上面两个页面都是 ViewPager + Fragment实现，Viewpager + Fragment情况下，fragment的生命周期因Viewpager的缓存机制而失去了具体意义
目前问题：每次进入4个Fragment的生命周期都会走一遍，数据同时请求，这就会造成UI初始化较慢
需求：每次进入只加载当前看到界面数据、切换时请求当前选项卡数据、重复切换只加载一次
具体实现原理：
使用Fragment类自带方法setUserVisibleHint()判断当前fragment是否对用户可见，根据回调的isVisibleToUser参数来进行相关的逻辑判断。重写该方法，创建变量isVisible拿到是否可见标志。
但是直接根据isVisible判断就加载数据，可能onCreateView()方法并未执行完毕,此时就会出现NullPointerException空指针异常。所以就需要满足控件初始化完成，用户可见，才能加载数据。
为了方便继承调用，将实现封装到了BaseFragment中
package com.jky.mobilebzt.fragment; import android.content.Context; import android.os.Bundle; import android.support.annotation.Nullable; import android.support.v4.app.Fragment; import android.text.TextUtils; import android.view.ContextThemeWrapper; import android.view.View; import android.widget.Toast; public class BaseNewFragment extends Fragment { private static final String TAG = BaseFragment.class.getSimpleName(); private boolean isFragmentVisible; private boolean isReuseView; private boolean isFirstVisible; private View rootView; public Context context; @Override public void setUserVisibleHint(boolean isVisibleToUser) { super.setUserVisibleHint(isVisibleToUser); //setUserVisibleHint()有可能在fragment的生命周期外被调用 if (rootView == null) { return; } if (isFirstVisible && isVisibleToUser) { onFragmentFirstVisible(); isFirstVisible = false; } if (isVisibleToUser) { onFragmentVisibleChange(true); isFragmentVisible = true; return; } if (isFragmentVisible) { isFragmentVisible = false; onFragmentVisibleChange(false); } } @Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); context = new ContextThemeWrapper(getActivity(), android.R.style.Theme_Holo_Light); initVariable(); } @Override public void onViewCreated(View view, @Nullable Bundle savedInstanceState) { if (rootView == null) { rootView = view; if (getUserVisibleHint()) { if (isFirstVisible) { onFragmentFirstVisible(); isFirstVisible = false; } onFragmentVisibleChange(true); isFragmentVisible = true; } } super.onViewCreated(isReuseView ? rootView : view, savedInstanceState); } @Override public void onDestroyView() { super.onDestroyView(); } @Override public void onDestroy() { super.onDestroy(); initVariable(); } private void initVariable() { isFirstVisible = true; isFragmentVisible = false; rootView = null; isReuseView = true; } private Toast toast; public void showShortToast(String text) { if (!TextUtils.isEmpty(text)) { if (toast != null) { toast.cancel(); toast = null; } if(getActivity()!=null){ toast = Toast.makeText(getActivity(), text, Toast.LENGTH_SHORT); toast.show(); } } } protected void reuseView(boolean isReuse) { isReuseView = isReuse; } protected void onFragmentVisibleChange(boolean isVisible) { } protected void onFragmentFirstVisible() { } protected boolean isFragmentVisible() { return isFragmentVisible; } }
在fragment中继承BaseFragment并实现两个方法 onFragmentVisibleChange 和onFragmentFirstVisible
@Override protected void onFragmentVisibleChange(boolean isVisible) { } @Override protected void onFragmentFirstVisible() { //这里面写请求数据的方法 PAGE_INDEX = 1; getData(0); }
