Android---Fragment的“懒加载“

基本APP都有个首页，如：

图片.png

实现的方式就是底部的几个tab和一个ViewPager，ViewPager中装在Fragment
。基本上，Fragment都会去请求数据。但是如图有4个Fragment，对应的网络请求都要去加载。这样子请求次数就大了很多。我们可以通过Fragment的“懒加载”
来减少请求次数，只在Fragment显示的加载数据。

我们可以通过Fragment的setUserVisibleHint方法控制

先看下上图的关键代码：

    ArrayList<Fragment> fragments = new ArrayList<>();
        TestFragment e = new TestFragment();
        e.TAG = "dfasdfdsafas TestFragment1";
        fragments.add(e);
        TestFragment e1 = new TestFragment();
        e1.TAG = "dfasdfdsafas TestFragment2";
        fragments.add(e1);
        TestFragment e2 = new TestFragment();
        e2.TAG = "dfasdfdsafas TestFragment3";
        fragments.add(e2);
        TestFragment e3 = new TestFragment();
        e3.TAG = "dfasdfdsafas TestFragment4";
        fragments.add(e3);

        CommonPagerAdapter commonPagerAdapter = new CommonPagerAdapter(getSupportFragmentManager(), fragments);
        activityMainViewPager.setAdapter(commonPagerAdapter);
        activityMainViewPager.setOffscreenPageLimit(4);
        activityMainViewPager.setCurrentItem(0);

activityMainViewPager.setOffscreenPageLimit(4);是为了保持各个tab的页面不被销毁。

再看下TestFragment的代码

public class TestFragment extends Fragment {

    public String TAG = "dfasdfdsafas";

    public TestFragment() {
        // Required empty public constructor
    }

    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        Log.i(TAG, "setUserVisibleHint=" + isVisibleToUser);

    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        Log.i(TAG, "onAttach");

    }

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.i(TAG, "onCreate");
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        Log.i(TAG, "onCreateView");
        return inflater.inflate(R.layout.fragment_test, container, false);
    }

    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        Log.i(TAG, "onViewCreated");
    }

    @Override
    public void onStart() {
        super.onStart();
        Log.i(TAG, "onStart");
    }

    @Override
    public void onResume() {
        super.onResume();
        Log.i(TAG, "onResume");
    }

    @Override
    public void onPause() {
        super.onPause();
        Log.i(TAG, "onPause");
    }

    @Override
    public void onStop() {
        super.onStop();
        Log.i(TAG, "onStop");
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        Log.i(TAG, "onDestroyView");
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.i(TAG, "onDestroy");
    }

    @Override
    public void onDetach() {
        super.onDetach();
        Log.i(TAG, "onDetach");
    }
}

App启动后，我们来看看Log

I/dfasdfdsafas TestFragment1: setUserVisibleHint=false
I/dfasdfdsafas TestFragment2: setUserVisibleHint=false
I/dfasdfdsafas TestFragment3: setUserVisibleHint=false
I/dfasdfdsafas TestFragment4: setUserVisibleHint=false
I/dfasdfdsafas TestFragment1: setUserVisibleHint=true
I/dfasdfdsafas TestFragment1: onAttach
I/dfasdfdsafas TestFragment1: onCreate
I/dfasdfdsafas TestFragment2: onAttach
I/dfasdfdsafas TestFragment2: onCreate
I/dfasdfdsafas TestFragment3: onAttach
I/dfasdfdsafas TestFragment3: onCreate
I/dfasdfdsafas TestFragment4: onAttach
I/dfasdfdsafas TestFragment4: onCreate
I/dfasdfdsafas TestFragment1: onCreateView
I/dfasdfdsafas TestFragment1: onViewCreated
I/dfasdfdsafas TestFragment1: onStart
I/dfasdfdsafas TestFragment1: onResume
I/dfasdfdsafas TestFragment2: onCreateView
I/dfasdfdsafas TestFragment2: onViewCreated
I/dfasdfdsafas TestFragment3: onCreateView
I/dfasdfdsafas TestFragment3: onViewCreated
I/dfasdfdsafas TestFragment4: onCreateView
I/dfasdfdsafas TestFragment4: onViewCreated
I/dfasdfdsafas TestFragment2: onStart
I/dfasdfdsafas TestFragment2: onResume
I/dfasdfdsafas TestFragment3: onStart
I/dfasdfdsafas TestFragment3: onResume
I/dfasdfdsafas TestFragment4: onStart
I/dfasdfdsafas TestFragment4: onResume

切换到tab2的Log

I/dfasdfdsafas TestFragment1: setUserVisibleHint=false
I/dfasdfdsafas TestFragment2: setUserVisibleHint=true

切换到tab3的log

I/dfasdfdsafas TestFragment2: setUserVisibleHint=false
I/dfasdfdsafas TestFragment3: setUserVisibleHint=true

切换到tab4的log

I/dfasdfdsafas TestFragment3: setUserVisibleHint=false
I/dfasdfdsafas TestFragment4: setUserVisibleHint=true

切换到tab1的log

I/dfasdfdsafas TestFragment4: setUserVisibleHint=false
I/dfasdfdsafas TestFragment1: setUserVisibleHint=true

所以，我们可以通过setUserVisibleHint来实现”懒加载“。可以设置一个flag,来记录是否加载过数据。如：

//用于懒加载，判断是否加载过
    protected boolean isLazyFragmentLoad = false;

然后Fragment的关键代码是大概是这样子的

   @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        initData(isVisibleToUser);
    }

    private void initData(boolean isVisibleToUser) {
        if (!isLazyFragmentLoad && isVisibleToUser) {
            isLazyFragmentLoad = true;
           //这边加载数据
        }
    }

发现setUserVisibleHint是优先于Fragment的任何的生命周期。

这边要注意的是，ViewPgaer的第一个页面，在setUserVisibleHint里不能加载数据，因为控件啥的还没有加载好。
解决方法：

1. ViewPager的第一个页面不用setUserVisibleHint方法，走正常流程。