android Fragment作为tab时的延迟加载

原文链接：http://www.jianshu.com/p/8a772b9df6d5

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();    }}

仿微信的Viewpager-Fragment惰性加载，延迟加载 源码地址 ：https://github.com/TellH/FragmentLazyLoading