FileSystemDirectoryReader.readEntries() - Web API 接口参考 编辑

这是一个实验中的功能
此功能某些浏览器尚在开发中，请参考浏览器兼容性表格以得到在不同浏览器中适合使用的前缀。由于该功能对应的标准文档可能被重新修订，所以在未来版本的浏览器中该功能的语法和行为可能随之改变。

非标准
该特性是非标准的，请尽量不要在生产环境中使用它！

The FileSystemDirectoryReader interface's readEntries() method retrieves the directory entries within the directory being read and delivers them in an array to a provided callback function. The objects in the array are all based upon FileSystemEntry. Generally, they are either FileSystemFileEntry objects, which represent standard files, or FileSystemDirectoryEntry objects, which represent directories.

Syntax

readEntries(successCallback[, errorCallback]);

Parameters

successCallback

A function which is called when the directory's contents have been retrieved. The function receives a single input parameter: an array of file system entry objects, each based on FileSystemEntry. Generally, they are either FileSystemFileEntry objects, which represent standard files, or FileSystemDirectoryEntry objects, which represent directories. If there are no files left, or you've already called readEntries() on this FileSystemDirectoryReader, the array is empty.

errorCallback 可选

A callback function which is called if an error occurs while reading from the directory. It receives one input parameter: a FileError object describing the error which occurred.

Return value

undefined.

Example

In this example, a drop zone is created, which responds to the drop event by scanning through the dropped files and directories, outputting a hierarchical directory listing.

HTML content

The HTML establishes the drop zone itself, which is a <div> element with the ID "dropzone", and an unordered list element with the ID "listing".

<p>Drag files and/or directories to the box below!</p>

<div id="dropzone">
  <div id="boxtitle">
    Drop Files Here
  </div>
</div>

<h2>Directory tree:</h2>

<ul id="listing">
</ul>

CSS content

The styles used by the example are shown here.

#dropzone {
  text-align: center;
  width: 300px;
  height: 100px;
  margin: 10px;
  padding: 10px;
  border: 4px dashed red;
  border-radius: 10px;
}

#boxtitle {
  display: table-cell;
  vertical-align: middle;
  text-align: center;
  color: black;
  font: bold 2em "Arial", sans-serif;
  width: 300px;
  height: 100px;
}

body {
  font: 14px "Arial", sans-serif;
}

JavaScript content

First, let's look at the recursive scanFiles() function. This function takes as input a FileSystemEntry representing an entry in the file system to be scanned and processed (the item parameter), and an element into which to insert the list of contents (the container parameter).

Note that to read all files in a directory, readEntries needs to be called repeatedly until it returns an empty array. In Chromium-based browsers, the following example will only return a max of 100 entries.

let dropzone = document.getElementById("dropzone");
let listing = document.getElementById("listing");

function scanFiles(item, container) {
  let elem = document.createElement("li");
  elem.innerHTML = item.name;
  container.appendChild(elem);

 if (item.isDirectory) {
    let directoryReader = item.createReader();
    let directoryContainer = document.createElement("ul");
    container.appendChild(directoryContainer);
    directoryReader.readEntries(function(entries) {
        entries.forEach(function(entry) {
          scanFiles(entry, directoryContainer);
      });
    });
  }
}

scanFiles() begins by creating a new <li> element to represent the item being scanned, inserts the name of the item into it as its text content, and then appends it to the container. The container is always a list element in this example, as you'll see shortly.

Once the current item is in the list, the item's isDirectory property is checked. If the item is a directory, we need to recurse into that directory. The first step is to create a FileSystemDirectoryReader to handle fetching the directory's contents. That's done by calling the item's createReader() method. Then a new <ul> is created and appended to the parent list; this will contain the directory's contents in the next level down in the list's hierarchy.

After that, directoryReader.readEntries() is called to read in all the entries in the directory. These are each, in turn, passed into a recursive call to scanFiles() to process them. Any of them which are files are simply inserted into the list; any which are directories are inserted into the list and a new level of the list's hierarchy is added below, and so forth.

Then come the event handlers. First, we prevent the dragover event from being handled by the default handler, so that our drop zone can receive the drop:

dropzone.addEventListener("dragover", function(event) {
    event.preventDefault();
}, false);

The event handler that kicks everything off, of course, is the handler for the drop event:

dropzone.addEventListener("drop", function(event) {
  let items = event.dataTransfer.items;

  event.preventDefault();
  listing.innerHTML = "";

  for (let i=0; i<items.length; i++) {
    let item = items[i].webkitGetAsEntry();

    if (item) {
        scanFiles(item, listing);
    }
  }
}, false);

This fetches the list of DataTransferItem objects representing the items dropped from event.dataTransfer.items. Then we call Event.preventDefault() to prevent the event from being handled further after we're done.

Now it's time to start building the list. First, the list is emptied by setting listing.innerHTML to be empty. That leaves us with an empty ul to begin inserting directory entries into.

Then we iterate over the items in the list of dropped items. For each one, we call its webkitGetAsEntry() method to obtain a FileSystemEntry representing the file. If that's successful, we call scanFiles() to process the item—either by adding it to the list if it's just a file or by adding it and walking down into it if it's a directory.

Result

You can see how this works by trying it out below. Find some files and directories and drag them in, and take a look at the resulting output.

Specifications

Specification	Status	Comment
File and Directory Entries API	Draft	Draft of proposed API

This API has no official W3C or WHATWG specification.

Browser compatibility

BCD tables only load in the browser

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

See also

• File and Directory Entries API
• Introduction to the File System API
• FileSystemDirectoryEntry
• FileSystem