【转】实现ViewPager懒加载的三种方法

时间：2023-02-09 19:22:45

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

123456789101112131415161718192021 public class FragmentSample extends Fragment{  .......   @Override  public void setUserVisibleHint(boolean isVisibleToUser) {  super.setUserVisibleHint(isVisibleToUser);  if (isVisibleToUser) {  requestData(); // 在此请求数据  }  }  ...... }

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。

123456789101112131415161718192021 // 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; // msstatic class ItemInfo {Object object;int position;boolean scrolling;}private static final Comparator<ItemInfo> COMPARATOR = new Comparator<ItemInfo>() {@Overridepublic int compare(ItemInfo lhs, ItemInfo rhs) {return lhs.position - rhs.position;}}; ............}

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:

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143 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; }}@Overridepublic void setAdapter(PagerAdapter adapter) {super.setAdapter(adapter); mLazyPagerAdapter = adapter != null && adapter instanceof LazyPagerAdapter ? (LazyPagerAdapter) adapter : null;}@Overrideprotected 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;}@Overridepublic void startUpdate(ViewGroup container) {}@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(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;}@Overridepublic 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);}} @Overridepublic 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;}@Overridepublic void finishUpdate(ViewGroup container) {if (mCurTransaction != null) {mCurTransaction.commitAllowingStateLoss();mCurTransaction = null;mFragmentManager.executePendingTransactions();}} @Overridepublic 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 { }}

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

标签：

相关文章