reference to custom AlertDialog button returns null when onTextChanged

Question

I am working on an application that uses a custom view for an AlertDialog. I want to make it so that the submit button is disabled if the edit text field is empty. I found a few examples online but my in my implementation keeps returning null on the reference to the submit button. as soon as I enter any text into the edittext the app crashes. here is alertdialog class with my failing attempt at grabbing the button commented out:

public class FileNameDialogFragment extends DialogFragment {
private static final String TAG = "dialogFragment";

private EditText namefield;
private Button submit;

@Override
public Dialog onCreateDialog(Bundle savedInstanceState){

    LayoutInflater inflater = LayoutInflater.from(getActivity());
    View v = inflater.inflate(R.layout.file_prompt_dialog, null, false);
    namefield = (EditText) v.findViewById(R.id.nameField);

    AlertDialog dialog = new AlertDialog.Builder(getActivity())
            .setTitle(R.string.dialog_title)
            .setView(v)
            .setIcon(R.drawable.icon)
            .setPositiveButton(R.string.btnOK, new DialogInterface.OnClickListener(){

                @Override
                public void onClick(DialogInterface dialog, int which) {
                    MainActivity.setFileName(namefield.getText().toString());
                    String fileName = namefield.getText().toString();
                    Toast.makeText(getActivity(), MainActivity.getUrl().toString(), Toast.LENGTH_SHORT).show();
                    MainActivity.download(MainActivity.getUrl(), getActivity(), MainActivity.getPath(), fileName);
                }
            })
            .setNegativeButton(R.string.btnCancel, new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    dismiss();
                }
            })
            .create();

            //I have also tried to place the code that is below here. neither works

    dialog.show();
    /*
     *  this is the section of code that i am having trouble with
     *
    submit = ((AlertDialog)dialog).getButton(AlertDialog.BUTTON_POSITIVE);
    namefield.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {
            //left intentionally blank
        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
            if (count == 0){
                submit.setEnabled(false);
            }else{
                submit.setEnabled(true);<--NullPointerException
            }
        }

        @Override
        public void afterTextChanged(Editable s) {
            //left intentionally blank
        }
    });*/
    return dialog;
}
}

I am hoping to find out how to make this work as a way to validate input. i don't want the user to be able to leave this field blank because the input is used to name a file that is downloaded from the internet.

Answer 1

You can try the code below, I have specified how you can disable custom button as well as dialog's positive/negative button depending on your editText value

@Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
    LayoutInflater inflater = LayoutInflater.from(getActivity());
    View v = inflater.inflate(R.layout.file_prompt_dialog, null, false);
    namefield = v.findViewById(R.id.nameField);

    //To get custom button of dialog
    submit = v.findViewById(R.id.submitBtn);

    final AlertDialog dialog = new AlertDialog.Builder(getActivity())
            .setTitle(R.string.dialog_title)
            .setView(v)
            .setIcon(R.mipmap.ic_launcher)
            .setPositiveButton(R.string.btnOK, new DialogInterface.OnClickListener(){

                @Override
                public void onClick(DialogInterface dialog, int which) {
                    MainActivity.setFileName(namefield.getText().toString());
                    String fileName = namefield.getText().toString();
                    Toast.makeText(getActivity(), "Positive Button", Toast.LENGTH_SHORT).show();
                    MainActivity.download(MainActivity.getUrl(), getActivity(), MainActivity.getPath(), fileName);
                }
            })
            .setNegativeButton(R.string.btnCancel, new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    dismiss();
                }
            })
            .create();

    dialog.show();

    //Dialog's positive button
    dialog.getButton(AlertDialog.BUTTON_POSITIVE).setEnabled(false);

    //Custom button
    submit.setEnabled(false);

    namefield.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {
            //left intentionally blank
        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
            if (count == 0){

                //To disable dialog's positive button
                dialog.getButton(AlertDialog.BUTTON_POSITIVE).setEnabled(false);

                //To disable custom button
                submit.setEnabled(false);
            }else{

                //To enable dialog/s positive button
                dialog.getButton(AlertDialog.BUTTON_POSITIVE).setEnabled(true);

                //To enable custom button
                submit.setEnabled(true);
            }
        }

        @Override
        public void afterTextChanged(Editable s) {
            //left intentionally blank
        }
    });

    return dialog;
}