Question

In a Windows 8 Metro application written in JS I open a file, get the stream, write some image data to it using the ‘promise – .then’ pattern. It works fine – the file is successfully saved to the file system, except after using the BitmapEncoder to flush the stream to the file, the stream is still open. ie; I can’t access the file until I kill the application, but the ‘stream’ variable is out of scope for me to reference, so I can’t close() it. Is there something comparable to the C# using statement that could be used?

...then(function (file) {
                return file.openAsync(Windows.Storage.FileAccessMode.readWrite);
            })
.then(function (stream) {
                //Create imageencoder object
                return Imaging.BitmapEncoder.createAsync(Imaging.BitmapEncoder.pngEncoderId, stream);
            })
.then(function (encoder) {
                //Set the pixel data in the encoder ('canvasImage.data' is an existing image stream)
                encoder.setPixelData(Imaging.BitmapPixelFormat.rgba8, Imaging.BitmapAlphaMode.straight, canvasImage.width, canvasImage.height, 96, 96, canvasImage.data);
                //Go do the encoding
                return encoder.flushAsync();
                //file saved successfully, 
                //but stream is still open and the stream variable is out of scope.
            };

Answer 1

This simple imaging sample from Microsoft might help. Copied below.

It looks like, in your case, you need to declare the stream before the chain of then calls, make sure you don’t name-collide with your parameter to your function accepting the stream (note the part where they do _stream = stream), and add a then call to close the stream.

function scenario2GetImageRotationAsync(file) { 
    var accessMode = Windows.Storage.FileAccessMode.read; 

    // Keep data in-scope across multiple asynchronous methods 
    var stream; 
    var exifRotation;
    return file.openAsync(accessMode).then(function (_stream) { 
        stream = _stream; 
        return Imaging.BitmapDecoder.createAsync(stream); 
    }).then(function (decoder) { 
        // irrelevant stuff to this question
    }).then(function () { 
        if (stream) { 
            stream.close(); 
        } 
        return exifRotation; 
    }); 
}