAsText</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> file, [encoding]</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 将文件读取为文本</td> 
  </tr> 
 </tbody> 
</table>

**readAsText：**该方法有两个参数，其中第二个参数是文本的编码方式，默认值为 UTF-8。这个方法非常容易理解，将文件以文本方式读取，读取的结果即是这个文本文件中的内容。  
**readAsBinaryString：**该方法将文件读取为二进制字符串，通常我们将它传送到后端，后端可以通过这段字符串存储文件。  
**readAsDataURL：**这是例子程序中用到的方法，该方法将文件读取为一段以 data: 开头的字符串，这段字符串的实质就是 Data URL，Data URL是一种将小文件直接嵌入文档的方案。这里的小文件通常是指图像与 html 等格式的文件。

3. 处理事件

FileReader 包含了一套完整的事件模型，用于捕获读取文件时的状态，下面这个表格归纳了这些事件。

<table style="margin:0px 0px 30px; padding:0px; border:0px; font-size:14px; font-family:'Microsoft Yahei',Arial; vertical-align:middle; border-spacing:0px; width:658px; border-collapse:collapse; color:rgb(8,45,63)"> 
 <thead style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
  <tr style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
   <th style="margin:0px; padding:10px 8px; border-width:0px 0px 2px; border-bottom-style:solid; border-bottom-color:rgb(164,198,23); font-weight:normal; font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; width:120px; color:rgb(122,142,18)"> 事件</th> 
   <th style="margin:0px; padding:10px 8px; border-width:0px 0px 2px; border-bottom-style:solid; border-bottom-color:rgb(164,198,23); font-weight:normal; font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 描述</th> 
  </tr> 
 </thead> 
 <tbody style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
  <tr style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> onabort</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 中断时触发</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> onerror</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 出错时触发</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> onload</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 文件读取成功完成时触发</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> onloadend</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 读取完成触发，无论成功或失败</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> onloadstart</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 读取开始时触发</td> 
  </tr> 
  <tr style="margin:0px; padding:0px; border:0px; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> onprogress</td> 
   <td style="margin:0px; padding:6px 8px; border-width:0px 0px 1px; border-bottom-style:solid; border-bottom-color:rgb(204,221,170); font-style:inherit; font-size:14px; font-family:inherit; vertical-align:middle; color:rgb(122,142,18)"> 读取中</td> 
  </tr> 
 </tbody> 
</table>

文件一旦开始读取，无论成功或失败，实例的 result 属性都会被填充。如果读取失败，则 result 的值为 null ，否则即是读取的结果，绝大多数的程序都会在成功读取文件的时候，抓取这个值。

fr.onload = function() {
        this.result;
    };

下面通过一个上传图片预览和带进度条上传来展示FileReader的使用。

如果要限定上传文件的类型，可以通过文件选择器获取文件对象并通过type属性来检查文件类型

if(!/image\/\w+/.test(file.type)){
        alert("请确保文件为图像类型");
        return false;
    }

不难发现这个检测是基于正则表达式的，因此可以进行各种复杂的匹配，非常有用。

如果要增加一个进度条，可以使用HTML 5的progress标签，通过下面的代码实现。

<form>
        <fieldset>
            <legend>分度读取文件：</legend>
            <input type="file" id="File" />
            <input type="button" value="中断" id="Abort" />
            <p>
                <label>读取进度：</label><progress id="Progress" value="0" max="100"></progress>
            </p>
            <p id="Status"></p>
        </fieldset>
    </form>