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js controls file dragging and obtains dragging content

小云云

Feb 23, 2018 pm 02:52 PM

javascriptcontentObtain

When the user drags a file to an element in the browser, js can monitor the events related to dragging and process the dragging results. This article discusses some issues related to dragging files. problem, but did not deal with too many issues regarding compatibility.

Drag events

jsThe events that can be monitored for drag and drop aredrag#、dragend、dragenter、dragexit(no browser implementation)、dragleave 、dragover、dragstart、 drop, please see MDN for details.

Among them, the events related to dragging files include dragenter(file dragging in), dragover(The file is dragged and dropped)、dragleave(The file is dragged and dropped) ,drop(drag and drop the file).
Drag events can be bound to specified DOM elements or to the entire page.

<span style="font-size: 14px;">var dropEle = document.querySelector('#dropZone');<br>dropEle.addEventListener('drop', function (e) {<br>    // <br>}, false);<br><br>document.addEventListener('drop', function (e) {<br>    // <br>}, false);<br></span>

Prevent default behavior

Generally speaking, we only need to write the business logic for handling drag and drop filesdrop can be done in the event, why do we need to bind dragenter、dragover、dragleaveWhat about these three events?

Because when you drag a file to a browser that does not handle the drag event, the browser will open the file. For example, if you drag a picture, the browser will Open this image and drag a PDF to the browser without a PDF reader, and the browser will open the PDF file.

If the browser opens the dragged file, the page will jump away. We hope to get the dragged file instead of letting the page jump away. As mentioned above, it is the default behavior of the browser to open the dragged file. If we need to prevent this default behavior, we need to prevent it in the above event.

<span style="font-size: 14px;">dropZone.addEventListener("dragenter", function (e) {<br>    e.preventDefault();<br>    e.stopPropagation();<br>}, false);<br><br>dropZone.addEventListener("dragover", function (e) {<br>    e.preventDefault();<br>    e.stopPropagation();<br>}, false);<br><br>dropZone.addEventListener("dragleave", function (e) {<br>    e.preventDefault();<br>    e.stopPropagation();<br>}, false);<br><br>dropZone.addEventListener("drop", function (e) {<br>    e.preventDefault();<br>    e.stopPropagation();<br>    // 处理拖拽文件的逻辑<br>}<br></span>

Actuallydragenter does not block the default behavior and does not trigger the browser to open the file. In order to prevent certain Some browsers may have compatibility issues that prevent all events in the drag cycle from default behavior and prevent event bubbling.

Get the dragged file

We will drop it inThe event object in the callback of this event can get the file object.

In the event object, a property like e.dataTransfer is a DataTransfer type of data has the following attributes

Attribute	Type	Description
<span style="font-size: 14px;">dropEffect</span>	String	Used to hack some compatibility issues
effectAllowed	String	Not used for the time being
<span style="font-size: 14px;">files</span>	FileList	Drag and drop file list
<span style="font-size: 14px;">items</span>	DataTransferItemList	Drag data (may be a string)
types	Array	The dragging data type should be Properties are confusing in Safari

在Chrome中我们用items对象获得文件，其他浏览器用files获得文件，主要是为了处理拖拽文件夹的问题，最好不允许用户拖拽文件夹，因为文件夹内可能还有文件夹，递归上传文件会很久，如果不递归查找，只上传目录第一层级的文件，用户可能以为上传功能了，但是没有上传子目录文件，所以还是禁止上传文件夹比较好，后面我会说要怎么处理。

Chrome获取文件

<span style="font-size: 14px;">dropZone.addEventListener("drop", function (e) {<br>    e.preventDefault();<br>    e.stopPropagation();<br>    <br>    var df = e.dataTransfer;<br>    var dropFiles = []; // 存放拖拽的文件对象<br>    <br>    if(df.items !== undefined) {<br>        // Chrome有items属性，对Chrome的单独处理<br>        for(var i = 0; i             var item = df.items[i];<br>            // 用webkitGetAsEntry禁止上传目录<br>            if(item.kind === "file" && item.webkitGetAsEntry().isFile) {<br>                var file = item.getAsFile();<br>                dropFiles.push(file);<br>            }<br>        }<br>    }<br>}<br></span>

其他浏览器获取文件

这里只测试了Safari，其他浏览器并没有测试，不过看完本文一定也有思路处理其他浏览器的兼容情况。

<span style="font-size: 14px;">dropZone.addEventListener("drop", function (e) {<br>    e.preventDefault();<br>    e.stopPropagation();<br>    <br>    var df = e.dataTransfer;<br>    var dropFiles = []; // 存放拖拽的文件对象<br>    <br>    if(df.items !== undefined) {<br>        // Chrome拖拽文件逻辑<br>    } else {<br>        for(var i = 0; i             dropFiles.push(df.files[i]);<br>        }<br>    }<br>}<br></span>

由于Safari没有item，自然也没有webkitGetAsEntry，所以在Safari无法确定拖拽的是否是文件还是文件夹。

非Chrome内核浏览器判断目录的方法

浏览器获取到的每个file对象有四个属性：lastModified、name、size、type，其中type是文件的MIME Type，文件夹的type是空的，但是有些文件没有MIME Type，如果按照type是否为空判断是不是拖拽的文件夹的话，会误伤一部分文件，所以这个方法行。

那么还有什么方法可以判断呢，思路大概是这样子的，用户拖拽的文件和文件夹应该是不一样的东西，用File API操作的时候应该会有区别，比如进行某些操作的时候，文件就能够正常操作，但是文件夹就会报错，通过错误的捕获就能够判断是文件还是文件夹了，好我们根据这个思路来写一下。

