抽屉图像你好朋友们，我正在我的项目中使用导航抽屉，但在尝试打开活动时遇到上述错误 listview-listitem（第二次）当我们单击列表项时，该应用程序在第一次完美运行，但是如果我们返回并单击抽屉菜单的任何项，然后如果我们尝试单击列表项，则会发生错误列表视图
FRAGMENT.java
package com.forever.technology.bscit;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.ListFragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import com.forever.technology.bscit.R;
/**
* A simple {@link Fragment} subclass.
*/
public class Sem1Fragment_list extends ListFragment {
public Sem1Fragment_list() {
// Required empty public constructor
}
ListView lv;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_sem1_list, container, false);
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
ArrayAdapter adapter = ArrayAdapter.createFromResource(getActivity(),
R.array.sem1, android.R.layout.simple_list_item_1);
setListAdapter(adapter);
}
@Override
public void onListItemClick(ListView l, View v, int position, long id) {
super.onListItemClick(l, v, position, id);
switch (position)
{
case 0:
Intent intent = new Intent(getContext(),Subject1Activity.class);
startActivity(intent);
}
}
}
主要活动
package com.forever.technology.bscit;
import android.content.ComponentName;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.view.View;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
public class MainActivity extends AppCompatActivity
implements NavigationView.OnNavigationItemSelectedListener {
NavigationView navigationView = null;
Toolbar toolbar = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//Set the fragment initially
HomeFragment fragment = new HomeFragment();
android.support.v4.app.FragmentTransaction fragmentTransaction =
getSupportFragmentManager().beginTransaction();
fragmentTransaction.replace(R.id.fragment_container, fragment);
fragmentTransaction.commit();
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
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) findViewById(R.id.nav_view);
//How to change elements in the header programatically
/* View headerView = navigationView.getHeaderView(0);
TextView emailText = (TextView) headerView.findViewById(R.id.email);
emailText.setText("newemail@email.com"); */
navigationView.setNavigationItemSelectedListener(this);
}
@Override
public void onBackPressed() {
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
} else {
super.onBackPressed();
}
}
@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);
return true;
}
@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();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
// Handle navigation view item clicks here.
int id = item.getItemId();
if (id == R.id.nav_home) {
//Set the fragment initially
HomeFragment fragment = new HomeFragment();
android.support.v4.app.FragmentTransaction fragmentTransaction =
getSupportFragmentManager().beginTransaction();
fragmentTransaction.replace(R.id.fragment_container, fragment);
fragmentTransaction.commit();
// Handle the camera action
} else if (id == R.id.nav_sem1) {
//Set the fragment initially
Sem1Fragment fragment = new Sem1Fragment();
android.support.v4.app.FragmentTransaction fragmentTransaction =
getSupportFragmentManager().beginTransaction();
fragmentTransaction.replace(R.id.fragment_container, fragment);
fragmentTransaction.commit();
// Handle the camera action
} else if (id == R.id.nav_sem2) {
//Set the fragment initially
Sem2Fragment fragment = new Sem2Fragment();
android.support.v4.app.FragmentTransaction fragmentTransaction =
getSupportFragmentManager().beginTransaction();
fragmentTransaction.replace(R.id.fragment_container, fragment);
fragmentTransaction.commit();
} else if (id == R.id.nav_sem3) {
} else if (id == R.id.nav_sem4) {
} else if (id == R.id.nav_sem5) {
} else if (id == R.id.nav_sem6) {
}
else if (id == R.id.nav_blogs) {
}
else if (id == R.id.nav_site) {
}
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}
}
主题1.活动
package com.forever.technology.bscit;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
public class Subject1Activity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.subject1);
}
}
洛格猫
03-23 01:06:33.111 11547-11547/com.compscitutorials.basigarcia.navigationdrawervideotutorial E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.compscitutorials.basigarcia.navigationdrawervideotutorial, PID: 11547
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.content.ComponentName.<init>(ComponentName.java:128)
at android.content.Intent.<init>(Intent.java:4449)
at com.forever.technology.bscit.Sem1Fragment_list.onListItemClick(Sem1Fragment_list.java:50)
at android.support.v4.app.ListFragment$2.onItemClick(ListFragment.java:58)
at android.widget.AdapterView.performItemClick(AdapterView.java:310)
at android.widget.AbsListView.performItemClick(AbsListView.java:1145)
at android.widget.AbsListView$PerformClick.run(AbsListView.java:3066)
at android.widget.AbsListView$3.run(AbsListView.java:3903)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
问题出在Sem1Fragment.java代码上。您拥有膨胀视图的静态引用"rootView"。再次从NavigationDrawer中选择相同的片段时，您再次实例化相同的片段，但由于膨胀的视图引用是静态的，它不会被刷新，并且将包含过时的上下文，因此getContext()为空。因此更改您的代码 -
