android - 无法在 dialogfragment 中的按钮上获取指针

Question

我正在尝试构建一个简单的 DialogFragment，其中包含一个 TextView 和两个按钮(发送和取消)。

我想在 textview 第一次为空时禁用按钮，但代码中的 positiveButton 变量始终为 null，并且出现以下异常:

 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setEnabled(boolean)' on a null object reference

这是代码:

public class SendFriendRequestFragment extends DialogFragment {
private TextView tvEmail = null;
Button positiveButton = null;

@Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
    // Use the Builder class for convenient dialog construction
    AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());

    // Get the layout inflater
    final LayoutInflater inflater = getActivity().getLayoutInflater();

    final View layout = inflater.inflate(R.layout.fragment_send_friend_request, null);

    // Inflate and set the layout for the dialog
    // Pass null as the parent view because its going in the dialog layout
    builder.setView(layout)
            .setTitle("Send request")
            .setPositiveButton(R.string.send, new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int id) {
                    String email = tvEmail.getText().toString().trim();

                    if (validateInput()) {
                        SendFriendRequest(getActivity(), email);
                    } else {
                        Toast.makeText(getActivity(), "Request cannot be sent", Toast.LENGTH_LONG).show();
                        return;
                    }

                }
            })
            .setNegativeButton(R.string.cancel, new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int id) {
                    // User cancelled the dialog
                }
            });

    Dialog dialog = builder.create();
    positiveButton = ((AlertDialog) dialog).getButton(AlertDialog.BUTTON_POSITIVE);

    positiveButton.setEnabled(false); // positiveButton is null and this call raises an exception !
    tvEmail = (TextView) layout.findViewById(R.id.email);
    tvEmail.addTextChangedListener(new InputTextWatcher(tvEmail));
    // Create the AlertDialog object and return it
    //return builder.create();
    return dialog;
}
// Other functions that use the two variables positiveButton and tvEmail ...
}

谁能告诉我如何解决我的问题，以及获取所用布局中包含的按钮和 View 指针的最佳方法是什么？

谢谢!

Answer 1

在显示 Dialog 之前，不会真正创建肯定的 Button。由于您在 DialogFragment 中，因此您不会直接处理 Dialog 上的 show() 调用。您可以在 AlertDialog 上设置一个 OnShowListener，但在 DialogFragment 中获取 Button 可能更简单s onStart() 方法。

@Override
public void onStart() {
    super.onStart();

    positiveButton = ((AlertDialog) getDialog()).getButton(AlertDialog.BUTTON_POSITIVE);
    ...
}