关于ViewPager+Fragment中Fragment不销毁/生命周期
今天我们的界面大幅改动，最终改成现在主流的Tab类型，和微信有点类似，下面有四个Tab对于四个不一样的模块。
但是这个四个界面都需要访问网络，并且我们开发框架用的是注解框架，导致对请求网络不好管理。，
而且又是viewpager+fragment
我们计划是在第一个页面将四个模板全部初始化，然后放到集合里面，这样即使用户任意切换都不会有问题，因为我们已经初始化了，
但是用过viewpager的就知道，他会预加载前面和后面的item，但是其他的会销毁掉。
这样可以避免内存浪费，但是我们不想这样，这样用户体验特别不好，别说产品和测试不同意，我自己也看不过去，
特别是每次一点入这个tab然后访问下网络，再更新下UI让我都莫名其妙。
最后还是解决了问题，一行代码。如下;
- 方案一：设置ViewPager的缓存界面数
此方案适用于界面数较少的情况，避免缓存界面太多导致内存吃紧。
方法：
mPager .setOffscreenPageLimit(2);
参数：int limit - 缓存当前界面每一侧的界面数
以上述为例，当前界面为1，limit = 2，表示缓存2、3两个界面。如此便避免了界面3被销毁。
- 方案二：保存状态并恢复
此方案适用于可用界面信息可由状态保存和恢复实现的情况。
在onDestroyView方法内保存相关信息，在onCreateView方法内恢复信息设置。
- 方案三（推荐）：复用Fragment的RootView
此方案适用通用场景，推荐使用。
步骤1：在onDestroyView方法内把Fragment的RootView从ViewPager中remove
@Override public void onDestroyView() { LogUtils.d(TAG , "-->onDestroyView"); super .onDestroyView(); if (null != FragmentView) { ((ViewGroup) mFragmentView.getParent()).removeView(mFragmentView); } }
步骤2：在onCreateView方法内复用RootView
@Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { LogUtils.d (TAG, "-->onCreateView"); if (null == mFragmentView) { mFragmentView = inflater.inflate(R.layout.fragment, container, false); mListView = (ListView) mFragmentView .findViewById(R.id.mm_listview); mListView.setAdapter(mAdapter); mPbar = (ProgressBar) mFragmentView.findViewById(R.id.pbar_mm_loading); mPbar.setVisibility(View.VISIBLE); } return mFragmentView ; }
我省事用了第一种，当然后面改成看第三种。
补充下，刚刚看了下第一种方式的源码：
显示这个方法
public void setOffscreenPageLimit(int limit) { if (limit < DEFAULT_OFFSCREEN_PAGES) { Log.w(TAG, "Requested offscreen page limit " + limit + " too small; defaulting to " + DEFAULT_OFFSCREEN_PAGES); limit = DEFAULT_OFFSCREEN_PAGES; } if (limit != mOffscreenPageLimit) { mOffscreenPageLimit = limit; populate(); } }
其中我分开放出来：
private int mOffscreenPageLimit = DEFAULT_OFFSCREEN_PAGES; 。。。。 private static final int DEFAULT_OFFSCREEN_PAGES = 1;
也就是说，我们设置的数据不是1的话，就调用了populate()方法。
下面是这个方法的代码：
首先 void populate() { populate(mCurItem); } 其中mCurItem是当前的item private int mCurItem; // Index of currently displayed page.
