这是我的代码：
public class LatestPostFragment extends Fragment {
private static final String TAG = LatestPostFragment.class.getSimpleName();
View rootView;
LinearLayout grid_view_wrapper;
private ViewPager viewPager;
private MyViewPagerAdapter myViewPagerAdapter;
private LinearLayout layout_dots;
private TextView[] dots;
private Integer viewPagerLength;
Typeface font;
ArrayList<Post>viewPagerItems = new ArrayList<Post>();
private List<Category> categoriesList;
public LatestPostFragment() {}
@Override
public void onResume() {
super.onResume();
//Get a Tracker (should auto-report)
if (Const.Analytics_ACTIVE) {
AnalyticsUtil.sendScreenName(getActivity(), TAG);
}
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
rootView = inflater.inflate(R.layout.fragment_latest, container, false);
//Forcing RTL Layout, If Supports and Enabled from Const file
Utils.forceRTLIfSupported(getActivity());
grid_view_wrapper = (LinearLayout) rootView.findViewById(R.id.grid_view_wrapper);
font = Typeface.createFromAsset(getActivity().getAssets(), "fonts/iran_sans.ttf");
ConnectionDetector cd = new ConnectionDetector(this.getActivity());
Boolean isInternetPresent = cd.isConnectingToInternet();
if (isInternetPresent) {
getFeaturedPosts();
categoriesList = AppController.getInstance().getPrefManger().getFeaturedCategories();
for (Category a : categoriesList) {
getLatestPosts(a.getId());
}
}
return rootView;
}
private void getFeaturedPosts(){
String featured_url = Const.URL_BLOG + Const.URL_BLOG_API + Const.URL_FEATURED + "?t=" + System.currentTimeMillis();
// making fresh volley request and getting json
JsonObjectRequest jsonReq = new JsonObjectRequest(Request.Method.GET, featured_url, null, new Response.Listener<JSONObject>() {
@Override
public void onResponse(JSONObject response) {
if (response != null) {
try {
if (response.has("error")) {
String error = response.getString("error");
Toast.makeText(getActivity().getApplicationContext(), error, Toast.LENGTH_LONG).show();
}else {
if (response.isNull("feed")) {
}else{
viewPagerLength = response.getInt("total_items");
JSONArray feedArray = response.getJSONArray("feed");
for (int i = 0; i < feedArray.length(); i++) {
JSONObject feedObj = (JSONObject) feedArray.get(i);
Post item = new Post();
item.setId(feedObj.getInt("id"));
item.setName(feedObj.getString("name"));
item.setCategory(feedObj.getString("category"));
String image = feedObj.isNull("image_big") ? null : feedObj.getString("image_big");
item.setImge(image);
item.setTimeStamp(feedObj.getString("timeStamp"));
item.setAuthor(feedObj.getString("author"));
// url might be null sometimes
String feedUrl = feedObj.isNull("url") ? null : feedObj.getString("url");
item.setUrl(feedUrl);
viewPagerItems.add(item);
}
setupViewPager();
}
}
}catch (JSONException es) {
es.printStackTrace();
Toast.makeText(getActivity().getApplicationContext(), "Featured: " + getString(R.string.unknown_error), Toast.LENGTH_LONG).show();
}
}else{
}
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
VolleyLog.d(TAG, "Error: " + error.getMessage());
}
}) {
/** Passing some request headers **/
@Override
public Map<String, String> getHeaders() throws AuthFailureError {
HashMap<String, String> headers = new HashMap<String, String>();
headers.put("Content-Type", "application/json");
headers.put("ApiKey", Const.AuthenticationKey);
return headers;
}
};
// Adding request to volley request queue
AppController.getInstance().addToRequestQueue(jsonReq);
}
public void setupViewPager(){
layout_dots = (LinearLayout) rootView.findViewById(R.id.layout_dots);
viewPager = (ViewPager) rootView.findViewById(R.id.view_pager);
addBottomDots(0);
// viewpager change listener
ViewPager.OnPageChangeListener viewPagerPageChangeListener = new ViewPager.OnPageChangeListener() {
@Override
public void onPageSelected(int position) {
addBottomDots(position);
}
@Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
