方法一
在Fragment可见时请求数据。此方案仍预加载了前后的页面，但是没有请求数据，只有进入到当前Framgent时才请求数据。
优点：实现了数据的懒加载
缺点：一次仍是三个Framgment对象，不是完全意义的懒加载
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public class FragmentSample extends Fragment{
.......
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if (isVisibleToUser) {
requestData(); // 在此请求数据
}
}
......
}
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方法二
直接修改ViewPager源码。通过查看ViewPager源码可知，控制其预加载的是一个常量 DEFAULT_OFFSCREEN_PAGES，其默认值为1，表示当前页面前后各预加载一个页面，在这里我们直接将其设置为0即可，即去掉预加载。但是，这样有一个问题，那就是在使用其他控件时需要传入ViewPager时，这个就不能用了。
优点：完全屏蔽掉了预加载
缺点：应用太受限制，比如使用ViewPagerIndicator时需要传入ViewPager对象，这时傻眼了。
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// 注意，这是直接拷贝的ViewPager的源码，只修改了注释处的代码
public class LazyViewPager extends ViewGroup {
private static final String TAG = "LazyViewPager";
private static final boolean DEBUG = false;
private static final boolean USE_CACHE = false;
// 默认为1，即前后各预加载一个页面，设置为0去掉预加载
private static final int DEFAULT_OFFSCREEN_PAGES = 0;
private static final int MAX_SETTLE_DURATION = 600; // ms
static class ItemInfo {
Object object;
int position;
boolean scrolling;
}
private static final Comparator<ItemInfo> COMPARATOR = new Comparator<ItemInfo>() {
public int compare(ItemInfo lhs, ItemInfo rhs) {
return lhs.position - rhs.position;
}
};
............
}
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方法三
直接继承ViewPager，结合PagerAdapter实现懒加载。该方案是我用到的最完善的方法，完全的懒加载，每次只会建立一个Fragment对象。
优点：完全屏蔽预加载
缺点：稍微复杂，需要自定义，开源库地址
这个库就4个类，作者通过继承ViewPager（保证其普适性）、自定义ViewPagerAdapter和 LazyFragmentPagerAdapter以及设置懒加载的标记接口，很好的实现了懒加载。感谢作者。
在此贴出关键代码，有兴趣的同学可以学习下。
LazyViewPager:
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public class LazyViewPager extends ViewPager {
private static final float DEFAULT_OFFSET = 0.5f;
private LazyPagerAdapter mLazyPagerAdapter;
private float mInitLazyItemOffset = DEFAULT_OFFSET;
public LazyViewPager(Context context) {
super(context);
}
public LazyViewPager(Context context, AttributeSet attrs) {
super(context, attrs);
TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.LazyViewPager);
setInitLazyItemOffset(a.getFloat(R.styleable.LazyViewPager_init_lazy_item_offset, DEFAULT_OFFSET));
a.recycle();
}
/**
* change the initLazyItemOffset
* @param initLazyItemOffset set mInitLazyItemOffset if {@code 0 < initLazyItemOffset <= 1}
*/
public void setInitLazyItemOffset(float initLazyItemOffset) {
if (initLazyItemOffset > 0 && initLazyItemOffset <= 1) {
mInitLazyItemOffset = initLazyItemOffset;
}
}
public void setAdapter(PagerAdapter adapter) {
super.setAdapter(adapter);
mLazyPagerAdapter = adapter != null && adapter instanceof LazyPagerAdapter ? (LazyPagerAdapter) adapter : null;
}
protected void onPageScrolled(int position, float offset, int offsetPixels) {
if (mLazyPagerAdapter != null) {
if (getCurrentItem() == position) {
int lazyPosition = position + 1;
if (offset >= mInitLazyItemOffset && mLazyPagerAdapter.isLazyItem(lazyPosition)) {
mLazyPagerAdapter.startUpdate(this);
mLazyPagerAdapter.addLazyItem(this, lazyPosition);
mLazyPagerAdapter.finishUpdate(this);
}
} else if (getCurrentItem() > position) {
int lazyPosition = position;
if (1 - offset >= mInitLazyItemOffset && mLazyPagerAdapter.isLazyItem(lazyPosition)) {
mLazyPagerAdapter.startUpdate(this);
mLazyPagerAdapter.addLazyItem(this, lazyPosition);
mLazyPagerAdapter.finishUpdate(this);
}
}
}
super.onPageScrolled(position, offset, offsetPixels);
}
}
public abstract class LazyFragmentPagerAdapter extends LazyPagerAdapter<Fragment> {
private static final String TAG = "LazyFragmentPagerAdapter";
private static final boolean DEBUG = false;
private final FragmentManager mFragmentManager;
private FragmentTransaction mCurTransaction = null;
public LazyFragmentPagerAdapter(FragmentManager fm) {
mFragmentManager = fm;
}
public void startUpdate(ViewGroup container) {
}
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(container, position);
if (fragment instanceof Laziable) {
mLazyItems.put(position, fragment);
} else {
mCurTransaction.add(container.getId(), fragment, name);
}
}
if (fragment != getCurrentItem()) {
fragment.setMenuVisibility(false);
fragment.setUserVisibleHint(false);
}
return fragment;
}
public void destroyItem(ViewGroup container, int position, Object object) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
if (DEBUG)
Log.v(TAG, "Detaching item #" + getItemId(position) + ": f=" + object + " v=" + ((Fragment) object).getView());
final long itemId = getItemId(position);
String name = makeFragmentName(container.getId(), itemId);
if (mFragmentManager.findFragmentByTag(name) == null) {
mCurTransaction.detach((Fragment) object);
} else {
mLazyItems.remove(position);
}
}
public Fragment addLazyItem(ViewGroup container, int position) {
Fragment fragment = mLazyItems.get(position);
if (fragment == null)
return null;
final long itemId = getItemId(position);
String name = makeFragmentName(container.getId(), itemId);
if (mFragmentManager.findFragmentByTag(name) == null) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
mCurTransaction.add(container.getId(), fragment, name);
mLazyItems.remove(position);
}
return fragment;
}
public void finishUpdate(ViewGroup container) {
if (mCurTransaction != null) {
mCurTransaction.commitAllowingStateLoss();
mCurTransaction = null;
mFragmentManager.executePendingTransactions();
}
}
public boolean isViewFromObject(View view, Object object) {
return ((Fragment) object).getView() == view;
}
public long getItemId(int position) {
return position;
}
private static String makeFragmentName(int viewId, long id) {
return "android:switcher:" + viewId + ":" + id;
}
/**
* mark the fragment can be added lazily
*/
public interface Laziable {
}
}
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友情提示：填充LazyViewPager的Fragment一定要实现接口LazyFragmentPagerAdapter.Laziable，其实就是一个标记。
原文链接：
http://answerzhao.github.io/2016/06/06/%E5%AE%9E%E7%8E%B0ViewPager%E6%87%92%E5%8A%A0%E8%BD%BD%E7%9A%84%E4%B8%89%E7%A7%8D%E6%96%B9%E6%B3%95/#
其他参考地址：
http://www.jianshu.com/p/104be7cd72b6
http://www.cnblogs.com/dasusu/p/5926731.html