viewpager通常和fragment配合使用，通过viewpager的滑动操作来是实现fragment的显示和隐藏．很多的场景中，我们都希望可以实现只有在viewpager滑动到当前fragment,也就是当前fragment可见时才去加载数据，渲染view,也就是fragment的懒加载．这可以在一定程度上提高性能．

主要代码如下：

public class LazyLoadFragment extends Fragment {

    private static final long TIME_DELAY = 300;
    private boolean mIsVisibleToUser;
    private boolean mIsViewCreated;
    private Handler mHandler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            switch (msg.what){
                case MSG_LOAD_DATA:
                    //加载数据逻辑                   
                    break;
            }
        }
    };
    private boolean mHasLoadData = false;
    public static final int MSG_LOAD_DATA = 1;

    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        this.mIsVisibleToUser = isVisibleToUser;
        if(isVisibleToUser){
            lazyLoadData();
        }else{
            stopLoadData();
        }
    }

    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        mIsViewCreated = true;
        lazyLoadData();
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        mIsViewCreated = false;
    }

    private void lazyLoadData() {
        if(mIsVisibleToUser && mIsViewCreated && !mHasLoadData){
            if(!mHandler.hasMessages(MSG_LOAD_DATA)){
                mHandler.sendEmptyMessageDelayed(MSG_LOAD_DATA,TIME_DELAY);
            }
        }
    }

    private boolean hasLoadData(){        
        //增加判断是否已经加载了数据
        //判断数据成员是否有值了,或者recyclerview中是否已经有子view了
        return false;
    }

    private void stopLoadData() {
        if(mHandler.hasMessages(MSG_LOAD_DATA)){
            mHandler.removeMessages(MSG_LOAD_DATA);
        }
    }
}