我目前正在为我的项目使用android，但我无法找到将变量从Custom类传递到Fragment的方法。我可以在MainActivity和Fragment之间交换变量，但我不能在Custom class和Fragment中使用相同的方法。
我用来在MainActivity和Fragment之间发送变量的方式是这样的
在片段文件中：
public class FourthFragment extends Fragment {
MainActivity activity;
@Override
public void onAttach(Context context) {
super.onAttach(context);
activity = (MainActivity) getActivity();
}
之后我可以在Fragment中使用MainActivity中的任何方法。它的工作正常！所以我尝试以同样的方式将变量从Custom类传递给Fragment，但它没有成功。
我试过的代码。 这是我的自定义类（MyAdapter.java）
public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
MainActivity activity;
FourthFragment fourthfragment = new FourthFragment();
private ArrayList<CreateList> galleryList;
private Context context;
public MyAdapter(Context context, ArrayList<CreateList> galleryList) {
this.galleryList = galleryList;
this.context = context;
}
@Override
public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.cell_layout, viewGroup, false);
return new ViewHolder(view);
}
@Override
public void onBindViewHolder(MyAdapter.ViewHolder viewHolder, final int i) {
viewHolder.title.setText(galleryList.get(i).getImage_title());
viewHolder.img.setScaleType(ImageView.ScaleType.CENTER_CROP);
viewHolder.img.setImageResource((galleryList.get(i).getImage_ID()));
viewHolder.img.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
AlertDialog.Builder builder1 = new AlertDialog.Builder(context);
builder1.setMessage("Are you sure you want to send this "+ galleryList.get(i).getImage_title());
builder1.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
Toast.makeText(context,"Yes",Toast.LENGTH_SHORT).show();
//TODO: Problem here NULL pointer exception
fourthfragment.imgSend();
}
});
builder1.setNegativeButton("No", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
Toast.makeText(context,"No",Toast.LENGTH_SHORT).show();
}
});
builder1.show();
Toast.makeText(context,"Image", Toast.LENGTH_SHORT).show();
}
});
}
@Override
public int getItemCount() {
return galleryList.size();
}
public class ViewHolder extends RecyclerView.ViewHolder{
private TextView title;
private ImageView img;
public ViewHolder(View view) {
super(view);
title = (TextView)view.findViewById(R.id.title);
img = (ImageView) view.findViewById(R.id.img);
}
}
}
这个是我的片段（FourthFragment.java）我只剪掉了一些重要部分。
public class FourthFragment extends Fragment {
MainActivity activity;
MyAdapter myAdapter;
RecyclerView recyclerView;
byte[] dataOut2 = {(byte)255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ,0};
@Override
public void onAttach(Context context) {
super.onAttach(context);
activity = (MainActivity) getActivity();
}
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
init(savedInstanceState);
if (savedInstanceState != null)
onRestoreInstanceState(savedInstanceState);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_fourth, container, false);
initInstances(rootView, savedInstanceState);
return rootView;
}
private void init(Bundle savedInstanceState) {
// Init Fragment level's variable(s) here
}
@SuppressWarnings("UnusedParameters")
private void initInstances(View rootView, Bundle savedInstanceState) {
// Init 'View' instance(s) with rootView.findViewById here
recyclerView = (RecyclerView) rootView.findViewById(R.id.imageGallery);
recyclerView.setHasFixedSize(true);
RecyclerView.LayoutManager layoutManager = new GridLayoutManager(getContext(),2);
recyclerView.setLayoutManager(layoutManager);
ArrayList<CreateList> createLists = prepareData();
MyAdapter adapter = new MyAdapter(getContext(), createLists);
recyclerView.setAdapter(adapter);
}
//Below this is a method that I create to recieve variable(i) from Custom Class (MyAdapter.java)
public void imgSend(int i)
{
if( i< 28)
{
dataOut2[8] = (byte)1;
int j = 0;
j = i + 1;
dataOut2[9] = (byte)j;
activity.sendMessage(dataOut2);
}
else if(i>=28)
{
dataOut2[8] = (byte)2;
int k = 0;
k = i - 27;
dataOut2[9]= (byte)k;
activity.sendMessage(dataOut2);
}
Toast.makeText(getContext(),""+dataOut2,Toast.LENGTH_SHORT).show();
}
当我运行所有程序时，它引起了这样的问题 当我开始点击“是”＆＃39;在Dialog中开始做一个方法 fourthFragment.imgSend（ⅰ）;
FATAL EXCEPTION: main
Process: myapplication.example.com.botcontrol, PID: 22397
java.lang.NullPointerException: Attempt to invoke virtual method 'void myapplication.example.com.botcontrol.MainActivity.sendMessage(byte[])' on a null object reference
at myapplication.example.com.botcontrol.Fragment.FourthFragment.imgSend(FourthFragment.java:156)
at myapplication.example.com.botcontrol.Fragment.MyAdapter$1$1.onClick(MyAdapter.java:70)
at android.support.v7.app.AlertController$ButtonHandler.handleMessage(AlertController.java:157)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:6939)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
那么我可以做什么或以其他方式发送变量？如果还需要其他任何东西，我会发布它！ 谢谢
答案 0 :(得分：0)
这是因为您将上下文强制转换为MainActivity，在这种情况下您需要更加通用。仅使用Activity作为变量。并在投射之前检查类型。
Activity activity;
...
@Override
public void onAttach(Context context) {
super.onAttach(context);
if(getActivity() instanceOf MainActivity){
activity = (MainActivity) getActivity();
} else if(getActivity() instanceOf CustomActivity){
activity = (CustomActivity) getActivity();
}
}
答案 1 :(得分：0)
您的activity变量为空。你有没有提交片段？我没有在你的代码中看到你这样做的地方。
使用FragmentManager提交片段并将其附加到其活动。通常，您将在Activity.onCreate()中提交片段，但您也可以根据您的使用情况在其他地方提交。
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if(savedInstanceState == null) {
getSupportFragmentManager().beginTransaction().add(R.id.my_fragment_container, new FourthFragment()).commitNow();
}
}
一旦提交并附加了Fragment，就不应该再获得空指针异常了。
For reference, here's the documentation related to attaching fragments