static View rootView;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
if (rootView == null) {
rootView = inflater.inflate(R.layout.fragment_sem1, container, false);
}
return rootView;
}
由
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_sem1, container, false);
}
再做一个改进
- 您应该
onNavigationItemSelected()检查您尝试实例化的片段是否已经可见，然后不要再次实例化它。
如果我的理解是正确的，你正在开始一个与MainActivity不同的片段Sem1Fragment。因此，具有列表的实际片段，即Sem1Fragment_list没有启动Subject1Activity所需的上下文。因此，只需将导航抽屉代码中的Sem1Fragment替换为Sem1Fragment_list，如下所示。
主活动.java
public class MainActivity extends AppCompatActivity
implements NavigationView.OnNavigationItemSelectedListener {
NavigationView navigationView = null;
Toolbar toolbar = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//Set the fragment initially
HomeFragment fragment = new HomeFragment();
android.support.v4.app.FragmentTransaction fragmentTransaction =
getSupportFragmentManager().beginTransaction();
fragmentTransaction.add(R.id.fragment_container, fragment);
fragmentTransaction.commit();
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
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) findViewById(R.id.nav_view);
//How to change elements in the header programatically
/* View headerView = navigationView.getHeaderView(0);
TextView emailText = (TextView) headerView.findViewById(R.id.email);
emailText.setText("newemail@email.com"); */
navigationView.setNavigationItemSelectedListener(this);
}
@Override
public void onBackPressed() {
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
} else {
super.onBackPressed();
}
}
@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);
return true;
}
@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();
//noinspection SimplifiableIfStatement
if (id == R.id.action_about) {
return true;
}
if (id == R.id.action_exit) {
AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setCancelable(false);
builder.setMessage("Do you want to Exit?");
builder.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
//if user pressed "yes", then he is allowed to exit from application
finish();
}
});
builder.setNegativeButton("No", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
//if user select "No", just cancel this dialog and continue with app
dialog.cancel();
}
});
AlertDialog alert = builder.create();
alert.show();
return true;
}
return super.onOptionsItemSelected(item);
}
@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
// Handle navigation view item clicks here.
int id = item.getItemId();
if (id == R.id.nav_home) {
//Set the fragment initially
HomeFragment fragment = new HomeFragment();
android.support.v4.app.FragmentTransaction fragmentTransaction =
getSupportFragmentManager().beginTransaction();
fragmentTransaction.replace(R.id.fragment_container, fragment);
fragmentTransaction.commit();
// Handle the camera action
} else if (id == R.id.nav_sem1) {
//Set the fragment initially
Sem1Fragment_list fragment = new Sem1Fragment_list();
android.support.v4.app.FragmentTransaction fragmentTransaction =
getSupportFragmentManager().beginTransaction();
fragmentTransaction.replace(R.id.fragment_container, fragment);
fragmentTransaction.addToBackStack(null); //add this to restore the fragment when back button is pressed
fragmentTransaction.commit();
// Handle the camera action
} else if (id == R.id.nav_sem2) {
//Set the fragment initially
Sem2Fragment fragment = new Sem2Fragment();
android.support.v4.app.FragmentTransaction fragmentTransaction =
getSupportFragmentManager().beginTransaction();
fragmentTransaction.replace(R.id.fragment_container, fragment);
fragmentTransaction.addToBackStack(null);
fragmentTransaction.commit();
} else if (id == R.id.nav_sem3) {
} else if (id == R.id.nav_sem4) {
} else if (id == R.id.nav_sem5) {
} else if (id == R.id.nav_sem6) {
}
else if (id == R.id.nav_blogs) {
try {
//If App installed
Intent intent;
intent = getPackageManager().getLaunchIntentForPackage("com.forevertech.app");
startActivity(intent);
} catch (Exception e) { //google play app is not installed
Intent intent = new Intent(Intent.ACTION_VIEW);
intent.setData(Uri.parse("https://play.google.com/store/apps/details?id=com.forevertech.app"));
startActivity(intent);
}
}
else if (id == R.id.nav_site) {
Intent intent = new Intent(Intent.ACTION_VIEW);
intent.setData(Uri.parse("http://www.abaagekya.com/"));
startActivity(intent);
}
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}
}
请参阅 Web 上的任何导航抽屉示例。您必须在基本活动中实现导航抽屉，并且您的所有活动都应扩展基本活动。
这是我的工作代码：
基本活动
package com.projects.activity;
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.ActionBarDrawerToggle;
import android.support.v4.widget.DrawerLayout;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.FrameLayout;
import android.widget.ListView;
import com.ajay.projects.adapters.NavigationDrawerListAdapter;
import com.ajay.projects.models.Items;
import java.util.ArrayList;
/**
* @author ajay.barve@gmail.com
* <p/>
* This activity will add Navigation Drawer for our application and all the code related to navigation drawer.
