//調整主會場位置偽代碼
marketingInfoList.add(new MarketingInfo("www.juejin.com", "主會場"))
for (int i = 0; i <= 3; i++) {
    //將放在前兩個主會場前面
    if (i < 2) {
        marketingInfoList.add(i, new MarketingInfo("www.baidu.com", "模擬" + i));
    } else {
    //后兩個往主會場后面添加
        marketingInfoList.add(new MarketingInfo("www.baidu.com", "模擬" + i));
    }
}
mPagerAdapter.notifyDataSetChanged();
//重新設置選中主會場
mViewPager.setCurrentItem(2);

public Object instantiateItem(ViewGroup container, int position) {
    if (mCurTransaction == null) {
        mCurTransaction = mFragmentManager.beginTransaction();
    }
    
    final long itemId = getItemId(position);

    //跟據(jù)itemId生成fragment名字，通過名字去查找fragment是否加載過
    String name = makeFragmentName(container.getId(), itemId);
    Fragment fragment = mFragmentManager.findFragmentByTag(name);
    //fragment加載過則直接attach，否則的話新生成一個fragment
    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;
 }

private static String makeFragmentName(int viewId, long id) {
    return "android:switcher:" + viewId + ":" + id;
}

public long getItemId(int position) {
    return position;
}

 public long getItemId(int position) {
     //可以直接使用后端給頁面ID
     return pageId;
     //后端不給也沒事，我們自己生成一個
     return data.get(position).getTitle().hashCode();
 }

public int getItemPosition(Object object) {
        return POSITION_UNCHANGED;
}

void dataSetChanged() {
        // This method only gets called if our observer is attached, so mAdapter is non-null.

        final int adapterCount = mAdapter.getCount();
        mExpectedAdapterCount = adapterCount;
        boolean needPopulate = mItems.size() < mOffscreenPageLimit * 2 + 1
                && mItems.size() < adapterCount;
        int newCurrItem = mCurItem;

        boolean isUpdating = false;
            //mItems為舊數(shù)據(jù)的容器
        for (int i = 0; i < mItems.size(); i++) {
            final ItemInfo ii = mItems.get(i);
          //返回刷新之前Tab項所處的位置
            final int newPos = mAdapter.getItemPosition(ii.object);
            //返回的位置等于POSITION_UNCHANGED（-1）表示當前頁未有變更，不做任何操作
            if (newPos == PagerAdapter.POSITION_UNCHANGED) {
                continue;
            }
            //如果返回的位置等于POSITION_NONE（-2）表示當前頁Tab項刷新后不存在，需要銷毀并重新加載新的頁面
            if (newPos == PagerAdapter.POSITION_NONE) {
                mItems.remove(i);
                i--;

                if (!isUpdating) {
                    mAdapter.startUpdate(this);
                    isUpdating = true;
                }

                mAdapter.destroyItem(this, ii.position, ii.object);
                needPopulate = true;

                if (mCurItem == ii.position) {
                    // Keep the current item in the valid range
                    newCurrItem = Math.max(0, Math.min(mCurItem, adapterCount - 1));
                    needPopulate = true;
                }
                continue;
            }
            //如果當前頁的新的位置和和舊位置不等，則說明調整了順序
            if (ii.position != newPos) {
            //這段代碼是將頁面定位到刷新之前的打開頁，據(jù)數(shù)據(jù)的position和mCurItem相等的話，則表示這個item是之前打開的，賦予它新位置的值
                if (ii.position == mCurItem) {
                    // Our current item changed position. Follow it.
                    newCurrItem = newPos;
                }

                ii.position = newPos;
                needPopulate = true;
            }
        }

        if (isUpdating) {
            mAdapter.finishUpdate(this);
        }

        Collections.sort(mItems, COMPARATOR);

        if (needPopulate) {
            // Reset our known page widths; populate will recompute them.
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (!lp.isDecor) {
                    lp.widthFactor = 0.f;
                }
            }

            setCurrentItemInternal(newCurrItem, false, true);
            requestLayout();
        }
    }