}
@Override
public void onPageScrollStateChanged(int arg0) {
}
};
myViewPagerAdapter = new MyViewPagerAdapter();
viewPager.setAdapter(myViewPagerAdapter);
viewPager.addOnPageChangeListener(viewPagerPageChangeListener);
}
private void addBottomDots(int currentPage) {
dots = new TextView[viewPagerLength];
layout_dots.removeAllViews();
for (int i = 0; i < Const.FeaturedView; i++) {
dots[i] = new TextView(getActivity());
dots[i].setText(Html.fromHtml("•"));
dots[i].setTextSize(35);
dots[i].setTextColor(getResources().getColor(R.color.viewpager_inactive));
layout_dots.addView(dots[i]);
}
if (dots.length > 0)
dots[currentPage].setTextColor(getResources().getColor(R.color.viewpager_active));
}
public class MyViewPagerAdapter extends PagerAdapter {
private LayoutInflater layoutInflater;
public MyViewPagerAdapter() {
}
@Override
public Object instantiateItem(ViewGroup container, int position) {
layoutInflater = (LayoutInflater) getActivity().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View view = layoutInflater.inflate(R.layout.viewpager_item, container, false);
final Post p = viewPagerItems.get(position);
TextView name = (TextView) view.findViewById(R.id.name);
name.setTypeface(font);
name.setTextSize(20);
name.setText(Html.fromHtml(p.getName()));
TextView category = (TextView) view.findViewById(R.id.category);
category.setText(Html.fromHtml(p.getCategory()));
ImageView img = (ImageView) view.findViewById(R.id.feedImage1);
Picasso.with(getActivity()).load(p.getImge()).into(img);
//TextView post_author = (TextView) view.findViewById(R.id.post_author);
//post_author.setText(String.format(getString(R.string.post_by), Html.fromHtml(p.getAuthor())));
CharSequence timeAgo = DateUtils.getRelativeTimeSpanString(
Long.parseLong(p.getTimeStamp()),
System.currentTimeMillis(), DateUtils.SECOND_IN_MILLIS);
TextView timestamp = (TextView) view.findViewById(R.id.timestamp);
// amin time
DateTime dateTime = new DateTime(Long.parseLong(p.getTimeStamp()));
dateTime.withZone(DateTimeZone.forID("Asia/Tehran"));
Roozh jCal = new Roozh();
jCal.GregorianToPersian(dateTime.getYear(), dateTime.getMonthOfYear(), dateTime.getDayOfMonth());
CharSequence timeAmin = jCal.toString();
// view time
if(System.currentTimeMillis() - Long.parseLong(p.getTimeStamp())<604800000)
timestamp.setText(String.format(getString(R.string.publish_on), timeAgo));
else
timestamp.setText(String.format(getString(R.string.publish_on), timeAmin));
view.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Intent i = new Intent();
i.setClass(getActivity(), PostViewActivity.class);
i.putExtra(PostViewActivity.TAG_SEL_POST_ID, "P" + p.getId());
i.putExtra(PostViewActivity.TAG_SEL_POST_TITLE, p.getName());
startActivity(i);
}
});
container.addView(view);
return view;
}
@Override
public int getCount() {
return viewPagerItems.size();
}
@Override
public boolean isViewFromObject(View view, Object obj) {
return view == obj;
}
@Override
public void destroyItem(ViewGroup container, int position, Object object) {
View view = (View) object;
container.removeView(view);
}
}
private void getLatestPosts(final String category_id){
String featured_url = Const.URL_BLOG + Const.URL_BLOG_API + Const.URL_FEATURED_CATEGORY_POST.replace("_CAT_ID_", category_id) + "?t=" + System.currentTimeMillis();
Log.d(TAG, featured_url);
// making fresh volley request and getting json
JsonObjectRequest jsonReq = new JsonObjectRequest(Request.Method.GET, featured_url, null, new Response.Listener<JSONObject>() {
@Override
public void onResponse(JSONObject response) {
if (response != null) {
try {
if (response.has("error")) {
String error = response.getString("error");
Toast.makeText(getActivity().getApplicationContext(), error, Toast.LENGTH_LONG).show();
}else {
if (response.isNull("feed")) {
}else{
LayoutInflater layoutInflater = (LayoutInflater) getActivity().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View view = layoutInflater.inflate(R.layout.gridview_category, null, false);
