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我们在做应用开发的时候，一个Activity里面可能会以viewpager（或其他容器）与多个Fragment来组合使用，而如果每个fragment都需要去加载数据，或从本地加载，或从网络加载！

那么在这个activity刚创建的时候就变成需要初始化大量资源，请求很多接口，这样的结果，我们当然不会满意。那么，能不能做到当切换到这个fragment的时候，它才去初始化呢？

ok 那就如图 我们展示一下 源代码 ：

MainActivity MyFragmentPagerAdapter

MoreFragment继承》LazyFragment 继承》父类BaseFragment

• MainActivity 这是平时 我们viewpager 配合fragment 的使用方法 我们新建工程的时候 9成都会用到此方法

public class MainActivity extends AppCompatActivity {
    @InjectView(R.id.viewPager2)
    ViewPager viewPager;
    private List<Fragment> fragmentList;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.inject(this);
        fragmentList = new ArrayList<>();
        fragmentList.add(MoreFragment.newInstance(1,false));
        fragmentList.add(MoreFragment.newInstance(2,true));
        fragmentList.add(MoreFragment.newInstance(3,true));
        fragmentList.add(MoreFragment.newInstance(4,true));
        FragmentManager supportFragmentManager = getSupportFragmentManager();
        MyFragmentPagerAdapter myFragmentPagerAdapter = new MyFragmentPagerAdapter(supportFragmentManager, fragmentList);
        viewPager.setAdapter(myFragmentPagerAdapter);
        viewPager.setOffscreenPageLimit(4);
    }
     class MyFragmentPagerAdapter extends FragmentPagerAdapter {
        private List<Fragment> fragmentList;
        public MyFragmentPagerAdapter(FragmentManager fm, List<Fragment> fragmentList) {
            super(fm);
            this.fragmentList = fragmentList;
        }
        @Override
        public Fragment getItem(int position) {
            return fragmentList.get(position);
        }
        @Override
        public int getCount() {
            return fragmentList.size();
        }
    }
}

• MoreFragment 这就是我们正常创建的fragment 和以往不同的是：

  onCreateView 方法 变成了 onCreateViewLazy

  并且 多了一个newInstance方法

public class MoreFragment extends LazyFragment {
    public static final String INTENT_INT_INDEX="index";
    public static MoreFragment newInstance(int tabIndex,boolean isLazyLoad) {
        Bundle args = new Bundle();
        args.putInt(INTENT_INT_INDEX, tabIndex);
        args.putBoolean(LazyFragment.INTENT_BOOLEAN_LAZYLOAD, isLazyLoad);
        MoreFragment fragment = new MoreFragment();
        fragment.setArguments(args);
        return fragment;
    }
    @Override
    protected void onCreateViewLazy(Bundle savedInstanceState) {
        super.onCreateViewLazy(savedInstanceState);
        setContentView(R.layout.fragment_tabmain_item);
    }
}

