Question

我正在粘贴所有代码，因为我不知道我哪里出错了。该应用程序正常工作，直到我让//mViewPager.setCurrentItem（tab.getPosition（））;进入行动，应用程序不再启动，我试图按照开发人员网站上的步骤。还将理解对该问题的解释。谢谢！

package com.basel.ribbit;

import android.app.ActionBar;
import android.app.ActionBar.Tab;
import android.app.FragmentTransaction;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;

import com.parse.ParseAnalytics;
import com.parse.ParseUser;

public class MainActivity extends FragmentActivity {



    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a {@link FragmentPagerAdapter}
     * derivative, which will keep every loaded fragment in memory. If this
     * becomes too memory intensive, it may be best to switch to a
     * {@link android.support.v13.app.FragmentStatePagerAdapter}.
     */
    SectionsPagerAdapter mSectionsPagerAdapter;

    /**
     * The {@link ViewPager} that will host the section contents.
     */
    ViewPager mViewPager;
    public static final String TAG = MainActivity.class.getSimpleName();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //createActionBar();

        ParseAnalytics.trackAppOpened(getIntent());
        createActionBar();

        ParseUser currentUser = ParseUser.getCurrentUser();

        if(currentUser == null){
            navigateToLogin();}else{
                Log.i(TAG,"A NEW USER HAS LOGGED IN    " +  currentUser.getUsername());
            }


        // Create the adapter that will return a fragment for each of the three
        // primary sections of the activity.
        mSectionsPagerAdapter = new SectionsPagerAdapter(this, getSupportFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mSectionsPagerAdapter);
        setPagerOnItemSelected();


    }

    private void setPagerOnItemSelected() {
        mViewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

            @Override
            public void onPageSelected(int position) {
                // TODO Auto-generated method stub
            //  getActionBar().setSelectedNavigationItem(position);

            }

            @Override
            public void onPageScrolled(int arg0, float arg1, int arg2) {
                // TODO Auto-generated method stub

            }

            @Override
            public void onPageScrollStateChanged(int arg0) {
                // TODO Auto-generated method stub

            }
        });
    }

private void createActionBar() {
        final ActionBar actionBar = getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
        ActionBar.TabListener tabListener = new ActionBar.TabListener() {

            @Override
            public void onTabUnselected(Tab tab, FragmentTransaction ft) {
                // TODO Auto-generated method stub

            }

            @Override
            public void onTabSelected(Tab tab, FragmentTransaction ft) {
                // TODO Auto-generated method stub
            //mViewPager.setCurrentItem(tab.getPosition());
            }

            @Override
            public void onTabReselected(Tab tab, FragmentTransaction ft) {
                // TODO Auto-generated method stub

            }
        };
        actionBar.addTab(actionBar.newTab().setText("Hello").setTabListener(tabListener));
        actionBar.addTab(actionBar.newTab().setText("Hi").setTabListener(tabListener));



    }

    private void navigateToLogin() {
        Intent intent = new Intent (this, LoginActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(intent);
    }

    @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 onMenuItemSelected(int featureId, MenuItem item) {
        // TODO Auto-generated method stub
        int id = item.getItemId();
        if(id == R.id.action_logout){
            ParseUser.logOut();
            navigateToLogin();

        }
        return super.onMenuItemSelected(featureId, item);


    enter code here
    }

    @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.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

}

Answer 1

在createActionBar();

之前移动此行

mViewPager = (ViewPager) findViewById(R.id.pager);

您正在崩溃，因为您尚未初始化mViewPager，并且您正在尝试使用它。如果你等到设置mViewPager之后，它应该可以工作。

另一种可能性是，activity_main.xml中的ViewPager标识为pager。

使用选项卡设置滑动视图时应用程序崩溃

1 个答案: