将图形保存到内部存储

Question

我按照在线教程制作了一个简单的绘图应用程序。一切正常，直到图形保存到内部存储的部分。出于某种原因，即使分配了明确的名称，图像名称也始终为空。我在运行Android Nougat的Axon 7 Mini上运行该应用程序。这是我无法工作的代码。我在Manifest中得到了适当的许可。如果将图像保存为JPEG与PNG，这是否重要？

Picture can't be saved into phone storage

else if(view.getId()==R.id.save_btn){
        //save drawing
        AlertDialog.Builder saveDialog = new AlertDialog.Builder(this);
        saveDialog.setTitle("Save drawing");
        saveDialog.setMessage("Save drawing to device Gallery?");
        saveDialog.setPositiveButton("Yes", new DialogInterface.OnClickListener(){
            public void onClick(DialogInterface dialog, int which){
                //save drawing
                drawView.setDrawingCacheEnabled(true);
                //attempt to save
                String imgSaved = null;
                OutputStream fOut = null;
                File mediaDirectory = new File(Environment.getExternalStorageDirectory().getPath()
                        + "DCIM/Camera/");
                mediaDirectory.mkdirs();
                File f = new File(mediaDirectory, UUID.randomUUID().toString() + ".png");
                try {
                    fOut = new FileOutputStream(f);
                    drawView.getDrawingCache().compress(Bitmap.CompressFormat.PNG, 100, fOut);
                    fOut.flush();
                    fOut.close();
                    imgSaved = "success";
                    sendBroadcast(new Intent(Intent.ACTION_MEDIA_MOUNTED, Uri.parse
                            ("file://" + Environment.getExternalStorageDirectory())));
                }
                catch (Exception e) {
                    e.printStackTrace();
                }
                //feedback
                if(imgSaved!=null){
                    Toast savedToast = Toast.makeText(getApplicationContext(),
                            "Drawing saved to Gallery!", Toast.LENGTH_SHORT);
                    savedToast.show();
                }
                else{
                    Toast unsavedToast = Toast.makeText(getApplicationContext(),
                            "Oops! Image could not be saved.", Toast.LENGTH_SHORT);
                    unsavedToast.show();
                }
                drawView.destroyDrawingCache();
            }
        });
        saveDialog.setNegativeButton("Cancel", new DialogInterface.OnClickListener(){
            public void onClick(DialogInterface dialog, int which){
                dialog.cancel();
            }
        });
        saveDialog.show();
    }