* We are going to extend all our other activities from this BaseActivity so that every activity will have Navigation Drawer in it.
* This activity layout contain one frame layout in which we will add our child activity layout.
*/
public class BaseActivity extends Activity implements View.OnClickListener, AdapterView.OnItemSelectedListener {
/**
* Static variable for selected item position. Which can be used in child activity to know which item is selected from the list.
*/
protected static int position;
/**
* This flag is used just to check that launcher activity is called first time so that we can open appropriate Activity on launch and make list item position selected accordingly.
*/
private static boolean isLaunch = true;
/**
* Frame layout: Which is going to be used as parent layout for child activity layout.
* This layout is protected so that child activity can access this
*/
protected FrameLayout frameLayout;
/**
* ListView to add navigation drawer item in it.
* We have made it protected to access it in child class. We will just use it in child class to make item selected according to activity opened.
*/
protected ListView mDrawerList;
/**
* List item array for navigation drawer items.
*/
protected String[] listArray = {"About CML Dashboard", "About Jaslok Onchology Department", "View Report", "Add Record", "CBC Trend", "Tabular Report"};
protected ArrayList<Items> _items;
boolean dataExists = false;
/**
* Base layout node of this Activity
*/
private DrawerLayout mDrawerLayout;
/**
* Drawer listner class for drawer open, close etc.
*/
private ActionBarDrawerToggle actionBarDrawerToggle;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.navigation_drawer_base_layout);
frameLayout = (FrameLayout) findViewById(R.id.content_frame);
mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
mDrawerList = (ListView) findViewById(R.id.left_drawer);
// set a custom shadow that overlays the main content when the drawer opens
//mDrawerLayout.setDrawerShadow(R.drawable.drawer_shadow, GravityCompat.START);
_items = new ArrayList<Items>();
_items.add(new Items("About Jaslok Hospital", "Lean about Jaslok Hospital & Jaslok onchology department", R.drawable.item_1));
_items.add(new Items("View Report", "View CBC Reports stored int he device", R.drawable.item_2));
_items.add(new Items("Add Record", "Add a new CBC Report", R.drawable.item_3));
_items.add(new Items("CBC Trend", "View trends of each parameter in CBC Report", R.drawable.item_4));
_items.add(new Items("Tabular Report", "View tabular CBC Report", R.drawable.item_5));
//Adding header on list view
View header = getLayoutInflater().inflate(R.layout.list_view_header_layout, null);
mDrawerList.addHeaderView(header);
// set up the drawer's list view with items and click listener
mDrawerList.setAdapter(new NavigationDrawerListAdapter(this, _items));
mDrawerList.setOnItemClickListener(new OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view,
int position, long id) {
openActivity(position);
Log.e("Logger:BaseActivity", "onItemClick-Enter");
}
});
// enable ActionBar app icon to behave as action to toggle nav drawer
getActionBar().setDisplayHomeAsUpEnabled(true);
getActionBar().setHomeButtonEnabled(true);
// ActionBarDrawerToggle ties together the the proper interactions between the sliding drawer and the action bar app icon
actionBarDrawerToggle = new ActionBarDrawerToggle(
this, /* host Activity */
mDrawerLayout, /* DrawerLayout object */
R.drawable.aj_launcher, /* nav drawer image to replace 'Up' caret */
R.string.open_drawer, /* "open drawer" description for accessibility */
R.string.close_drawer) /* "close drawer" description for accessibility */ {
@Override
public void onDrawerClosed(View drawerView) {
getActionBar().setTitle(listArray[position]);
invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
super.onDrawerClosed(drawerView);
}
@Override
public void onDrawerOpened(View drawerView) {
getActionBar().setTitle(getString(R.string.app_name));
invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
super.onDrawerOpened(drawerView);
}
@Override
public void onDrawerSlide(View drawerView, float slideOffset) {
super.onDrawerSlide(drawerView, slideOffset);
}
@Override
public void onDrawerStateChanged(int newState) {
super.onDrawerStateChanged(newState);
}
};
mDrawerLayout.setDrawerListener(actionBarDrawerToggle);
}
/*End of OnCreate*/
/*Start OnClick*/
/* Comment : Have to implement with the OnClickListner onClick is called when a view has been clicked.*/
public void onClick(View v) { // Parameter v stands for the view that was clicked.
