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Detailed introduction to HTML5 FileReader interface

黄舟Original: 2017-04-13 10:31:021529browse

1. FileReader Overview

FileReader object allows a web application to asynchronously read the contents of a file (or raw data buffer) stored on the user's computer, using File or Blob object specifies the file or data to be read.

The File object can be a FileList object returned after the user selects a file on an d5fd7aea971a85678ba271703566ebfd element, or it can be generated by a drag-and-drop operation. The DataTransfer object can also be the result returned after executing the mozGetAsFile() method on a HTMLCanvasElement.

2. FileReader interface method

Method name	Parameters	Description
##readAsBinaryString	file	Read the file as binary code
readAsText	file,[encoding]	Read file as text
readAsDataURL	file	Read file as DataURL
readAsArrayBuffer	file	Read the file as an ArrayBuffer object
abort	(none)	Abort read operation

3. FileReader interface

Event

##onabortTriggered when data reading is interruptedonerrorTriggered when data reading error occurs onloadstartTriggered when data reading startsonprogress Data readingonloadTriggered when data reading is successfully completedonloadendTriggered when data reading is completed, regardless of success or failure

event	Description

4. Usage examples

<!DOCTYPE html>
<html lang="zh-cn">
<head>
    <meta http-equiv="content-type" content="text/html; charset=utf-8">
    <title>Html5Test</title>
</head>
<body>
    <article>
        <header></header>
        <section>
            <p>
                <lable>请选择一个文件：</lable>
                <input type="file" id="file_reader">
                <input type="button" value="读取图像" onclick="readAsDataUrl();">
                <input type="button" value="读取二进制数据" onclick="readAsBinaryString();">
                <input type="button" value="读取文本文件" onclick="readAsText();">
            </p>
            <p id="file_reader_result" name="file_reader_result">
                <!-- 这里用来显示读取结果 -->
            </p>
            <script>
            var file_reader_result = document.getElementById("file_reader_result");
            // 检测浏览器是否支持FileReader
            if (typeof FileReader == "undefined") {
                file_reader_result.innerHTML = "您的浏览器不支持FileReader";
                file.setAttribute(&#39;disabled&#39;, &#39;disabled&#39;);
            }
            // 将文件以Data Url形式读入页面
            function readAsDataUrl(){
                // 检查是否为图像文件
                var f = document.getElementById("file_reader").files[0];
                if (!/image\/\w+/.test(f.type)) {
                    file_reader_result.innerHTML = "请上传图片文件！";
                    return false;
                };
                var reader = new FileReader();
                // 将文件以Data Url形式读入页面
                reader.readAsDataURL(f);
                reader.onload = function(e){
                    file_reader_result.innerHTML = &#39;<img src="&#39; + this.result + &#39;" alt="图片" />&#39;;
                }
            }
            // 将文件以二进制形式读入页面
            function readAsBinaryString(){
                // 检查是否为图像文件
                var f = document.getElementById("file_reader").files[0];
                if (!/image\/\w+/.test(f.type)) {
                    file_reader_result.innerHTML = "请上传图片文件！";
                    return false;
                };
                var reader = new FileReader();
                // 将文件以二进制形式读入页面
                reader.readAsBinaryString(f);
                reader.onload = function(e){
                    file_reader_result.innerHTML = this.result;
                }
            }
            // 将文件以文本形式读入页面 目前测试仅支持txt文件
            function readAsText(){
                // 检查是否是文本文件
                var f = document.getElementById("file_reader").files[0];
                if (!/text\/\w+/.test(f.type)) {
                    file_reader_result.innerHTML = "请上传文本文件！";
                    return false;
                };
                var reader = new FileReader();
                // 将文件以文本形式读入页面
                reader.readAsText(f);
                reader.onload = function(e){
                    file_reader_result.innerHTML = this.result;
                }
            }
            </script>
        </section>
    </article>
</body>
</html>

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