DailyWork02_ViewPager中Fragment实现懒加载

##一、Requirement
适用场景：

• ViewPager加载Fragment
• fragment显示的时候才加载数据
• ViewPager切换，隐藏的fragment可以取消
• 跳转其他页面请求取消，
• 返回页面重新请求

**注意：**普通Activity中fragment懒加载不用此种方式显示

##二、Demo
FragmentExchangeActivity

##三、Theory

1. viewPager加载和切换fragment的时候setUserVisibleHint会调用，参数isVisibleToUser告知会显示还是隐藏

2. 第一次加载fragment的时候setUserVisibleHint会调用两次，同时Fragment的view不一定创建了，

3. fragment的onResume和onPause受到Activity的方法影响

4. 定义onVisible和onInVisible方法，表示fragment的显示与消失

5. 主要靠setUserVisibleHint来调用onVisible和onInVisible，定义标记flag，在onResume和onPause中补充

##三 CoreCode

public class ViewPagerLazyFragment extends Fragment {

    public static final String TAG  = "ViewPagerListener";

    private String mType;

    //标记需要等待回调onMyResume;
    private boolean isWaitingForOnMyResume = false;

    public static ViewPagerLazyFragment newInstance(String type) {
        Bundle args = new Bundle();
        args.putString("type", type);
        ViewPagerLazyFragment fragment = new ViewPagerLazyFragment();
        fragment.setArguments(args);
        return fragment;
    }

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.view_pager_listener_fragment, null);
        mType = getArguments().getString("type");
        ((TextView) view.findViewById(R.id.tv_tag)).setText(mType);

        view.findViewById(R.id.tv_jump).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                getContext().startActivity(new Intent(getContext(),OtherActivity.class));
            }
        });
        return view;
    }

    @Override
    public void onResume() {
        super.onResume();
        if (isWaitingForOnMyResume && getUserVisibleHint()) {
            isWaitingForOnMyResume = false;
            onVisible();
        }
    }



    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        Log.i(TAG,"ViewPagerListenerFragment setUserVisibleHint() type = " + mType + " isVisibleToUser="+isVisibleToUser);
        if (isVisibleToUser) {
            if (isResumed()) {
                onVisible();
            } else {
                isWaitingForOnMyResume = true;
            }
        } else {
            if (isResumed())
                onInVisible();
        }
    }

    @Override
    public void onPause() {
        super.onPause();
        if(getUserVisibleHint()){
            isWaitingForOnMyResume = true;
            onInVisible();
        }

        Log.i(TAG,"ViewPagerListenerFragment onPause() type = "+mType);
    }

    /**
     * viewPager中界面每次可见调用；
     */
    public void onVisible() {
        Log.i(TAG,"ViewPagerListenerFragment onVisible() type = " + mType);
    }


    /**
     * viewPager中界面每次不可见调用；
     */
    public void onInVisible() {
        Log.i(TAG,"ViewPagerListenerFragment onInVisible() type = " + mType);
    }
}