我想恢复我在viewpager中存储在共享偏好中的活动。我已将活动详细信息存储在onSaveState中，并在onResume中检索活动getpreferences。但不知何故，它返回错误，键在onSaveState中为空值。如何在viewpager中保存首选项？
这是代码
public class FullScreenImageActivity extends AppCompatActivity {
ViewPager viewPager;
ImageView favBtn;
private SharedPreference sharedPreference;
Activity context = this;
private Fragment contentFragment;
@TargetApi(Build.VERSION_CODES.GINGERBREAD)
public void onCreate(Bundle savedInstanceState) {
this.requestWindowFeature(Window.FEATURE_NO_TITLE);
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_fullscreen_image); //TBS
FragmentManager fragmentManager = getSupportFragmentManager();
if (Build.VERSION.SDK_INT > 9) {
StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
StrictMode.setThreadPolicy(policy);
}
if (savedInstanceState.containsKey("content")) {
String content = savedInstanceState.getString("content");
if (content.equals(FavouriteListFragment.ARG_ITEM_ID)) {
if (fragmentManager.findFragmentByTag(FavouriteListFragment.ARG_ITEM_ID) != null) {
contentFragment = fragmentManager
.findFragmentByTag(FavouriteListFragment.ARG_ITEM_ID);
}
}
}
Intent i = getIntent();
int position = i.getIntExtra("position", 0);
String[] arr=i.getStringArrayExtra("array");
String url = i.getStringExtra("url");
sharedPreference = new SharedPreference();
viewPager = (ViewPager) findViewById(R.id.slider);
viewPager.setAdapter(new GalleryViewPagerAdapter(this, arr, position, url));
viewPager.setCurrentItem(position);
}
class GalleryViewPagerAdapter extends PagerAdapter {
private Context context;
LayoutInflater inflater;
private String[] imageArrayList;
private String mUrl;
private int mPosition;
private ProgressDialog mProgress;
public GalleryViewPagerAdapter(Context _context, String[] imageArrayList, int position, String url) {
context = _context;
this.imageArrayList = imageArrayList; //null;
this.mPosition=position;
inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
mUrl=url;
}
@Override
public int getCount() {
return imageArrayList.length;
}
@Override
public Object instantiateItem(final ViewGroup container, final int position) {
showProgress();
favBtn = (ImageView) findViewById(R.id.btn_favourite);
String favoriteUrl=sharedPreference.getPrefsFavUrl(context);
//Boolean stateBtnNow=sharedPreference.getBtnState(context,mUrl);//why mUrl change to same url with favourite
Boolean stateBtnNow=false;
if(stateBtnNow) {
favBtn.setColorFilter(Color.argb(255, 249, 0, 0));//red
}
else
{
favBtn.setColorFilter(Color.argb(255, 192, 192, 192));
}
favBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
//Boolean stateBtn = sharedPreference.getBtnState(context, mUrl);
Boolean stateBtn=false;
if (!stateBtn) {
sharedPreference.save(context, mUrl);
//edit to save url in array
//sharepreference.add(context, product);
sharedPreference.saveBtnState(context, stateBtn);
Toast.makeText(context,
"Added to Favourite!",
Toast.LENGTH_SHORT).show();
favBtn.setColorFilter(Color.argb(255, 249, 0, 0));//red
Toast.makeText(context,
"Button state is " + stateBtn,
Toast.LENGTH_SHORT).show();
} else {
sharedPreference.saveBtnState(context, stateBtn);
favBtn.setColorFilter(Color.argb(255, 192, 192, 192));//grey
}
}
});
View photoRow = inflater.inflate(R.layout.item_fullscreen_image, container,
false);
TouchImageView fullScreenImg;
fullScreenImg =(TouchImageView)photoRow.findViewById(R.id.img_flickr);
// added imageloader for better performance
StaggeredDemoApplication.getImageLoader().get(imageArrayList[position],
ImageLoader.getImageListener(fullScreenImg, R.drawable.bg_no_image, android.R.drawable.ic_dialog_alert), container.getWidth(), 0);
((ViewPager) container).addView(photoRow);
stopProgress();
return photoRow;
}
private void stopProgress() {
mProgress.cancel();
}
private void showProgress() {
mProgress = ProgressDialog.show(FullScreenImageActivity.this, "", "Loading...");
}
@Override
public boolean isViewFromObject(View view, Object object) {
return view == ((RelativeLayout) object);
}
@Override
