安卓：Fragment的懒加载

Android应用开发过程中，ViewPager同时加载多个fragment，以实现多tab页面快速切换, 但是fragment初始化时若加载的内容较多，就可能导致整个应用启动速度缓慢，影响用户体验。
为了提高用户体验，我们会使用一些懒加载方案，实现加载延迟。这时我们会用到getUserVisibleHint()与setUserVisibleHint()这两个方法。

/**** @param isVisibleToUser true if this fragment's UI is currently visible to the user (default),*                        false if it is not.*/public void setUserVisibleHint(boolean isVisibleToUser) {   if (!mUserVisibleHint && isVisibleToUser && mState < STARTED) {       mFragmentManager.performPendingDeferredStart(this);   }   mUserVisibleHint = isVisibleToUser;   mDeferStart = !isVisibleToUser;}/** * @return The current value of the user-visible hint on this fragment. * @see #setUserVisibleHint(boolean) */public boolean getUserVisibleHint() {    return mUserVisibleHint;}

从上述源码注释我们可以看出，当fragment被用户可见时，setUserVisibleHint()会调用且传入true值，当fragment不被用户可见时，setUserVisibleHint()则得到false值。而在传统的fragment生命周期里也看不到这个函数。

那么，问题来了，

fragment是如何知道自己时候用户可见？
setUserVisibleHint() 在上图所示fragment的生命周期的什么位置?

先说结论：

1. viewpager监听切换tab事件，tab切换一次，执行一次setUserVisibleHint()方法
2. setUserVisibleHint() 在 上图所示fragment所有生命周期之前，无论viewpager是在activity哪个生命周期里初始化。
3. activity生命周期 和 fragment生命周期 时序并不是按序来的，也就是说fragment的oncreate方法时序并不一定在activity的oncreate方法之后。

具体原因，我们从应用场景开始一点一点的分析。

1. 我们的使用场景：

public class MainActivity extends FragmentActivity {    private ViewPager viewPager;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        viewPager = (ViewPager) findViewById(R.id.viewpager);        vpOrder.setAdapter(new MainFragmentPagerAdapter(getSupportFragmentManager()));        vpOrder.setOffscreenPageLimit(5);        vpOrder.setCurrentItem(0);    }｝

2.ViewPager ,FragmentPagerAdapter

• /frameworks/base/core/java/android/app/Activity.java

//每次切换ViewPager的Tab时调用的方法void populate(int newCurrentItem) {        mAdapter.startUpdate(this);        //......        addNewItem(mCurItem, curIndex);        // mCurItem 为当前可见Fragment        // 调用setUserVisibleHint(true)        mAdapter.setPrimaryItem(this, mCurItem, curItem != null ? curItem.object : null);         mAdapter.finishUpdate(this);        //..... }ItemInfo addNewItem(int position, int index) {        ItemInfo ii = new ItemInfo();        ii.position = position;        //初始化fragment, 调用setUserVisibleHint(false)        ii.object = mAdapter.instantiateItem(this, position);        ii.widthFactor = mAdapter.getPageWidth(position);        if (index < 0 || index >= mItems.size()) {            mItems.add(ii);        } else {            mItems.add(index, ii);        }        return ii;}

3. Activity , FragmentManager

• /frameworks/support/v4/java/android/support/v4/app/FragmentManager.java

   @Overridepublic 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);        //将fragment添加到FragmentManager里面        mCurTransaction.add(container.getId(), fragment,                makeFragmentName(container.getId(), itemId));    }    if (fragment != mCurrentPrimaryItem) {        fragment.setMenuVisibility(false);        //我们要找的方法在这里        fragment.setUserVisibleHint(false);     }    return fragment;}@Overridepublic void setPrimaryItem(ViewGroup container, int position, Object object) {    Fragment fragment = (Fragment)object;    if (fragment != mCurrentPrimaryItem) {        if (mCurrentPrimaryItem != null) {            mCurrentPrimaryItem.setMenuVisibility(false);            mCurrentPrimaryItem.setUserVisibleHint(false);        }        if (fragment != null) {            //我们要找的方法在这里            fragment.setMenuVisibility(true);            fragment.setUserVisibleHint(true);        }        mCurrentPrimaryItem = fragment;    }}

/frameworks/support/v4/java/android/support/v4/app/FragmentManager.java

//分发onCreate事件函数public void dispatchCreate() {    mStateSaved = false;    moveToState(Fragment.CREATED, false);}//moveToState 重载 1void moveToState(int newState, boolean always) {    moveToState(newState, 0, 0, always);}//moveToState 重载 2void moveToState(int newState, int transit, int transitStyle, boolean always) {    if (mHost == null && newState != Fragment.INITIALIZING) {        throw new IllegalStateException("No host");    }    if (!always && mCurState == newState) {        return;    }    mCurState = newState;    //若mActive为null,就算Activtiy里面调用了dispatchOnCreate()也不会执行Fragment    //的OnAttach和onCreate等方法。    //只有mActive非null,即addFragment()执行后，才会真正进入到生命周期。    //而根据FragmentPagerAdapter可知，只有当viewpager调用setAdapter方法，才会添加fragment到FramentManager。    //执行setAdapter的时候，会调用setUserVisibleHint()方法，并且，只有当setAdapter方法执行完之后，才会进入到Fragment到生命周期，因此setUserVisibleHint()方法在所有生命周期之前被调用。    if (mActive != null) {        boolean loadersRunning = false;        for (int i=0; i<mActive.size(); i++) {            Fragment f = mActive.get(i);            if (f != null) {                moveToState(f, newState, transit, transitStyle, false);                if (f.mLoaderManager != null) {                    loadersRunning |= f.mLoaderManager.hasRunningLoaders();                }            }        }        if (!loadersRunning) {            startPendingDeferredFragments();        }        if (mNeedMenuInvalidate && mHost != null && mCurState == Fragment.RESUMED) {            mHost.onSupportInvalidateOptionsMenu();            mNeedMenuInvalidate = false;        }    }}//moveToState 重载 3void moveToState(Fragment f, int newState, int transit, int transitionStyle,         //...         f.onAttach();         //...         f.performOnCreate();         //其他生命周期 }//添加fragment到FragmentManagerpublic void addFragment(Fragment fragment, boolean moveToStateNow) {    if (mAdded == null) {        mAdded = new ArrayList<Fragment>();    }    if (DEBUG) Log.v(TAG, "add: " + fragment);    //激活fragment    makeActive(fragment);    if (!fragment.mDetached) {        if (mAdded.contains(fragment)) {            throw new IllegalStateException("Fragment already added: " + fragment);        }        mAdded.add(fragment);        fragment.mAdded = true;        fragment.mRemoving = false;        if (fragment.mHasMenu && fragment.mMenuVisible) {            mNeedMenuInvalidate = true;        }        if (moveToStateNow) {            moveToState(fragment);        }    }}void makeActive(Fragment f) {    if (f.mIndex >= 0) {        return;    }    if (mAvailIndices == null || mAvailIndices.size() <= 0) {        if (mActive == null) {            //激活Fragment            mActive = new ArrayList<Fragment>();            }        f.setIndex(mActive.size(), mParent);        mActive.add(f);    } else {        f.setIndex(mAvailIndices.remove(mAvailIndices.size()-1), mParent);        mActive.set(f.mIndex, f);    }    if (DEBUG) Log.v(TAG, "Allocated fragment index " + f);}