纯JS实作照片上传、下载与预览

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完整code可以到以下gist

Client端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>
        img {
            width: 150px;
          }
    </style>
</head>
<body>
    <input type="file" id="file-uploader" accept="image/png" multiple="multiple"/>
    <img id='preview'> 
</body>
</html>

input tag
- type设成file，让使用者能选择档案
- accept设定使用者能选的格式，如果要所有照片格式就改成image/*
- multiple让使用者能选择多个档案

Client端JS

我们会用JSON来传递资料(也可以用form-data)

取得使用者选择的照片

读进来的每个档案会是blob（Binary Large Object）

const fileUploader = document.querySelector('#file-uploader');
fileUploader.addEventListener('change', (e) => {
   console.log(e.target.files); // get list of file objects
});

e.target.files是fileList

将照片转成base64 string

function display_img(curFiles) {
    const curFile = curFiles[0];
    const reader = new FileReader();
    // 这会在readAS後才执行
    reader.onload = function (e) {
        // console.log('file:', e.target.result); // base64
        document.querySelector('#preview').src = e.target.result;
    };

    // to data url
    reader.readAsDataURL(curFile);
}

img.src能显示url with base64

将blob转成array buffer

相较於buffer, blob保留了照片的资讯，像是档名、MIME type等

function blob2buffer(blob) {
    return new Promise((resolve, reject) => {
        var arrayBuffer;
        var fileReader = new FileReader();
        fileReader.onload = function (event) {
            arrayBuffer = event.target.result;
            resolve(arrayBuffer);
        };

        fileReader.readAsArrayBuffer(blob);
        return arrayBuffer;
    });
}

先将blob转成buffer，方便後续建成array

将array转成字串post到server

buffer不能直接操作，要先转成指定type的array

arrayBuffer = await blob2buffer(pic);

fetch(url, {
    method: 'POST',
    // flask need to set header of json
    headers: {
        'content-type': 'application/json'
    },
    body: JSON.stringify({
        item_id: 1,
        format: 'png',
        img: Array.from(new Uint8Array(arrayBuffer)),
    }),
})

转成array前要先将buffer转成typed array

读取server端传回来的array

产生blob的url (不同於前面产base64的fileReader)

function display_arr_img(arr) {
    document.querySelector('#preview').src = URL.createObjectURL(new Blob([new Uint8Array(arr)], {type: "image/png"}));
}

将array转回typed array，再将其变回blob

Server端API

这里我们用python + flask作为例子

DataBase

这里我们用MySQL，然後照片的栏位type是blob，要注意blob有分可以存的size

TINYBLOB: maximum length of 255 bytes
BLOB: maximum length of 65,535 bytes => 64kb
MEDIUMBLOB: maximum length of 16,777,215 bytes
LONGBLOB: maximum length of 4,294,967,295 bytes

连接到资料库

import pymysql

connection = pymysql.connect(host='localhost', user='root', password='password', db='db01', charset='', cursorclass=pymysql.cursors.DictCursor)

注意pymysql不支援connection string

Python Code

所需套件在requirements.txt内

存到blob的好处是不会让照片被额外index (会导致档案大小增幅30%)

@app.route('/upload/json/img_raw', methods=['POST'])
def raw_img():
    json_data = flask.request.json

    # save to db
    with connection.cursor() as cursor:
        sql = "UPDATE table1 SET `img_raw`=%s WHERE id=%s"
        try:
            # img现在是list, 可以用json string/ byterarray/ base64存进资料库
            cursor.execute(sql, (json.dumps(json_data['img']), json_data['item_id']))
            connection.commit()
        except Exception as err:
            connection.rollback()
            return flask.jsonify({'err': err})

        # get a picture
        sql = "SELECT * FROM table1"
        cursor.execute(sql)
        result = cursor.fetchone()
        r = result[0]['img_raw']
        r = json.loads(r.decode())  # utf-8, 也可用ascii
        
    return flask.jsonify({'img': r})

存进mysql blob栏位後，会变成binary object