public void destroyItem(ViewGroup container, int position, Object object) {
// Remove viewpager_item.xml from ViewPager
((ViewPager) container).removeView((RelativeLayout) object);
}
}
@Override
protected void onSaveInstanceState(Bundle outState) {
outState.putString("content", FavouriteListFragment.ARG_ITEM_ID);
super.onSaveInstanceState(outState);
}
@Override
public void onResume() {
super.onResume();
}
}
以下是sharepreferences中的代码
public class SharedPreference {
public static final String PREFS_NAME = "AOP_PREFS";
public static final String PREFS_STATE="AOP_BTN";
public static final String PREFS_FAV_URL="AOP_FAV_URL";
public static final String PREFS_KEY = "AOP_PREFS_String";
public static final String PREF_BTN_KEY = "AOP_PREF_BTN";
public static final String PREF_URL_KEY = "AOP_PREF_URL";
public SharedPreference() {
super();
}
public void save(Context context, String text) {
SharedPreferences settings;
Editor editor;
//edit here to add image array list
//settings = PreferenceManager.getDefaultSharedPreferences(context);
settings = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE); //1
editor = settings.edit(); //2
editor.putString(PREFS_KEY, text); //3
editor.commit(); //4
}
public String getValue(Context context) {
SharedPreferences settings;
String text;
//settings = PreferenceManager.getDefaultSharedPreferences(context);
settings = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
text = settings.getString(PREFS_KEY, null);
return text;
}
public String getPrefsFavUrl(Context context){
SharedPreferences settings;
String favUrl;
settings = context.getSharedPreferences(PREFS_FAV_URL, Context.MODE_PRIVATE);
favUrl = settings.getString(PREF_URL_KEY, null);
return favUrl;
}
public void clearSharedPreference(Context context) {
SharedPreferences settings;
Editor editor;
//settings = PreferenceManager.getDefaultSharedPreferences(context);
settings = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
editor = settings.edit();
editor.clear();
editor.commit();
}
public void removeValue(Context context) {
SharedPreferences settings;
Editor editor;
settings = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
editor = settings.edit();
editor.remove(PREFS_KEY);
editor.commit();
}
public void saveBtnState(Context context, Boolean stateBtn) {
SharedPreferences settings;
Editor editor;
//settings = PreferenceManager.getDefaultSharedPreferences(context);
settings = context.getSharedPreferences(PREFS_STATE, Context.MODE_PRIVATE); //1
editor = settings.edit(); //2
editor.putBoolean(PREF_BTN_KEY, stateBtn);//added state for button
editor.commit(); //4
}
public boolean getBtnState(Context context,String text)//edit to store url here and check the boolean here
{
SharedPreferences prefs;
prefs = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
String favUrl = getPrefsFavUrl(context);
boolean switchState = false;
if(favUrl.equals(text)) {
switchState=true;
}
return switchState;
}
public void saveFavourite(FragmentActivity activity, String favouriteUrl) {
SharedPreferences settings;
Editor editor;
settings = activity.getSharedPreferences(PREFS_FAV_URL, Context.MODE_PRIVATE); //1
editor = settings.edit(); //2
editor.putString(PREF_URL_KEY, favouriteUrl); //3
editor.commit(); //4
}
}
这是错误日志文件
02-03 09:48:57.928 15368-15368/com.example.myapp W/System.err: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.lang.Class.newInstance()' on a null object reference
02-03 09:48:57.928 15368-15368/com.example.myapp W/System.err: at com.example.myapp.MainActivity.displayView(MainActivity.java:141)
02-03 09:48:57.928 15368-15368/com.example.myapp W/System.err: at com.example.myapp.MainActivity.onCreate(MainActivity.java:41)
02-03 09:48:57.928 15368-15368/com.example.myapp W/System.err: at android.app.Activity.performCreate(Activity.java:5990)
这是我的MainActivity计划
public class MainActivity extends AppCompatActivity
implements NavigationView.OnNavigationItemSelectedListener, FragmentFlickrGridImage.OnFragmentInteractionListener, ShareActionProvider.OnShareTargetSelectedListener {
