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。
fragment生命周期
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：
- viewpager监听切换tab事件，tab切换一次，执行一次setUserVisibleHint()方法
- 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。
- 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
@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);
//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;
}
@Override
public 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 1
void moveToState(int newState, boolean always) {
moveToState(newState, 0, 0, always);
}
//moveToState 重载 2
void 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();
}
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();
}
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);
}