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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</h1> 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();
}
}
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