• LazyFragment

懒加载Fragment 只有创建并显示的时候才会调用onCreateViewLazy方法

• 懒加载的原理onCreateView的时候Fragment有可能没有显示出来。

• 但是调用到setUserVisibleHint(boolean isVisibleToUser),isVisibleToUser =

  true的时候就说明有显示出来

• 但是要考虑onCreateView和setUserVisibleHint的先后问题所以才有了下面的代码

《1》原先的Fragment的回调方法名字后面要加个Lazy，比如Fragment的onCreateView方法， 就写成onCreateViewLazy

《2》使用该LazyFragment会导致多一层布局深度

public class LazyFragment extends BaseFragment {
    private boolean isInit = false;//真正要显示的View是否已经被初始化（正常加载）
    private Bundle savedInstanceState;
    public static final String INTENT_BOOLEAN_LAZYLOAD = "intent_boolean_lazyLoad";
    private boolean isLazyLoad = true;
    private FrameLayout layout;
    private boolean isStart = false;//是否处于可见状态，in the screen
    @Override
    protected final void onCreateView(Bundle savedInstanceState) {
        Log.d("TAG", "onCreateView() : " + "getUserVisibleHint():" + getUserVisibleHint());
        super.onCreateView(savedInstanceState);
        Bundle bundle = getArguments();
        if (bundle != null) {
            isLazyLoad = bundle.getBoolean(INTENT_BOOLEAN_LAZYLOAD, isLazyLoad);
        }
        //判断是否懒加载
        if (isLazyLoad) {
            //一旦isVisibleToUser==true即可对真正需要的显示内容进行加载
            if (getUserVisibleHint() && !isInit) {
                this.savedInstanceState = savedInstanceState;
                onCreateViewLazy(savedInstanceState);
                isInit = true;
            } else {
                //进行懒加载
                layout = new FrameLayout(getApplicationContext());
                layout.setLayoutParams(new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));
                View view = LayoutInflater.from(getApplicationContext()).inflate(R.layout.fragment_lazy_loading, null);
                layout.addView(view);
                super.setContentView(layout);
            }
        } else {
            //不需要懒加载，开门江山，调用onCreateViewLazy正常加载显示内容即可
            onCreateViewLazy(savedInstanceState);
            isInit = true;
        }
    }
    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        Log.d("TAG", "setUserVisibleHint() called with: " + "isVisibleToUser = [" + isVisibleToUser + "]");
        //一旦isVisibleToUser==true即可进行对真正需要的显示内容的加载
        //可见，但还没被初始化
        if (isVisibleToUser && !isInit && getContentView() != null) {
            onCreateViewLazy(savedInstanceState);
            isInit = true;
            onResumeLazy();
        }
        //已经被初始化（正常加载）过了
        if (isInit && getContentView() != null) {
            if (isVisibleToUser) {
                isStart = true;
                onFragmentStartLazy();
            } else {
                isStart = false;
                onFragmentStopLazy();
            }
        }
    }
    @Override
    public void setContentView(int layoutResID) {
        //判断若isLazyLoad==true,移除所有lazy view，加载真正要显示的view
        if (isLazyLoad && getContentView() != null && getContentView().getParent() != null) {
            layout.removeAllViews();
            View view = inflater.inflate(layoutResID, layout, false);
            layout.addView(view);
        }
        //否则，开门见山，直接加载
        else {
            super.setContentView(layoutResID);
        }
    }
    @Override
    public void setContentView(View view) {
        //判断若isLazyLoad==true,移除所有lazy view，加载真正要显示的view
        if (isLazyLoad && getContentView() != null && getContentView().getParent() != null) {
            layout.removeAllViews();
            layout.addView(view);
        }
        //否则，开门见山，直接加载
        else {
            super.setContentView(view);
        }
    }
    @Deprecated
    @Override
    public final void onStart() {
        Log.d("TAG", "onStart() : " + "getUserVisibleHint():" + getUserVisibleHint());
        super.onStart();
        if (isInit && !isStart && getUserVisibleHint()) {
            isStart = true;
            onFragmentStartLazy();
        }
    }
    @Deprecated
    @Override
    public final void onStop() {
        super.onStop();
        Log.d("TAG", "onStop() called: " + "getUserVisibleHint():" + getUserVisibleHint());
        if (isInit && isStart && getUserVisibleHint()) {
            isStart = false;
            onFragmentStopLazy();
        }
    }
    //当Fragment被滑到可见的位置时，调用
    protected void onFragmentStartLazy() {
        Log.d("TAG", "onFragmentStartLazy() called with: " + "");
    }
    //当Fragment被滑到不可见的位置，offScreen时，调用
    protected void onFragmentStopLazy() {
        Log.d("TAG", "onFragmentStopLazy() called with: " + "");
    }
    protected void onCreateViewLazy(Bundle savedInstanceState) {
        Log.d("TAG", "onCreateViewLazy() called with: " + "savedInstanceState = [" + savedInstanceState + "]");
    }
    protected void onResumeLazy() {
        Log.d("TAG", "onResumeLazy() called with: " + "");
    }
    protected void onPauseLazy() {
        Log.d("TAG", "onPauseLazy() called with: " + "");
    }
    protected void onDestroyViewLazy() {
    }
    @Override
    @Deprecated
    public final void onResume() {
        Log.d("TAG", "onResume() : " + "getUserVisibleHint():" + getUserVisibleHint());
        super.onResume();
        if (isInit) {
            onResumeLazy();
        }
    }
    @Override
    @Deprecated
    public final void onPause() {
        Log.d("TAG", "onPause() : " + "getUserVisibleHint():" + getUserVisibleHint());
        super.onPause();
        if (isInit) {
            onPauseLazy();
        }
    }
    @Override
    @Deprecated
    public final void onDestroyView() {
        Log.d("TAG", "onDestroyView() : " + "getUserVisibleHint():" + getUserVisibleHint());
        super.onDestroyView();
        if (isInit) {
            onDestroyViewLazy();
        }
        isInit = false;
    }
}

• BaseFragment

public class BaseFragment extends Fragment {
    protected LayoutInflater inflater;
    private View contentView;
    private Context context;
    private ViewGroup container;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        context = getActivity().getApplicationContext();
    }
    //子类通过重写onCreateView，调用setOnContentView进行布局设置，否则contentView==null，返回null
    @Override
    public final View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        this.inflater = inflater;
        this.container = container;
        onCreateView(savedInstanceState);
        if (contentView == null)
            return super.onCreateView(inflater, container, savedInstanceState);
        return contentView;
    }
    protected void onCreateView(Bundle savedInstanceState) {
    }
    @Override
    public void onDestroyView() {
        super.onDestroyView();
        contentView = null;
        container = null;
        inflater = null;
    }
    public Context getApplicationContext() {
        return context;
    }
    public void setContentView(int layoutResID) {
        setContentView((ViewGroup) inflater.inflate(layoutResID, container, false));
    }
    public void setContentView(View view) {
        contentView = view;
    }
    public View getContentView() {
        return contentView;
    }
    public View findViewById(int id) {
        if (contentView != null)
            return contentView.findViewById(id);
        return null;
    }
    // http://stackoverflow.com/questions/15207305/getting-the-error-java-lang-illegalstateexception-activity-has-been-destroyed
    @Override
    public void onDetach() {
        Log.d("TAG", "onDetach() : ");
        super.onDetach();
        try {
            Field childFragmentManager = Fragment.class.getDeclaredField("mChildFragmentManager");
            childFragmentManager.setAccessible(true);
            childFragmentManager.set(this, null);
        } catch (NoSuchFieldException e) {
            throw new RuntimeException(e);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }
    @Override
    public void onDestroy() {
        Log.d("TAG", "onDestroy() : ");
        super.onDestroy();
    }
}

来自：http://www.jianshu.com/p/8a772b9df6d5