其中BaseFragment是我在项目中使用的一个基类就是继承了v4包中的Fragment，代码不贴了，你的项目中应该也会自己的基类，继承你自己的基类就好了。上面代码中主要在两个方法中进行了控制，onViewCreated()和setUserVisibleHint, 另外这里我还使用了标志位，多重条件保证Fragment创建时数据不会被预加载且Fragment可见时只加载一次。
为什么这样可以在ViewPager中防止数据预加载呢，先来看一下setUserVisibleHint这个方法：
/**
-
Set a hint to the system about whether this fragment’s UI is currently visible
-
to the user. This hint defaults to true and is persistent across fragment instance
-
state save and restore.
-
An app may set this to false to indicate that the fragment's UI is
-
scrolled out of visibility or is otherwise not directly visible to the user.
-
This may be used by the system to prioritize operations such as fragment lifecycle updates
-
or loader ordering behavior.
-
Note: This method may be called outside of the fragment lifecycle.
-
and thus has no ordering guarantees with regard to fragment lifecycle method calls.
-
@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 != null && isAdded()) {
mFragmentManager.performPendingDeferredStart(this);
}
mUserVisibleHint = isVisibleToUser;
mDeferStart = mState < STARTED && !isVisibleToUser;
}
以上是来自support v4包中的Fragment源码, 大概的意思是调用这个方法来设置Fragment当前是否是对用户可见的，这个方法只有一个参数，visible就传true，否则就传false。可是在Fragment源码搜索发现并没有地方调用这个方法，那这个方法可能是给用户来调用的。
由此，我们可以猜想肯定是在ViewPager使用的过程中的某个地方调用了这个方法，我们看下ViewPager使用Fragment的流程一般是：
mAdapter = new BaseFragmentPagerAdapter(getSupportFragmentManager(), mFragmentList);
mViewPager.setAdapter(mAdapter);
于是，我先到FragmentPagerAdapter的源码中搜索了下，果不其然，发现了踪迹：
@SuppressWarnings(“ReferenceEquality”)
@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);
} el
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se {
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;
}
在instantiateItem方法的最后我们发现了调用的地方：
if (fragment != mCurrentPrimaryItem) {
fragment.setMenuVisibility(false);
fragment.setUserVisibleHint(false);
}
这里有个判断fragment != mCurrentPrimaryItem这个时候会把Fragment的Visible设为false, 那这个mCurrentPrimaryItem又是什么呢，继续搜索源码：
@SuppressWarnings(“ReferenceEquality”)
@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) {
fragment.setMenuVisibility(true);
fragment.setUserVisibleHint(true);
}
mCurrentPrimaryItem = fragment;
}
}
在一个setPrimaryItem的方法中找到这个变量的赋值，这个方法将传进来的fragment的visible设置为true, 同时会更新mCurrentPrimaryItem变量的值。我们再继续搜索setPrimaryItem这个方法的调用，结果在当前FragmentPagerAdapter的源码中没有找到，但是在它的父类PagerAdapter的源码中找到了它的定义：
/**
-
Called to inform the adapter of which item is currently considered to
-
be the “primary”, that is the one show to the user as the current page.
-
@param container The containing View from which the page will be removed.
-
@param position The page position that is now the primary.
-
@param object The same object that was returned by
-
{@link #instantiateItem(View, int)}.
*/
public void setPrimaryItem(ViewGroup container, int position, Object object) {
setPrimaryItem((View) container, position, object);
}
看注释大概明白了这个方法的含义是设置为ViewPager中当前展示给用户的那一页。
继续到ViewPager的源码中搜索，找到了调用它的地方：
是在一个populate()的方法中调用的，搜索发现好多地方调用了它，但是发现了有两个关键的地方：
/**
-
Set a PagerAdapter that will supply views for this pager as needed.
-
@param adapter Adapter to use
*/
public void setAdapter(PagerAdapter adapter) {
//…忽略部分源码
if (mAdapter != null) {
//…忽略部分源码
if (mRestoredCurItem >= 0) {
mAdapter.restoreState(mRestoredAdapterState, mRestoredClassLoader);
setCurrentItemInternal(mRestoredCurItem, false, true);
mRestoredCurItem = -1;
mRestoredAdapterState = null;
mRestoredClassLoader = null;
} else if (!wasFirstLayout) {
populate();
} else {
requestLayout();
}
}
//…忽略部分源码
}
/**
-
Set the currently selected page.
-
@param item Item index to select
-
@param smoothScroll True to smoothly scroll to the new item, false to transition immediately
*/
public void setCurrentItem(int item, boolean smoothScroll) {
mPopulatePending = false;
setCurrentItemInternal(item, smoothScroll, false);
}