从输入流读取

Question

I am developing a library for my Android application where I am appending extra info to the end of a PNG image. I am creating a DataOutputStream variable and writing extra info to the end of it to use when I open the PNG and convert it to a Bitmap using a DataInputStream . I added a marker to distinguish when the image code finishes and my extra info starts.

The extra data is correctly being appended after the marker. The problem is reading ONLY the PNG data of the DataInputStream to convert it into a Bitmap. All of the DataInputStream is being read (even if I add a large amount of placeholder bytes before the marker).

The implementation I am using to read the PNG portion of the stream is:

Bitmap image = BitmapFactory.decodeStream(inputStream);

I am wondering if there is another way I should be implementing this to stop reading the stream after the PNG data bytes.

If there isn't a better way, the route I would be taking is copying the input stream into an array. I would then read all of the data until I reach the marker.

Answer 1

You can create a wrapper InputStream that would then report EOF before reading the entire stream. This lets you avoid having to read the whole stream into a byte array.

class MarkerInputStream extends FilterInputStream {
    MarkerInputStream(InputStream in) {
        super(in);
    }

    @Override
    public int read() throws IOException {
        if (isAtMarker()) {
            return -1;
        }
        // may need to read from a cache depending on what isAtMarker method does.
        return super.read();
    }

    private boolean isAtMarker() {
        // logic for determining when you're at the end of the image portion
        return false;
     }
}