final TextView category_button = (TextView) view.findViewById(R.id.btnCategory);
final String category_name = response.getString("category");
TextView category_title = (TextView) view.findViewById(R.id.category_title);
category_title.setText(Html.fromHtml(category_name));
category_title.setTypeface(font);
category_title.setTextSize(20);
category_button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
((AppCompatActivity) getActivity()).getSupportActionBar().setSubtitle(category_name);
Fragment fragment = GridFragment.newInstance(GridFragment.PageType.CATEGORY, category_id, category_name);
FragmentManager fragmentManager = getFragmentManager();
fragmentManager.beginTransaction()
.replace(R.id.frame_container, fragment).commit();
}
});
LinearLayout hsv = (LinearLayout) view.findViewById(R.id.recycler_view);
JSONArray feedArray = response.getJSONArray("feed");
for (int i = 0; i < feedArray.length(); i++) {
JSONObject feedObj = (JSONObject) feedArray.get(i);
View lvg = layoutInflater.inflate(R.layout.listview_item_grid, null, false);
TextView post_name = (TextView) lvg.findViewById(R.id.post_name);
post_name.setText(Html.fromHtml(feedObj.getString("name")));
ImageView img = (ImageView) lvg.findViewById(R.id.feedImage1);
Picasso.with(getActivity()).load(feedObj.getString("image_big")).into(img);
final Post item = new Post();
item.setId(feedObj.getInt("id"));
item.setName(feedObj.getString("name"));
/*item.setCategory(feedObj.getString("category"));
String image = feedObj.isNull("image_big") ? null : feedObj.getString("image_big");
item.setImge(image);
item.setTimeStamp(feedObj.getString("timeStamp"));
// url might be null sometimes
String feedUrl = feedObj.isNull("url") ? null : feedObj.getString("url");
item.setUrl(feedUrl);
postList.add(item);*/
lvg.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Intent i = new Intent();
i.setClass(getActivity(), PostViewActivity.class);
i.putExtra(PostViewActivity.TAG_SEL_POST_ID, "P" + item.getId());
i.putExtra(PostViewActivity.TAG_SEL_POST_TITLE, item.getName());
startActivity(i);
}
});
hsv.addView(lvg);
}
grid_view_wrapper.addView(view);
}
}
}catch (JSONException es) {
es.printStackTrace();
Toast.makeText(getActivity().getApplicationContext(), getString(R.string.unknown_error), Toast.LENGTH_LONG).show();
}
}else{
}
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
VolleyLog.d(TAG, "Error: " + error.getMessage());
}
}) {
/** Passing some request headers **/
@Override
public Map<String, String> getHeaders() throws AuthFailureError {
HashMap<String, String> headers = new HashMap<String, String>();
headers.put("Content-Type", "application/json");
headers.put("ApiKey", Const.AuthenticationKey);
return headers;
}
};
// Adding request to volley request queue
AppController.getInstance().addToRequestQueue(jsonReq);
}
}
我在这行中有错误：
LayoutInflater layoutInflater = (LayoutInflater) getActivity().getSystemService(Context.LAYOUT_INFLATER_SERVICE)
扔掉这个：
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.app.Activity.getSystemService(java.lang.String)' on a null object reference
答案 0 :(得分：1)
如果在长时间运行的后台调用（如运行getActivity()后）上下文丢失，则null可以返回jsonRequest。
也许用户在请求运行时单击主页按钮，当它在onResponse方法中返回时，活动已被销毁，因此getActivity（）调用返回null对象。
这有多种解决方案;最简单的是getActivity()返回后的空检查。你也可以check this stackoverflow question
答案 1 :(得分：1)
我建议给PagerAdapter提供一个Context
public class MyViewPagerAdapter extends PagerAdapter {
private Context mContext;
public MyViewPagerAdapter(Context ctx) {
this.mContext = ctx;
}
@Override
public Object instantiateItem(ViewGroup container, int position) {
LayoutInflater inflater = LayoutInflater.from(mContext);
View view = inflater.inflate(R.layout.viewpager_item, container, false);
现在，您可以在任何上下文中使用该类，但是从Fragment中可以使用
myViewPagerAdapter = new MyViewPagerAdapter(getActivity());
尽管如此，出于其他原因，这可能仍然是空的。