private int backstack_count;
private boolean viewIsAtHome;
private Fragment contentFragment;
private ShareActionProvider share;
private Intent shareIntent=new Intent(Intent.ACTION_SEND);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
if (savedInstanceState == null) {
displayView(R.id.recent_picture);
}
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
drawer.setDrawerListener(toggle);
toggle.syncState();
NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
navigationView.setItemIconTintList(null);
navigationView.setNavigationItemSelectedListener(this);
shareIntent.setType("text/plain");
}
private void replaceFragment(Fragment fragment, boolean isAddToBackStack) {
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
transaction.replace(R.id.flContent, fragment);
if (isAddToBackStack) {
transaction.addToBackStack(fragment.getClass().toString());
backstack_count ++;
}
transaction.commit();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
MenuItem item = menu.findItem(R.id.share);
share = new ShareActionProvider(this);
MenuItemCompat.setActionProvider(item, share);
return(super.onCreateOptionsMenu(menu));
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.share) {
onShareAction();
return true;
}
return super.onOptionsItemSelected(item);
}
private void onShareAction() {
String yourShareText = "test";
Intent shareIntent = ShareCompat.IntentBuilder.from(this).setType("text/plain").setText(yourShareText).getIntent();
// Set the share Intent
if (share != null) {
share.setShareIntent(shareIntent);
}
}
@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
// Handle navigation view item clicks here.
int id = item.getItemId();
displayView(item.getItemId());
return true;
}
private void displayView(int itemId) {
Fragment fragment = null;
Class fragmentClass = null;
String title="";
switch (itemId){
case R.id.one:
fragment = new FragmentFlickrGridImage("one");
title="one";
viewIsAtHome=true;
break;
case R.id.two:
fragment = new FragmentFlickrGridImage(" two");
title ="two";
viewIsAtHome=false;
break;
case R.id.three:
fragment = new FragmentFlickrGridImage("three");
title="three";
viewIsAtHome=false;
break;
case R.id.four:
fragment=new FavouriteListFragment();
title="Favourite";
viewIsAtHome=false;
break;
}
try {
fragment = (Fragment) fragmentClass.newInstance();
} catch (Exception e) {
e.printStackTrace();
}
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();
getSupportActionBar().setTitle(title);
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
}
@Override
public void onFragmentInteraction(Uri uri) {
}
@Override
public void onBackPressed() {
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
}
if (!viewIsAtHome) { //if the current view is not the Recent Hot Girl fragment
displayView(R.id.recent_picture);
} else {
DialogInterface.OnClickListener dialogClickListener = new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
switch (which){
case DialogInterface.BUTTON_POSITIVE:
//Yes button clicked
finish();
break;
case DialogInterface.BUTTON_NEGATIVE:
//No button clicked
break;
}
}
};
AlertDialog.Builder builder = new AlertDialog.Builder(this)
.setMessage("Are you sure?")
.setPositiveButton("Yes", dialogClickListener)
.setNegativeButton("No", dialogClickListener)
.setCancelable(false)
.setTitle("Exit");
builder.show();
}
}
@Override
public boolean onShareTargetSelected(ShareActionProvider source,
Intent intent) {
Toast.makeText(this, intent.getComponent().toString(),
Toast.LENGTH_LONG).show();
return(false);
}
}
答案 0 :(得分：0)
错误行是：
fragment = (Fragment) fragmentClass.newInstance();
fragmentClass在null中使用fragmentClass.newInstance()对象时始终会被分配null，并且它始终具有 try {
fragment = (Fragment) fragmentClass.newInstance();
} catch (Exception e) {
e.printStackTrace();
}
值，因为您从未为其分配其他值。尝试删除这些行：
if (fragment != null){
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();
}
无论如何，对片段使用非空构造函数不是一个好习惯。您应该使用this pattern代替。
修改
检查片段是否为空：
Projectile