How to implement progress bar for uploading files with Ajax Codular

This time I will show you how Ajax implements the progress bar Codular for uploading files. What are the precautions for Ajax to implement the progress bar Codular for uploading files. Here is a practical case, let's take a look.

Nowadays, people like to do some other things while browsing a web page without leaving the web page, which is usually achieved through ajax. Most of the time, people use jQuery to achieve it, but with the advancement of browsers, People than don't need to do this. Here we will cover how to upload files to the server without leaving the page, we will use the same backend PHP code that we used in our previous article. The The script will upload the file to the server, display the upload progress, and finally return the link address of the uploaded file. In some cases, you may want to return the id of the uploaded file or other application information. Note: This code does not support older The ie browser, through Can I use we only support ie10+

Let's Code

We will start with the HTML structure, then the JavaScript, I will then provide you with the PHP code, which is adapted from the previous tutorial - there won't be much explanation of the PHP code.

HTML

We only need to use two input boxes, one is the file type file, and the other is just a button button, so that we can listen to it being clicked to send File upload request. We'll also have a p that we change the width to highlight the status of the upload.

As shown below:

nbsp;html>


  <meta>
  <title>JS File Upload with Progress</title>
  <style>
  .container {
    width: 500px;
    margin: 0 auto;
  }
  .progress_outer {
    border: 1px solid #000;
  }
  .progress {
    width: 20%;
    background: #DEDEDE;
    height: 20px; 
  }
  </style>


  <p>
    </p><p>
      Select File: <input> <input>
    </p>
    <p>
      </p><p></p>
    
  
  <script></script>

You will see that we wrote a little progress bar style and added a script file at the bottom to process file upload and progress bar display.

JavaScript

First, we need to get the tags we are going to use, they are already tagged with ids.

var _submit = document.getElementById('_submit'), 
_file = document.getElementById('_file'), 
_progress = document.getElementById('_progress');

Next step, give_ submit adds a click event to upload the file we selected. To do this, we will use the addEventListener method and let it call the upload method after clicking the button.

_submit.addEventListener('click', upload);

Now we can continue to process the upload, there are the following steps:

1. Check the selected file

2. Dynamicly create the file data to be sent

3. Through js Create XMLHttpRequest

4. Upload file

Check the selected file

Our file input box _file has a parameter files to query the selected file queue - if you set the multiple parameter, you can select multiple files. We do a simple check and judge. If the length of the array is greater than 0, continue, otherwise return directly.

if(_file.files.length === 0){
  return;
}

Now that we can ensure that a file is selected, we will assume that there is a file, please remember that the index of the array starts with 0.

Dynamicly create the file data to be sent

To do this, we need to use FormData and add the data to it. Next, we can send our FormData in the request generated in step 3. The append method we use, the first parameter is the same as the input The name attribute of the box is similar, the second parameter is the value. Here, we set value to the first file we selected.

var data = new FormData();
data.append('SelectedFile', _file.files[0]);

We will use this when sending data to the server later .

Creating XMLHttpRequest via upload script

This part is very basic, we will create a new XMLHttpRequest and set some settings. First we will modify the value of onreadystatechange to define the callback function when requesting a state change. This method will check readyState when the state changes to ensure that the value is what we want - in In this example, it is 4, which means the request is completed.

In the second step, we will add the progress event on the upload attribute. In this way, we can get the upload progress and use it to update the progress bar.

var request = new XMLHttpRequest();
request.onreadystatechange = function(){
  if(request.readyState == 4){
    try {
      var resp = JSON.parse(request.response);
    } catch (e){
      var resp = {
        status: 'error',
        data: 'Unknown error occurred: [' + request.responseText + ']'
      };
    }
    console.log(resp.status + ': ' + resp.data);
  }
};

When After the request is successful, we use try...catch to wrap the process of parsing the return value. If the parsing fails, we will create our own return object so that the subsequent code will not report an error. You can decide how to handle the return value. Here we just Output it to the console.

Now let's deal with the progress bar:

request.upload.addEventListener('progress', function(e){
  _progress.style.width = Math.ceil(e.loaded/e.total) * 100 + '%';
}, false);

这里有一点点复杂,我们监听一个事件,该事件对象有两个我们比较关注的属性,loaded和total.loaded表示已经上传到服务器端的数值,total表示要发送的总数值,我们可以根据这两个值计算一个百分比,来设置进度条的宽度.

Note: 这里没有加入任何动画特效,但你可以根据需要自定义动画效果.

上传文件

现在我们可以发送请求，我们将通过POST请求到一个名为upload.php的文件，并使用send（）方法，参数为data，这样我们就可以发送数据:

request.open('POST', 'upload.php');
request.send(data);

下面给出完整的JavaScript代码:

var _submit = document.getElementById('_submit'), 
_file = document.getElementById('_file'), 
_progress = document.getElementById('_progress');
var upload = function(){
  if(_file.files.length === 0){
    return;
  }
  var data = new FormData();
  data.append('SelectedFile', _file.files[0]);
  var request = new XMLHttpRequest();
  request.onreadystatechange = function(){
    if(request.readyState == 4){
      try {
        var resp = JSON.parse(request.response);
      } catch (e){
        var resp = {
          status: 'error',
          data: 'Unknown error occurred: [' + request.responseText + ']'
        };
      }
      console.log(resp.status + ': ' + resp.data);
    }
  };
  request.upload.addEventListener('progress', function(e){
    _progress.style.width = Math.ceil(e.loaded/e.total) * 100 + '%';
  }, false);
  request.open('POST', 'upload.php');
  request.send(data);
}
_submit.addEventListener('click', upload);

现在到了PHP...

PHP

这是我们使用的代码,你将注意到一些区别,主要是我们用最上面的JSON方法来返回值都作为JSON格式输出.这个PHP与之前文章中的代码相同,这也就意味着该方法只适用于小于500Kb的PNG图片.此外,成功信息将返回已上传文件的路径:

<?php // Output JSON
function outputJSON($msg, $status = &#39;error&#39;){
  header(&#39;Content-Type: application/json&#39;);
  die(json_encode(array(
    &#39;data&#39; => $msg,
    'status' => $status
  )));
}
// Check for errors
if($_FILES['SelectedFile']['error'] > 0){
  outputJSON('An error ocurred when uploading.');
}
if(!getimagesize($_FILES['SelectedFile']['tmp_name'])){
  outputJSON('Please ensure you are uploading an image.');
}
// Check filetype
if($_FILES['SelectedFile']['type'] != 'image/png'){
  outputJSON('Unsupported filetype uploaded.');
}
// Check filesize
if($_FILES['SelectedFile']['size'] > 500000){
  outputJSON('File uploaded exceeds maximum upload size.');
}
// Check if the file exists
if(file_exists('upload/' . $_FILES['SelectedFile']['name'])){
  outputJSON('File with that name already exists.');
}
// Upload file
if(!move_uploaded_file($_FILES['SelectedFile']['tmp_name'], 'upload/' . $_FILES['SelectedFile']['name'])){
  outputJSON('Error uploading file - check destination is writeable.');
}
// Success!
outputJSON('File uploaded successfully to "' . 'upload/' . $_FILES['SelectedFile']['name'] . '".', 'success');

如果将所有内容都放在一起，您应该可以将文件上传到您期望的位置，并在浏览器的控制台内成功返回。

结束语

还有一些比较容易而又有效的方式来增强用户体验.通过将文件队列的多个文件加入到FormData,可以实现多文件上传. 有一点要说明,如果你是在本地做测试,你可能没办法看到进度条逐步变化,这取决于你本地机器的上传速度,我建议在服务器上使用较大的png文件做测试.

相信看了本文案例你已经掌握了方法，更多精彩请关注php中文网其它相关文章！

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