我有包含大量 fragment 和子 fragment 的复杂应用程序,并且我需要监听器在 fragment 中而不是在 Activity 中监听。这通常有效,但不适用于日期或时间选择器。
这是一个示例应用程序,其 Activity 包含一个 fragment 膨胀 - TestFragmentMain。该膨胀的 fragment 还有两个 fragment - 一个具有单个 EditText (TestFragment_InputBox),另一个具有单个 TextView (TestFragment_DateBox),它用于监听事件(TextChangeListener 和 DateChangeListener)。
在第一个 fragment 中的文本更改时,一切都像魅力一样工作,主要 fragment 正在接收结果。
但是,在日期更改时,我收到错误:
java.lang.NullPointerException: Attempt to invoke interface method 'void com.gmail.xxx.xxx.test.DateChangeListener.dateChanged(int, int, int)' on a null object reference
at com.gmail.xxx.xxx.test.TestFragment_DatePicker.onDateSet(TestFragment_DatePicker.java:32)
我真的不明白为什么。如有任何帮助,我们将不胜感激。
主要 Activity :
public class TestClass extends AppCompatActivity {
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.test_activity);
TestFragmentMain testFragmentMain = new TestFragmentMain();
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.replace(R.id.test_fragment_container, testFragmentMain);
ft.commitAllowingStateLoss();
}
}
主要 fragment ,带有听众
public class TestFragmentMain extends Fragment implements TextChangeListener, DateChangeListener {
@Override
public View onCreateView(@NotNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.test_fragment, container, false);
TestFragment_InputBox testFragmentInputBox = new TestFragment_InputBox();
FragmentTransaction ft = getChildFragmentManager().beginTransaction();
ft.replace(R.id.test_fragment_inputbox_container, testFragmentInputBox);
ft.commitAllowingStateLoss();
TestFragment_DateBox testFragmentDateBox = new TestFragment_DateBox();
ft = getChildFragmentManager().beginTransaction();
ft.replace(R.id.test_fragment_date_container, testFragmentDateBox);
ft.commitAllowingStateLoss();
return view;
}
@Override
public void onAttach(@NotNull Context context) {
super.onAttach(context);
}
@Override
public void textChanged() {
Log.d("LISTEN","Text has been changed...");
}
@Override
public void dateChanged(int year, int month, int day) {
Log.d("LISTEN","Date has been changed to ...");
}
}
带有输入框的 fragment :
public class TestFragment_InputBox extends Fragment {
private TextChangeListener textChangeListener;
@Override
public View onCreateView(@NotNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.test_input_box, container, false);
EditText editText = view.findViewById(R.id.input_view);
editText.addTextChangedListener(new TextWatcher() {
@Override
public void beforeTextChanged(CharSequence s, int start, int count, int after) {
}
@Override
public void onTextChanged(CharSequence s, int start, int before, int count) {
}
@Override
public void afterTextChanged(Editable s) {
textChangeListener.textChanged();
}
});
return view;
}
@Override
public void onAttach(@NotNull Context context) {
super.onAttach(context);
try {
textChangeListener = (TextChangeListener) getParentFragment();
} catch (ClassCastException e) {
e.printStackTrace();
}
}
}
带有日期框的 fragment :
public class TestFragment_DateBox extends Fragment implements DateChangeListener {
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.test_date_box, container, false);
TextView textView = view.findViewById(R.id.date_view);
textView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
DialogFragment datePicker = new TestFragment_DatePicker();
datePicker.show(getActivity().getSupportFragmentManager(), "datePicker");
}
});
return view;
}
@Override
public void onAttach(Context context) {
super.onAttach(context);
}
@Override
public void dateChanged(int year, int month, int day) {
Log.d("LISTEN","Date has been changed in box fragment with box ...");
}
}
日期选择器 fragment
public class TestFragment_DatePicker extends DialogFragment implements DatePickerDialog.OnDateSetListener {
public DateChangeListener dateChangeListener;
@NotNull
@Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
final Calendar c = Calendar.getInstance();
int year = c.get(Calendar.YEAR);
int month = c.get(Calendar.MONTH);
int day = c.get(Calendar.DAY_OF_MONTH);
return new DatePickerDialog(Objects.requireNonNull(getContext()), this, year, month, day);
}
@Override
public void onDateSet(DatePicker view, int year, int month, int day) {
dateChangeListener.dateChanged(year,month,day);
}
@Override
public void onAttach(@NotNull Context context) {
super.onAttach(context);
try {
dateChangeListener = (DateChangeListener) getParentFragment();
} catch (ClassCastException e) {
e.printStackTrace();
}
}
}
还有听众:
public interface TextChangeListener {
void textChanged();
}
public interface DateChangeListener {
void dateChanged(int year,int month,int day);
}
最佳答案
您的 fragment TestFragment_DateBox
调用日期选择器并继承DatePickerDialog.OnDateSetListener
public class TestFragment_DateBox extends Fragment implements DatePickerDialog.OnDateSetListener
并像这样显示日期选择器 fragment
TestFragment_DatePicker().show(this.childFragmentManager, "datepicker")
并覆盖 onDateSet
@Override
public void onDateSet(DatePicker view, int year, int month, int day) {
// As you are in the calling fragment you can use the values as you see fit
TextView textView = view.findViewById(R.id.date_view);
textView.text = year.toString()
}
在TestFragment_DatePicker
中有一个监听器
private lateinit var listener: DatePickerDialog.OnDateSetListener
然后重写 onAttach
override fun onAttach(context: Context) {
super.onAttach(context)
// Verify that the dialog parent implements the callback interface
try {
// Instantiate the OnDateSetListener so we can send events to the host
listener = parentFragment as DatePickerDialog.OnDateSetListener
} catch (e: ClassCastException) {
// The parent doesn't implement the interface, throw exception
throw ClassCastException(("$context must implement OnDateSetListener"))
}
}
对 java 和 kotlin 的混合表示歉意,但你明白了!
关于java - 如何正确获取父 fragment 而不是父 Activity 中的 DatePicker 值,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/59681970/