ViewPager.SimpleOnPageChangeListener Does Not Function

问题

I have a ViewPager / PagerAdapter which should allow me to swipe through a series of footer images in order to change the station (by changing the string PLAYLIST):

...however when I swipe through the images - nothing seems to happen.

Any suggestions are greatly appreciated!

SOURCE:

public class Home extends YouTubeBaseActivity implements

VideoClickListener {

    private VideosListView listView;
    private ActionBarDrawerToggle actionBarDrawerToggle;
    public static final String API_KEY = "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";
    public static final String VIDEO_ID = "o7VVHhK9zf0";
    private int mCurrentTabPosition = NO_CURRENT_POSITION;
    private static final int NO_CURRENT_POSITION = -1;
    private DrawerLayout drawerLayout;
    private ListView drawerListView;
    private String[] drawerListViewItems;
    private ViewPager mPager;
    ScrollView mainScrollView;
    Button fav_up_btn1;
    Button fav_dwn_btn1;
    String TAG = "DEBUG THIS";
    String PLAYLIST = "EminemVEVO‎";
    private OnPageChangeListener mPageChangeListener;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.home);
        final ActionBar actionBar = getActionBar();
        ViewPager viewPager = (ViewPager) findViewById(R.id.view_pager);
        ImagePagerAdapter adapter = new ImagePagerAdapter();
        viewPager.setAdapter(adapter);

        actionBar.setCustomView(R.layout.actionbar_custom_view_home);
        actionBar.setDisplayShowTitleEnabled(false);
        actionBar.setDisplayShowCustomEnabled(true);


        drawerListViewItems = getResources().getStringArray(R.array.items);
        drawerListView = (ListView) findViewById(R.id.left_drawer);
        drawerListView.setAdapter(new ArrayAdapter<String>(this,
                R.layout.drawer_listview_item, drawerListViewItems));

        drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
        actionBarDrawerToggle = new ActionBarDrawerToggle(this, /* host Activity */
                drawerLayout, /* DrawerLayout object */
                R.drawable.ic_drawer, /* nav drawer icon to replace 'Up' caret */
                R.string.drawer_open, /* "open drawer" description */
                R.string.drawer_close /* "close drawer" description */
                );
        drawerLayout.setDrawerListener(actionBarDrawerToggle);

        getActionBar().setDisplayHomeAsUpEnabled(true);

        drawerLayout.setDrawerShadow(R.drawable.drawer_shadow,
                GravityCompat.START);

        mainScrollView = (ScrollView) findViewById(R.id.groupScrollView);
        listView = (VideosListView) findViewById(R.id.videosListView);
        listView.setOnVideoClickListener(this);
        new GetYouTubeUserVideosTask(responseHandler, PLAYLIST).execute();
    }

    Handler responseHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            populateListWithVideos(msg);
        };
    };

    private void populateListWithVideos(Message msg) {
        Library lib = (Library) msg.getData().get(
                GetYouTubeUserVideosTask.LIBRARY);
        listView.setVideos(lib.getVideos());
    }

    @Override
    protected void onStop() {
        responseHandler = null;
        super.onStop();
    }

    @Override
    public void onVideoClicked(Video video) {
        Intent intent = new Intent(Intent.ACTION_VIEW);
        intent.setData(Uri.parse(video.getUrl()));
        startActivity(intent);
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        actionBarDrawerToggle.onConfigurationChanged(newConfig);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu items for use in the action bar
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.main, menu);
        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        if (actionBarDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    private class ImagePagerAdapter extends PagerAdapter {


        public ImagePagerAdapter() {
            super();

        }

        private int[] mImages = new int[] { R.drawable.selstation_up_btn,
                R.drawable.classical_up_btn, R.drawable.country_up_btn,
                R.drawable.dance_up_btn, R.drawable.hiphop_up_btn };

        @Override
        public int getCount() {
            return mImages.length;
        }

        @Override
        public boolean isViewFromObject(View view, Object object) {
            return view == ((ImageView) object);
        }

        @Override
        public Object instantiateItem(ViewGroup container, int position) {
            Context context = Home.this;
            ImageView imageView = new ImageView(context);

            imageView.setImageResource(mImages[position]);
            ((ViewPager) container).addView(imageView, 0);
            return imageView;
        }

        @Override
        public void destroyItem(ViewGroup container, int position, Object object) {
            ((ViewPager) container).removeView((ImageView) object);
        }

        private final ViewPager.SimpleOnPageChangeListener mPageChangeListener = new ViewPager.SimpleOnPageChangeListener() {

            @Override
            public void onPageSelected(final int position) {
                onTabChanged(mPager.getAdapter(), mCurrentTabPosition, position);
                mCurrentTabPosition = position;

            }
        };

        protected void onTabChanged(final PagerAdapter adapter,
                final int oldPosition, final int newPosition) {

            if (oldPosition > newPosition) {
                // left to right
            } else {
                // right to left
                String PLAYLIST = "TimMcGrawVEVO‎";

                View vg = findViewById(R.layout.home);
                vg.invalidate();
            }
            final ViewPager viewPager = (ViewPager) findViewById(R.id.view_pager);

            viewPager.setOnPageChangeListener(new OnPageChangeListener() {

                int oldPos = viewPager.getCurrentItem();

                @Override
                public void onPageScrolled(int position, float arg1, int arg2) {

                    if (position > oldPos) {
                        // Moving to the right

                    } else if (position < oldPos) {
                        // Moving to the Left
                        String PLAYLIST = "TimMcGrawVEVO‎";

                        View vg = findViewById(R.layout.home);
                        vg.invalidate();
                    }
                }

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

                }

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

                }

            });

        }
    }
}

回答1:

You're not adding the Listener to your Pager.

You need to call setOnPageChangeListener(mPageChangeListener) on your pagerAdapter.

For example in the Constructor:

public ImagePagerAdapter(){
   super();
   setOnPageChangeListener(mPageChangeListener);
}

for more info, check the docs. http://developer.android.com/reference/android/support/v4/view/ViewPager.html

来源：https://stackoverflow.com/questions/20427059/viewpager-simpleonpagechangelistener-does-not-function