简介

HTML5通过File API提供了一套读取文件的标准方法.

选择文件

首先我们通过以下代码检测你的浏览器是否支持File API:

if (window.File && window.FileReader && window.FileList && window.Blob){
//你的浏览完全支持这些API
}else{
//你的浏览器不完全支持这些API
}

通过input方式上传文件

<input id="files" type="file" name="files&#91;&#93;" />
<output id="list"></output>
<script type="text/javascript">
function handlerFileSelect(e){
  var files = e.target.files;//从event中获取FileList
  var output = [];
  for (var c = 0, f; f = files[c]; ++c){
    output.push("<li><b>", f.name, "</b> (", f.type || "n/a", ")-", f.size, " bytes, last modified: ", f.lastModifiedDate.toLocaleDateString(), "</li>");
  }
  document.getElementByID("list").innerHTML = "<ul>" + output.join("") + "</ul>";
}
document.getElementByID("files").addEventListener("change", handlerFileSelect, false);
</script>

示例: 现在我们就来尝试一下.

通过拖放功能上传文件

这个是html5新增的一个功能, 可以让你直接从桌面拖文件到指定区域.

<div id="drop_zone">将文件拖放到此处</div>
<output id="list"></output>

<script>
function handleFileSelect(e){
  e.stopPropagation();
  e.preventDefault();

  var files = e.dataTransfer.files;
  var output = [];
  for (var c =0, f; f = files[c]; ++c){
    output.push("<li><b>", f.name, "</b> (", f.type || "n/a", ")-", f.size, " bytes, last modified: ", f.lastModifiedDate.toLocaleDateString(), "</li>");
  }
  document.getElementByID("list").innerHTML = "<ul>" + output.join("") + "</ul>";
}

function hanleDragOver(e){
  e.stopPragation();
  e.preventDefault();
  e.dataTransfer.dropEffect = "copy";
}

var dropZone = document.getElementByID("drop_zone");
dropZone.addEventListener("dropover", handleDragOver, false);
dropZone.addEventListener("drop", handleFileSelect, false);
</script>

备注: 有些浏览器

<input type="file">

直接支持原生的拖放.

读取文件

最最重要部分来了.
在你获取文件后, 就要开始使用FileReader读取文件了.
FileReader共有4种方式:

1. FileReader.readAsBinaryString(Blob|File) –
2. FileReader.readAsText(Blob|File, opt_encoding) –
3. FileReader.readAsDataURL(Blob|File) –
4. FileReader.readAsArrayBuffer(Blob|File) –

<style>
  .thumb {
    height: 75px;
    border: 1px solid #000;
    margin: 10px 5px 0 0;
  }
</style>

<input type="file" id="files" name="files&#91;&#93;" multiple />
<output id="list"></output>

<script>
  function handleFileSelect(evt) {
    var files = evt.target.files; // FileList object

    // Loop through the FileList and render image files as thumbnails.
    for (var i = 0, f; f = files[i]; i++) {

      // Only process image files.
      if (!f.type.match('image.*')) {
        continue;
      }

      var reader = new FileReader();

      // Closure to capture the file information.
      reader.onload = (function(theFile) {
        return function(e) {
          // Render thumbnail.
          var span = document.createElement('span');
          span.innerHTML = ['<img class="thumb" src="', e.target.result,
                            '" title="', theFile.name, '"/>'].join('');
          document.getElementById('list').insertBefore(span, null);
        };
      })(f);

      // Read in the image file as a data URL.
      reader.readAsDataURL(f);
    }
  }

  document.getElementById('files').addEventListener('change', handleFileSelect, false);
</script>

解析文件

二进制读取

if (file.webkitSlice) {
  var blob = file.webkitSlice(startingByte, endindByte);
} else if (file.mozSlice) {
  var blob = file.mozSlice(startingByte, endindByte);
}
reader.readAsBinaryString(blob);

<style>
  #byte_content {
    margin: 5px 0;
    max-height: 100px;
    overflow-y: auto;
    overflow-x: hidden;
  }
  #byte_range { margin-top: 5px; }
</style>

