如果我在一个继承自AppCompatActivity的类中编写这段代码，它可以运行。但是在fragment类中，我不知道为什么它不能正常工作。在包含此fragment的activity中，我使用另一个适配器来显示具有相同代码的产品，并且它可以工作。
public class Categroy extends Fragment{
// CONNECTION_TIMEOUT and READ_TIMEOUT are in milliseconds
public static final int CONNECTION_TIMEOUT = 10000;
public static final int READ_TIMEOUT = 15000;
public RecyclerView mRVcategoryList;
public AdapterCategory mAdapter;
private String myurl="http://10.0.3.2/mobilaApp/category.php";
// private String categoryurl="http://10.0.3.2/mobilaApp/category.php";
public LinearLayoutManager layoutManager;
List<DataCategory> data=new ArrayList<>();
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// execute asyncLogin
new AsyncLogin().execute();
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_categroy, container, false);
RecyclerView recyclerView = (RecyclerView) rootView.findViewById(R.id.rvcat);
// 2. set layoutManger
recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
// 3. create an adapter
AdapterCategory mAdapter = new AdapterCategory(getActivity().getApplicationContext(), data);
// 4. set adapter
recyclerView.setAdapter(mAdapter);
recyclerView.setItemAnimator(new DefaultItemAnimator());
return rootView;
}
private class AsyncLogin extends AsyncTask<String, String, String> {
ProgressDialog pdLoading = new ProgressDialog(getActivity());
HttpURLConnection conn;
URL url = null;
@Override
protected void onPreExecute() {
super.onPreExecute();
//this method will be running on UI thread
pdLoading.setMessage("\tLoading...");
pdLoading.setCancelable(false);
pdLoading.show();
}
@Override
protected String doInBackground(String... params) {
try {
// Enter URL address where your json file resides
// Even you can make call to php file which returns json data
url = new URL(myurl);
// urlcat = new URL(categoryurl);
} catch (MalformedURLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
return e.toString();
}
try {
// Setup HttpURLConnection class to send and receive data from php and mysql
conn = (HttpURLConnection) url.openConnection();
conn.setReadTimeout(READ_TIMEOUT);
conn.setConnectTimeout(CONNECTION_TIMEOUT);
conn.setRequestMethod("GET");
// setDoOutput to true as we recieve data from json file
conn.setDoOutput(true);
} catch (IOException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
return e1.toString();
}
try {
int response_code = conn.getResponseCode();
// Check if successful connection made
if (response_code == HttpURLConnection.HTTP_OK) {
// Read data sent from server
InputStream input = conn.getInputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(input));
StringBuilder result = new StringBuilder();
String line;
while ((line = reader.readLine()) != null) {
result.append(line);
}
// Pass data to onPostExecute method
return (result.toString());
} else {
return ("unsuccessful");
}
} catch (IOException e) {
e.printStackTrace();
return e.toString();
} finally {
conn.disconnect();
}
}
@Override
protected void onPostExecute(String result) {
//this method will be running on UI thread
// dississ dialog pdLoading.dismiss();
pdLoading.dismiss();
try {
JSONArray jArray = new JSONArray(result);
// Extract data from json and store into ArrayList as class objects
for(int i=0;i<jArray.length();i++){
JSONObject json_data = jArray.getJSONObject(i);
DataCategory categorydata = new DataCategory();
categorydata.name_cat= json_data.getString("name_cat");
data.add(categorydata);
}
mRVcategoryList = (RecyclerView)getActivity(). findViewById(R.id.rvcat);
mAdapter = new AdapterCategory(getActivity().getApplicationContext(), data);
mRVcategoryList.setAdapter(mAdapter);
mRVcategoryList.setLayoutManager(new LinearLayoutManager(getActivity().getApplicationContext()));
} catch (JSONException e) {
Toast.makeText(getActivity().getApplicationContext(), e.toString(), Toast.LENGTH_LONG).show();
}
}
}
}
/* if i write this code in a class extends AppCompatActivity it
works but in a fragment class i don't know why it doesn't work .
in the activity that has this fragment i use another adapter
to show products with the same code and it works
*/
Adapter is here
public AdapterCategory(Context context, List<DataCategory> data){
this.context=context;
inflater= LayoutInflater.from(context);
this.data=data;
}
// Inflate the layout when viewholder created
@Override
public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view=inflater.inflate(R.layout.categoryrow, parent,false);
MyHolder holder=new MyHolder(view);
return holder;
}
// Bind data
@Override
public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
// Get current position of item in recyclerview to bind data and assign values from list
MyHolder myHolder= (MyHolder) holder;
DataCategory current=data.get(position);
myHolder.namecat.setText(current.name_cat);
}
// return total item from List
@Override
public int getItemCount() {
return data.size();
}
class MyHolder extends RecyclerView.ViewHolder{
TextView namecat;
// TextView textPrice;
// create constructor to get widget reference
public MyHolder(View itemView) {
super(itemView);
namecat= (TextView) itemView.findViewById(R.id.category_name);
}
}
}
onCreate方法是在布局被创建之前执行的。因此，“mRVcategoryList =（RecyclerView）getActivity（）。findViewById（R.id.rvcat）;”可能会为空值。 - SergRecyclerView抛出了异常？ - faranjit