如何在安卓系统的PrintManager中添加缩放功能？

public class CustomPrintDocumentAdapter extends PrintDocumentAdapter {
    private Context mContext;
    private Bitmap mContent;

    public CustomPrintDocumentAdapter(Context context, Bitmap content) {
        mContext = context;
        mContent = content;
    }

    @Override
    public void onLayout(PrintAttributes oldAttributes, PrintAttributes newAttributes, CancellationSignal cancellationSignal, LayoutResultCallback callback, Bundle extras) {
        if (cancellationSignal.isCanceled()) {
            callback.onLayoutCancelled();
            return;
        }

        PrintDocumentInfo.Builder builder = new PrintDocumentInfo.Builder("print_output.pdf")
                .setContentType(PrintDocumentInfo.CONTENT_TYPE_DOCUMENT)
                .setPageCount(1);

        PrintDocumentInfo info = builder.build();
        callback.onLayoutFinished(info, true);
    }

    @Override
    public void onWrite(PageRange[] pages, ParcelFileDescriptor destination, CancellationSignal cancellationSignal, WriteResultCallback callback) {
        if (cancellationSignal.isCanceled()) {
            callback.onWriteCancelled();
            return;
        }

        PrintedPdfDocument document = new PrintedPdfDocument(mContext, newAttributes);

        try {
            PdfDocument.Page page = document.startPage(0);
            Canvas canvas = page.getCanvas();

            // 缩放画布
            float scale = 0.5f; // 设置缩放比例为50%
            canvas.scale(scale, scale);

            Rect srcRect = new Rect(0, 0, mContent.getWidth(), mContent.getHeight());
            RectF dstRect = new RectF(0, 0, canvas.getWidth(), canvas.getHeight());
            canvas.drawBitmap(mContent, srcRect, dstRect, null);

            document.finishPage(page);
            document.writeTo(new FileOutputStream(destination.getFileDescriptor()));
            callback.onWriteFinished(new PageRange[]{PageRange.ALL_PAGES});
        } catch (IOException e) {
            callback.onWriteFailed(e.toString());
            return;
        } finally {
            document.close();
        }
    }
}

// 创建打印适配器
PrintDocumentAdapter printAdapter = new CustomPrintDocumentAdapter(context, bitmap);

// 获取PrintManager实例
PrintManager printManager = (PrintManager) context.getSystemService(Context.PRINT_SERVICE);

// 打印设置
PrintAttributes attributes = new PrintAttributes.Builder()
        .setMediaSize(PrintAttributes.MediaSize.ISO_A4)
        .setResolution(new PrintAttributes.Resolution("res1", "Resolution", 300, 300))
        .setMinMargins(PrintAttributes.Margins.NO_MARGINS)
        .build();

// 打印任务
PrintJob printJob = printManager.print("Document", printAdapter, attributes);

// 监听打印任务完成
printJob.addPrintJobStateChangeListener(new PrintJobStateChangeListener() {
    @Override
    public void onPrintJobStateChanged(PrintJobId printJobId) {
        if (printJob.isCompleted()) {
            // 打印完成
        } else if (printJob.isFailed()) {
            // 打印失败
        }
    }
});