Log.e("Logger:BaseActivity", "onClick:Enter");
final Context context = this;
}
/*End of onClick*/
/**
* @param position Launching activity when any list item is clicked.
*/
protected void openActivity(int position) {
Log.e("Logger:BaseActivity", "openActivity-Enter");
/**
* We can set title & itemChecked here but as this BaseActivity is parent for other activity,
* So whenever any activity is going to launch this BaseActivity is also going to be called and
* it will reset this value because of initialization in onCreate method.
* So that we are setting this in child activity.
*/
// mDrawerList.setItemChecked(position, true);
// setTitle(listArray[position]);
mDrawerLayout.closeDrawer(mDrawerList);
BaseActivity.position = position; //Setting currently selected position in this field so that it will be available in our child activities.
switch (position) {
case 0:
startActivity(new Intent(this, HomeActivity.class));
break;
case 1:
startActivity(new Intent(this, AboutActivity.class));
break;
case 2:
startActivity(new Intent(this, MainActivity.class));
break;
case 3:
startActivity(new Intent(this, AddActivity.class));
break;
case 4:
startActivity(new Intent(this, TrendActivity.class));
break;
case 5:
startActivity(new Intent(this, TabularActivity.class));
break;
default:
break;
}
// Toast.makeText(this, "Selected Item Position::"+position, Toast.LENGTH_LONG).show();
}
//@Override
public boolean onCreateOptionsMenu(Menu menu) {
Log.e("Logger:BaseActivity", "onCreateOptionsMenu-Enter");
getMenuInflater().inflate(R.menu.main, menu);
return super.onCreateOptionsMenu(menu);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
Log.e("Logger:BaseActivity", "onOptionsItemSelected-Enter");
// The action bar home/up action should open or close the drawer.
// ActionBarDrawerToggle will take care of this.
if (actionBarDrawerToggle.onOptionsItemSelected(item)) {
Log.e("Logger:BaseActivity", "onOptionsItemSelected-Enter"+item);
return true;
}
switch (item.getItemId()) {
case R.id.action_settings:
Log.e("Logger:BaseActivity", "onOptionsItemSelected-getItemId"+item.getItemId());
return true;
default:
return super.onOptionsItemSelected(item);
}
}
/* Called whenever we call invalidateOptionsMenu() */
//@Override
public boolean onPrepareOptionsMenu(Menu menu) {
Log.e("Logger:BaseActivity", "onPrepareOptionsMenu-Enter");
// If the nav drawer is open, hide action items related to the content view
boolean drawerOpen = mDrawerLayout.isDrawerOpen(mDrawerList);
menu.findItem(R.id.action_settings).setVisible(!drawerOpen);
return super.onPrepareOptionsMenu(menu);
}
/* We can override onBackPressed method to toggle navigation drawer*/
@Override
public void onBackPressed() {
Log.e("Logger:BaseActivity", "onBackPressed-Enter");
if (mDrawerLayout.isDrawerOpen(mDrawerList)) {
mDrawerLayout.closeDrawer(mDrawerList);
} else {
mDrawerLayout.openDrawer(mDrawerList);
}
}
@Override
public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
Log.e("Logger:BaseActivity", "onItemSelected-Enter");
}
@Override
public void onNothingSelected(AdapterView<?> adapterView) {
}
}
主页活动
package com.projects.activity;
import android.os.Bundle;
import android.widget.ListView;
import com.ajay.projects.adapters.HomeScreenGridViewAdapter;
/**
* @author ajay.barve@gmail.com
*/
public class HomeActivity extends BaseActivity {
//private GridView gridview;
private ListView gridview;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
/**
* We will not use setContentView in this activty Rather than we will
* use layout inflater to add view in FrameLayout of our base activity
* layout
*/
/**
* Adding our layout to parent class frame layout.
*/
getLayoutInflater().inflate(R.layout.home_layout, frameLayout);
/**
* Setting title and itemChecked
*/
mDrawerList.setItemChecked(position, true);
setTitle(listArray[position]);
initializeActivity();
}
private void initializeActivity() {
gridview = (ListView) findViewById(R.id.gridview);
gridview.setAdapter(new HomeScreenGridViewAdapter(this, _items));
}
}
代码是不言自明的。希望这个例子有帮助。