Android中ViewPager你所不知道的优化技巧分享

//调整主会场位置伪代码
marketingInfoList.add(new MarketingInfo("www.juejin.com", "主会场"))
for (int i = 0; i <= 3; i++) {
    //将放在前两个主会场前面
    if (i < 2) {
        marketingInfoList.add(i, new MarketingInfo("www.baidu.com", "模拟" + i));
    } else {
    //后两个往主会场后面添加
        marketingInfoList.add(new MarketingInfo("www.baidu.com", "模拟" + i));
    }
}
mPagerAdapter.notifyDataSetChanged();
//重新设置选中主会场
mViewPager.setCurrentItem(2);

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

    //跟据itemId生成fragment名字，通过名字去查找fragment是否加载过
    String name = makeFragmentName(container.getId(), itemId);
    Fragment fragment = mFragmentManager.findFragmentByTag(name);
    //fragment加载过则直接attach，否则的话新生成一个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) {
     //可以直接使用后端给页面ID
     return pageId;
     //后端不给也没事，我们自己生成一个
     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为旧数据的容器
        for (int i = 0; i < mItems.size(); i++) {
            final ItemInfo ii = mItems.get(i);
          //返回刷新之前Tab项所处的位置
            final int newPos = mAdapter.getItemPosition(ii.object);
            //返回的位置等于POSITION_UNCHANGED（-1）表示当前页未有变更，不做任何操作
            if (newPos == PagerAdapter.POSITION_UNCHANGED) {
                continue;
            }
            //如果返回的位置等于POSITION_NONE（-2）表示当前页Tab项刷新后不存在，需要销毁并重新加载新的页面
            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;
            }
            //如果当前页的新的位置和和旧位置不等，则说明调整了顺序
            if (ii.position != newPos) {
            //这段代码是将页面定位到刷新之前的打开页，据数据的position和mCurItem相等的话，则表示这个item是之前打开的，赋予它新位置的值
                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();
        }
    }