ViewPager的适配器的编写

  public class FixPagerAdapter extends FragmentStatePagerAdapter {

//顶部TabLayout的文字
private String[] titles;
//保存Fragment的集合
private List<Fragment> fragments = null;

public void setFragments(List<Fragment> fragments) {
    this.fragments = fragments;

}

public void setTitles(String[] titles){
    this.titles=titles;

}

public FixPagerAdapter(FragmentManager fm) {
    super(fm);
}

@Override
public Fragment getItem(int position) {
    return fragments.get(position);
}

@Override
public int getCount() {
    return fragments.size();
}

@Override
public Object instantiateItem(ViewGroup container, int position) {
    Fragment fragment = null;
    try {
        fragment = (Fragment) super.instantiateItem(container, position);

    } catch (Exception e) {

    }

    return fragment;
}

@Override
public void destroyItem(ViewGroup container, int position, Object object) {

}

@Override
public CharSequence getPageTitle(int position) {
    return titles[position];
}

}

ViewPager继承FragmentPagerAdapter和FragmentStatePagerAdapter两种的区别

 两种适配器的写法是一样的，显示出来的效果基本是一样的，
 FragmentPagerAdapter适用于当viewpager中fragment数量少的
情况，FragmentStatePagerAdapter适用于fragment数量多的情况
下使用
两种adapter的创建，储存方式不同
两种adapter的销毁方式不同

FragmentPagerAdapter

创建
@SuppressWarnings("ReferenceEquality")
@Override
public Object instantiateItem(ViewGroup container, int position) {
    if (mCurTransaction == null) {
        mCurTransaction = mFragmentManager.beginTransaction();
    }

    final long itemId = getItemId(position);

    // Do we already have this fragment?
    String name = makeFragmentName(container.getId(), itemId);
    Fragment fragment = mFragmentManager.findFragmentByTag(name);
    if (fragment != null) {
        if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
        mCurTransaction.attach(fragment);
    } else {
        fragment = getItem(position);
        if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
        mCurTransaction.add(container.getId(), fragment,
                makeFragmentName(container.getId(), itemId));
    }
    if (fragment != mCurrentPrimaryItem) {
        fragment.setMenuVisibility(false);
        fragment.setUserVisibleHint(false);
    }

    return fragment;
}

销毁
@Override
public void destroyItem(ViewGroup container, int position, Object object) {
    if (mCurTransaction == null) {
        mCurTransaction = mFragmentManager.beginTransaction();
    }
    if (DEBUG) Log.v(TAG, "Detaching item #" + getItemId(position) + ": f=" + object
            + " v=" + ((Fragment)object).getView());
    mCurTransaction.detach((Fragment)object);
}
创建时，以键值对的形式，存储id，fragment，fragment名字
销毁时，不是将不可见的fragment销毁，而是
仅仅将该fragment从页面中detach掉，fragment还是在manager
中保存，内存没有被释放，所以FragmentPagerAdapter不适合
fragment数量多的情况下使用，因
为未被释放的fragment会占用大量内存。

FragmentStatePagerAdapter

创建
 @Override
public Object instantiateItem(ViewGroup container, int position) {
    // If we already have this item instantiated, there is nothing
    // to do.  This can happen when we are restoring the entire pager
    // from its saved state, where the fragment manager has already
    // taken care of restoring the fragments we previously had instantiated.
    if (mFragments.size() > position) {
        Fragment f = mFragments.get(position);
        if (f != null) {
            return f;
        }
    }

    if (mCurTransaction == null) {
        mCurTransaction = mFragmentManager.beginTransaction();
    }

    Fragment fragment = getItem(position);
    if (DEBUG) Log.v(TAG, "Adding item #" + position + ": f=" + fragment);
    if (mSavedState.size() > position) {
        Fragment.SavedState fss = mSavedState.get(position);
        if (fss != null) {
            fragment.setInitialSavedState(fss);
        }
    }
    while (mFragments.size() <= position) {
        mFragments.add(null);
    }
    fragment.setMenuVisibility(false);
    fragment.setUserVisibleHint(false);
    mFragments.set(position, fragment);
    mCurTransaction.add(container.getId(), fragment);

    return fragment;
}

销毁
  @Override
public void destroyItem(ViewGroup container, int position, Object object) {
    Fragment fragment = (Fragment) object;

    if (mCurTransaction == null) {
        mCurTransaction = mFragmentManager.beginTransaction();
    }
    if (DEBUG) Log.v(TAG, "Removing item #" + position + ": f=" + object
            + " v=" + ((Fragment)object).getView());
    while (mSavedState.size() <= position) {
        mSavedState.add(null);
    }
    mSavedState.set(position, fragment.isAdded()
            ? mFragmentManager.saveFragmentInstanceState(fragment) : null);
    mFragments.set(position, null);

    mCurTransaction.remove(fragment);
}

创建时会保存fragment位置状态，fragment,
销毁时会保存fragment位置状态，这样就会保存当前fragment的   
位置状态，创建时候，在当前位置创建，节约资源