我们看下一个参数的populate()方法：
void populate(int newCurrentItem) { ItemInfo oldCurInfo = null; int focusDirection = View.FOCUS_FORWARD; if (mCurItem != newCurrentItem) { focusDirection = mCurItem < newCurrentItem ? View.FOCUS_RIGHT : View.FOCUS_LEFT; oldCurInfo = infoForPosition(mCurItem); mCurItem = newCurrentItem; } if (mAdapter == null) { sortChildDrawingOrder(); return; } // Bail now if we are waiting to populate. This is to hold off // on creating views from the time the user releases their finger to // fling to a new position until we have finished the scroll to // that position, avoiding glitches from happening at that point. if (mPopulatePending) { if (DEBUG) Log.i(TAG, "populate is pending, skipping for now..."); sortChildDrawingOrder(); return; } // Also, don't populate until we are attached to a window. This is to // avoid trying to populate before we have restored our view hierarchy // state and conflicting with what is restored. if (getWindowToken() == null) { return; } mAdapter.startUpdate(this); final int pageLimit = mOffscreenPageLimit; final int startPos = Math.max(0, mCurItem - pageLimit); final int N = mAdapter.getCount(); final int endPos = Math.min(N-1, mCurItem + pageLimit); if (N != mExpectedAdapterCount) { String resName; try { resName = getResources().getResourceName(getId()); } catch (Resources.NotFoundException e) { resName = Integer.toHexString(getId()); } throw new IllegalStateException("The application's PagerAdapter changed the adapter's" + " contents without calling PagerAdapter#notifyDataSetChanged!" + " Expected adapter item count: " + mExpectedAdapterCount + ", found: " + N + " Pager id: " + resName + " Pager class: " + getClass() + " Problematic adapter: " + mAdapter.getClass()); } // Locate the currently focused item or add it if needed. int curIndex = -1; ItemInfo curItem = null; for (curIndex = 0; curIndex < mItems.size(); curIndex++) { final ItemInfo ii = mItems.get(curIndex); if (ii.position >= mCurItem) { if (ii.position == mCurItem) curItem = ii; break; } } if (curItem == null && N > 0) { curItem = addNewItem(mCurItem, curIndex); } // Fill 3x the available width or up to the number of offscreen // pages requested to either side, whichever is larger. // If we have no current item we have no work to do. if (curItem != null) { float extraWidthLeft = 0.f; int itemIndex = curIndex - 1; ItemInfo ii = itemIndex >= 0 ? mItems.get(itemIndex) : null; final int clientWidth = getClientWidth(); final float leftWidthNeeded = clientWidth <= 0 ? 0 : 2.f - curItem.widthFactor + (float) getPaddingLeft() / (float) clientWidth; for (int pos = mCurItem - 1; pos >= 0; pos--) { if (extraWidthLeft >= leftWidthNeeded && pos < startPos) { if (ii == null) { break; } if (pos == ii.position && !ii.scrolling) { mItems.remove(itemIndex); mAdapter.destroyItem(this, pos, ii.object); if (DEBUG) { Log.i(TAG, "populate() - destroyItem() with pos: " + pos + " view: " + ((View) ii.object)); } itemIndex--; curIndex--; ii = itemIndex >= 0 ? mItems.get(itemIndex) : null; } } else if (ii != null && pos == ii.position) { extraWidthLeft += ii.widthFactor; itemIndex--; ii = itemIndex >= 0 ? mItems.get(itemIndex) : null; } else { ii = addNewItem(pos, itemIndex + 1); extraWidthLeft += ii.widthFactor; curIndex++; ii = itemIndex >= 0 ? mItems.get(itemIndex) : null; } } float extraWidthRight = curItem.widthFactor; itemIndex = curIndex + 1; if (extraWidthRight < 2.f) { ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null; final float rightWidthNeeded = clientWidth <= 0 ? 0 : (float) getPaddingRight() / (float) clientWidth + 2.f; for (int pos = mCurItem + 1; pos < N; pos++) { if (extraWidthRight >= rightWidthNeeded && pos > endPos) { if (ii == null) { break; } if (pos == ii.position && !ii.scrolling) { mItems.remove(itemIndex); mAdapter.destroyItem(this, pos, ii.object); if (DEBUG) { Log.i(TAG, "populate() - destroyItem() with pos: " + pos + " view: " + ((View) ii.object)); } ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null; } } else if (ii != null && pos == ii.position) { extraWidthRight += ii.widthFactor; itemIndex++; ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null; } else { ii = addNewItem(pos, itemIndex); itemIndex++; extraWidthRight += ii.widthFactor; ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null; } } } calculatePageOffsets(curItem, curIndex, oldCurInfo); } if (DEBUG) { Log.i(TAG, "Current page list:"); for (int i=0; i<mItems.size(); i++) { Log.i(TAG, "#" + i + ": page " + mItems.get(i).position); } } mAdapter.setPrimaryItem(this, mCurItem, curItem != null ? curItem.object : null); mAdapter.finishUpdate(this); // Check width measurement of current pages and drawing sort order. // Update LayoutParams as needed. final int childCount = getChildCount(); for (int i = 0; i < childCount; i++) { final View child = getChildAt(i); final LayoutParams lp = (LayoutParams) child.getLayoutParams(); lp.childIndex = i; if (!lp.isDecor && lp.widthFactor == 0.f) { // 0 means requery the adapter for this, it doesn't have a valid width. final ItemInfo ii = infoForChild(child); if (ii != null) { lp.widthFactor = ii.widthFactor; lp.position = ii.position; } } } sortChildDrawingOrder(); if (hasFocus()) { View currentFocused = findFocus(); ItemInfo ii = currentFocused != null ? infoForAnyChild(currentFocused) : null; if (ii == null || ii.position != mCurItem) { for (int i=0; i<getChildCount(); i++) { View child = getChildAt(i); ii = infoForChild(child); if (ii != null && ii.position == mCurItem) { if (child.requestFocus(focusDirection)) { break; } } } } } }
真长。。。。
其实也是他自身保存了ItemInfo，然后实现了效果
然后，。。
我又用了第一种方法，google写好的，省的麻烦不用白不用，还没有bug.