<span style="font-size: 14px;">dropZone.addEventListener("drop", function (e) {<br>    e.preventDefault();<br>    e.stopPropagation();<br><br>    var df = e.dataTransfer;<br>    var dropFiles = [];<br>    <br>    if(df.items !== undefined){<br>        // Chrome拖拽文件逻辑<br>    } else {<br>        for(var i = 0; i             var dropFile = df.files[i];<br>            if ( dropFile.type ) {<br>                // 如果type不是空串，一定是文件<br>                dropFiles.push(dropFile);<br>            } else {<br>                try {<br>                    var fileReader = new FileReader();<br>                    fileReader.readAsDataURL(dropFile.slice(0, 3));<br><br>                    fileReader.addEventListener('load', function (e) {<br>                        console.log(e, 'load');<br>                        dropFiles.push(dropFile);<br>                    }, false);<br><br>                    fileReader.addEventListener('error', function (e) {<br>                        console.log(e, 'error，不可以上传文件夹');<br>                    }, false);<br><br>                } catch (e) {<br>                    console.log(e, 'catch error，不可以上传文件夹');<br>                }<br>            }<br>        }<br>    }<br>}, false);<br></span>

上面代码创建了一个FileReader实例，通过这个实例对文件进行读取，我测试读取一个1G多的文件要3S多，时间有点长，就用slice截取了前3个字符，为什么是前3个不是前2个或者前4个呢，因为代码是我写的，我开心这么写呗~

如果load事件触发了，就说明拖拽过来的东西是文件，如果error事件触发了，就说明是文件夹，为了防止其他可能的潜在错误，用try包起来这段代码。

三方应用的一点点小hack

经过测试发现通过Mac的Finder拖拽文件没有问题，但是有时候文件并不一定在Finder中，也可能在某些应用中，有一个应用叫做圈点，这个应用的用户反馈文件拖拽失效，去看了其他开源文件上传的源码，发现了这样一行代码：

<span style="font-size: 14px;">dropZone.addEventListener("dragover", function (e) {<br>    e.dataTransfer.dropEffect = 'copy'; // 兼容某些三方应用，如圈点<br>    e.preventDefault();<br>    e.stopPropagation();<br>}, false);<br></span>

需要把dropEffect置为copy，上网搜了下这个问题，源码文档中也没有说为什么要加这个，有兴趣的同学可以找一下为什么。

可以拿来就用的代码

由于用了FileReader去读取文件，这是一个异步IO操作，为了记录当前处理了多少个文件，以及什么时候触发拖拽结束的回调，写了一个checkDropFinish的方法一直去比较处理的文件数量和文件总数，确定所有文件处理完了后就去调用完成的回调。

另外，我在最后调试异步处理的时候，用的断点调试，发现断点调试在Safari中会导致异步回调不触发，需要自己调试定制功能的同学注意下。

<span style="font-size: 14px;">// 获得拖拽文件的回调函数<br>function getDropFileCallBack (dropFiles) {<br>    console.log(dropFiles, dropFiles.length);<br>}<br><br>var dropZone = document.querySelector("#dropZone");<br>dropZone.addEventListener("dragenter", function (e) {<br>    e.preventDefault();<br>    e.stopPropagation();<br>}, false);<br><br>dropZone.addEventListener("dragover", function (e) {<br>    e.dataTransfer.dropEffect = 'copy'; // 兼容某些三方应用，如圈点<br>    e.preventDefault();<br>    e.stopPropagation();<br>}, false);<br><br>dropZone.addEventListener("dragleave", function (e) {<br>    e.preventDefault();<br>    e.stopPropagation();<br>}, false);<br><br>dropZone.addEventListener("drop", function (e) {<br>    e.preventDefault();<br>    e.stopPropagation();<br><br>    var df = e.dataTransfer;<br>    var dropFiles = []; // 拖拽的文件，会放到这里<br>    var dealFileCnt = 0; // 读取文件是个异步的过程，需要记录处理了多少个文件了<br>    var allFileLen = df.files.length; // 所有的文件的数量，给非Chrome浏览器使用的变量<br><br>    // 检测是否已经把所有的文件都遍历过了<br>    function checkDropFinish () {<br>        if ( dealFileCnt === allFileLen-1 ) {<br>            getDropFileCallBack(dropFiles);<br>        }<br>        dealFileCnt++;<br>    }<br><br>    if(df.items !== undefined){<br>        // Chrome拖拽文件逻辑<br>        for(var i = 0; i             var item = df.items[i];<br>            if(item.kind === "file" && item.webkitGetAsEntry().isFile) {<br>                var file = item.getAsFile();<br>                dropFiles.push(file);<br>                console.log(file);<br>            }<br>        }<br>    } else {<br>        // 非Chrome拖拽文件逻辑<br>        for(var i = 0; i             var dropFile = df.files[i];<br>            if ( dropFile.type ) {<br>                dropFiles.push(dropFile);<br>                checkDropFinish();<br>            } else {<br>                try {<br>                    var fileReader = new FileReader();<br>                    fileReader.readAsDataURL(dropFile.slice(0, 3));<br><br>                    fileReader.addEventListener('load', function (e) {<br>                        console.log(e, 'load');<br>                        dropFiles.push(dropFile);<br>                        checkDropFinish();<br>                    }, false);<br><br>                    fileReader.addEventListener('error', function (e) {<br>                        console.log(e, 'error，不可以上传文件夹');<br>                        checkDropFinish();<br>                    }, false);<br><br>                } catch (e) {<br>                    console.log(e, 'catch error，不可以上传文件夹');<br>                    checkDropFinish();<br>                }<br>            }<br>        }<br>    }<br>}, false);<br></span>

相关推荐：

如何实现文件拖拽上传

html5文件拖拽上传的示例代码分享

Dropzone.js实现文件拖拽上传功能

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