<input type="file" id="files" name="file" /> Read bytes: 
<span class="readBytesButtons">
  <button data-startbyte="0" data-endbyte="4">1-5</button>
  <button data-startbyte="5" data-endbyte="14">6-15</button>
  <button data-startbyte="6" data-endbyte="7">7-8</button>
  <button>entire file</button>
</span>
<div id="byte_range"></div>
<div id="byte_content"></div>

<script>
  function readBlob(opt_startByte, opt_stopByte) {

    var files = document.getElementById('files').files;
    if (!files.length) {
      alert('Please select a file!');
      return;
    }

    var file = files[0];
    var start = parseInt(opt_startByte) || 0;
    var stop = parseInt(opt_stopByte) || file.size - 1;

    var reader = new FileReader();

    // If we use onloadend, we need to check the readyState.
    reader.onloadend = function(evt) {
      if (evt.target.readyState == FileReader.DONE) { // DONE == 2
        document.getElementById('byte_content').textContent = evt.target.result;
        document.getElementById('byte_range').textContent = 
            ['Read bytes: ', start + 1, ' - ', stop + 1,
             ' of ', file.size, ' byte file'].join('');
      }
    };

    if (file.webkitSlice) {
      var blob = file.webkitSlice(start, stop + 1);
    } else if (file.mozSlice) {
      var blob = file.mozSlice(start, stop + 1);
    }
    reader.readAsBinaryString(blob);
  }
  
  document.querySelector('.readBytesButtons').addEventListener('click', function(evt) {
    if (evt.target.tagName.toLowerCase() == 'button') {
      var startByte = evt.target.getAttribute('data-startbyte');
      var endByte = evt.target.getAttribute('data-endbyte');
      readBlob(startByte, endByte);
    }
  }, false);
</script>

边上传边读取

<style>
  #progress_bar {
    margin: 10px 0;
    padding: 3px;
    border: 1px solid #000;
    font-size: 14px;
    clear: both;
    opacity: 0;
    -moz-transition: opacity 1s linear;
    -o-transition: opacity 1s linear;
    -webkit-transition: opacity 1s linear;
  }
  #progress_bar.loading {
    opacity: 1.0;
  }
  #progress_bar .percent {
    background-color: #99ccff;
    height: auto;
    width: 0;
  }
</style>

<input type="file" id="files" name="file" />
<button onclick="abortRead();">Cancel read</button>
<div id="progress_bar"><div class="percent">0%</div></div>

<script>
  var reader;
  var progress = document.querySelector('.percent');

  function abortRead() {
    reader.abort();
  }

  function errorHandler(evt) {
    switch(evt.target.error.code) {
      case evt.target.error.NOT_FOUND_ERR:
        alert('File Not Found!');
        break;
      case evt.target.error.NOT_READABLE_ERR:
        alert('File is not readable');
        break;
      case evt.target.error.ABORT_ERR:
        break; // noop
      default:
        alert('An error occurred reading this file.');
    };
  }

  function updateProgress(evt) {
    // evt is an ProgressEvent.
    if (evt.lengthComputable) {
      var percentLoaded = Math.round((evt.loaded / evt.total) * 100);
      // Increase the progress bar length.
      if (percentLoaded < 100) {
        progress.style.width = percentLoaded + '%';
        progress.textContent = percentLoaded + '%';
      }
    }
  }

  function handleFileSelect(evt) {
    // Reset progress indicator on new file selection.
    progress.style.width = '0%';
    progress.textContent = '0%';

    reader = new FileReader();
    reader.onerror = errorHandler;
    reader.onprogress = updateProgress;
    reader.onabort = function(e) {
      alert('File read cancelled');
    };
    reader.onloadstart = function(e) {
      document.getElementById('progress_bar').className = 'loading';
    };
    reader.onload = function(e) {
      // Ensure that the progress bar displays 100% at the end.
      progress.style.width = '100%';
      progress.textContent = '100%';
      setTimeout("document.getElementById('progress_bar').className='';", 2000);
    }

    // Read in the image file as a binary string.
    reader.readAsBinaryString(evt.target.files&#91;0&#93;);
  }

  document.getElementById('files').addEventListener('change', handleFileSelect, false);
</script>