gulp plugin to generate manifest for cordova-app-loader

updateManifest.js

/***
Gulp plugin to generate a manifest.json for cordova-app-loader.

Example usage:

```
var updateManifest = require('./updateManifest');
gulp.task('manifest', function () {
    gulp.src(['www/app.js', 'www/style.css'])
    .pipe(updateManifest('manifest.json', { load: ['www/app.js'] })
    .pipe(gulp.dest('www'));
});
```

Produces the following in `www/manifest.json`:

```
{
  "files": {
    "app.js": {
      "version": "4004535bdc97a3b764cb1d0343d1bc813cbdce93",
      "filename": "app.js"
    },
    "style.css": {
      "version": "2eea6c3fa942a7386bebefddf84c0df214c90248",
      "filename": "style.css"
    }
  },
  "load": [
    "app.js"
  ],
  "version": "d21f0e85ad97235a7f934d6e0fc0b87025c447ec"
}
```

***/

var crypto = require('crypto');
var through = require('through2');
var gutil = require('gulp-util');
var PluginError = gutil.PluginError;
var File = gutil.File;

function checksum(data) {
    return crypto.createHash('sha1').update(data).digest('hex');
}

module.exports = function (manifestFile, options) {
    var versionChecksum = '';
    var files = {};

    function processFile(file, encoding, callback) {
        if (file.isNull()) {
            // ignore empty files
            callback();
            return;
        }

        if (file.isStream()) {
            this.emit('error', new PluginError('gulp-cordova-app-loader-manifest', 'Streaming not supported'));
            callback();
            return;
        }

        var version = checksum(file.contents);
        versionChecksum += version;
        files[file.relative] = {
            version: version,
            filename: file.relative,
        };

        callback();
    }

    function endStream(callback) {
        var contents = JSON.stringify({
            files: files,
            load: options.load || [],
            version: checksum(versionChecksum),
        }, null, 2);
        this.push(new File({ path: manifestFile, contents: new Buffer(contents) }));
        this.push(null);
        callback();
    }

    return through.obj(processFile, endStream);
};