用HTML5 FileWriter覆盖文件 [英] overwrite a file with HTML5 FileWriter

问题描述

我正在使用 HTML5 FileWriter API 保存我的webapp状态。我有一些JS定期调用 FileWriter.write 来做到这一点（所以，随着时间的推移，写方法是多次打电话）。默认情况下，FileWriter API使用'append'方法来编写不符合我需要的文件，因为我不想覆盖文件内容。

I'm using HTML5 FileWriter API to save the state of my webapp. I have bit of JS that periodically calls FileWriter.write to do that (so , over time, the write method is called several times). By default FileWriter API use an 'append' approach to writing files which does not suits my needs since I wan't to overwrite the file content.

我首先尝试这样做：

this._writer.seek(0);
this._writer.write(content);

当您编写的文本短于文件内容时，这不起作用。然后我尝试了这个：

This is not working when you are writing a text shorter than the file content. I then tried this:

this._writer.truncate(0);
this._writer.write(content);

这段代码应该清除文件，然后写下我的新内容，但我得到以下内容调用 write 方法时出错：

This code is supposed to clear the file and then write my new content but I'm getting the following error when write method is called:

Uncaught InvalidStateError: An operation that depends on state cached in an interface object was made but the state had changed since it was read from disk.

奇怪的是：当我调试代码（带断点）时，错误不会发生，因为if FileWriter.truncate 是一个异步方法......

Odd thing: when I debug the code (with a breakpoint), the error does not occur, as if FileWriter.truncate was an asynchronous method...

我被困在这里，有什么想法吗？

I am stuck here, any ideas?

我使用的是Chrome 30.0.1599.69

I am using Chrome 30.0.1599.69

推荐答案

这是一个正确的代码，不会浪费500毫秒等待

Here is a correct code that won't waste 500ms on waiting

fileWriter.onwriteend = function() {
    if (fileWriter.length === 0) {
        //fileWriter has been reset, write file
        fileWriter.write(blob);
    } else {
        //file has been overwritten with blob
        //use callback or resolve promise
    }
};
fileWriter.truncate(0);

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