Problems using phonegap / cordova file plugin part 1

Question

I want to add some simple logging capabilities to my cordova app.

So I added the file plugin, implemented a super simple log method and tested.

My configuration:

    $ cordova --version
    3.5.0-0.2.7

    $ cordova plugins
    org.apache.cordova.file 1.3.0 "File"

The test device is a Huawei u8850, running Android 2.3.5

The Logger:

window.MyLog = {
    log: function(line){
        window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, function(FS) {
            FS.root.getFile('the_log1.txt', {"create":true, "exclusive":false},
                function(fileEntry) {
                    fileEntry.createWriter(
                        function(writer) {
                            console.log(line);
                            writer.seek(writer.length);     // append to eof
                            writer.write(line + '\n');      // write the line
                        }, fail);
                }, fail);
        }, fail);
    }
};

Testing:

    MyLog.log(new Date().toLocaleTimeString());

All seems fine:

• file was created, line was inserted
• when starting the app again, the line was appended

But then: I tried to write some extra chars:

window.MyLog = {
    log: function(line){
        window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, function(FS) {
            FS.root.getFile('the_log2.txt', {"create":true, "exclusive":false},
                function(fileEntry) {
                    fileEntry.createWriter(
                        function(writer) {
                            console.log(line);
                            writer.seek(writer.length);     // append to eof
                            writer.write(line + '\n');      // write the line
                            writer.write('----' + '\n');    // extra write
                        }, fail);
                }, fail);
        }, fail);
    }
};

I found this log cat output:

    16:00:00
    processMessage failed: Error: [object Object]
    processMessage failed: Stack: undefined
    processMessage failed: Message: S01 File218896726 {"lastModifiedDate":1409839200000,"fullPath":"\/the_log2.txt","type":"text\/plain","name":"the_log2.txt","size":0}

-> seemingly it is not possible to use the writer.write command more than once

Q:

• Why is this happening? Is it a bug or a feature?
• is there any documentation / sample code available on the usage of this plugin? (the docs found here: https://github.com/apache/cordova-plugin-file/blob/master/doc/index.md are not as comprehensive as I would like)

Answer 1

I think you have to implement and wait for the onwriteend() event to fire after writing anything to a file.

Once the onwriteend() fired you can write again to the file.

What I was doing when I created a logging feature was something like this (you can use it as a guide, i'm just typing it out of my head):

function fileItemSuccessWrite(writer){
    writer.seek(writer.length);

    writer.onwriteend = function(e) {
         // TODO WRITE AGAIN
    };

    writer.onerror = function(e) {
         // HANDLE ERROR
    };

    writer.write(YOUR_LOG_TO_WRITE);

}
fileEntry.createWriter(fileItemSuccessWrite, fileItemFailWrite);

Also what I have created was a cache (a tmp_variable) for the content I wanted to write in a log file. Once this tmp_variable reached a specific size I dumped it in the log file. So it was lighter on fileSystem access.

You can read about the writer and write on W3.org