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<title>
Fragment 懒加载失效, 多次加载数据 - MayGodBlessYou
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<h1>Fragment 懒加载失效, 多次加载数据</h1>
<div class="read-more clearfix">
<span class="date">2016/05/29</span>
<span>posted in </span>
<span class="posted-in"><a href='Android.html'>Android</a></span>
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</span>
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<p>ViewPager和Fragment一起使用时, 为了防止一进入Activity就加载所有Fragment的数据, 减轻Activity初始化加载的负担, 需要用懒加载的方式去加载Fragment.</p>
<span id="more"></span><!-- more -->
<p>懒加载的方法如下:</p>
<p>让Fragment继承BaseFragment, BaseFragment代码如下所示:</p>
<pre><code class="language-plain_text"> /**
* 应用程序的基类Fragment, 实现懒加载
* Created by Jack on 2016/5/17 08:31.1
* Email: zhuochangjing@foxmail.com
*/
public abstract class BaseFragment extends Fragment {
protected boolean isVisible;
protected Dialog mDialog;
/**
* 在这里实现Fragment数据的懒加载
*
* @param isVisibleToUser Fragment UI对用户是否可见
*/
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if (getUserVisibleHint()) {
isVisible = true;
onVisible();
} else {
isVisible = false;
onInvisible();
}
}
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle
savedInstanceState) {
View view = inflater.inflate(getContentView(), container, false);
initView(view);
mDialog = DialogUtil.createLoadingDialog(getActivity(), "正在加载数据中......");
return view;
}
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
lazyLoad();
}
protected void onVisible() {
lazyLoad();
}
protected void onInvisible() {
}
protected abstract void lazyLoad();
protected abstract int getContentView();
protected abstract void initView(View view);
}
</code></pre>
<p>子Fragment代码如下:</p>
<pre><code class="language-plain_text"> public class LogisticsFragment extends BaseFragment{
private boolean isPrepared;
@Override
protected void lazyLoad() {
if (isPrepared && isVisible) {
//做加载数据的网络操作
isPrepared = false;
}
}
@Override
protected void initView(View view) {
//做初始化View的操作
isPrepared = true;
}
}
</code></pre>
<p>但是现在由于ViewPager和Fragment一起用, 由于ViewPager默认只能缓存Fragment左右两边各一个Fragment, 也就是说缓存值是1, 设置缓存页面的方法如下:</p>
<pre><code class="language-plain_text">mViewpager.setOffscreenPageLimit(2);
</code></pre>
<p>如果未调用上面的设置缓存页面的方法, 默认缓存值就是1, 此时如果你的Fragment页在3个及以上时, 当你切换Fragment时, 就会造成未缓存的Fragment会执行onDestroyView, 然而再次切换到这个未缓存的Fragment或者切换到与这个未缓存的Fragment相邻的Fragment时, 他就会再次执行onCreateView方法, 也就是说再次initView, 这样isPrepared又会被置为true, 因此导致lazyLoad方法里的判断加载数据的表达式为true, 加载数据就会再次执行了.</p>
<p>这里懒加载并不是说让lazyLoad方法只执行一次, 而是通过判断让加载数据只执行一次.</p>
<p>ViewPager的设置缓存页面的方法具体细节可以翻阅官方文档: <a href="https://developer.android.google.cn/reference/android/support/v4/view/ViewPager.html#setOffscreenPageLimit(int)">setOffscreenPageLimit</a></p>
<p>参考链接: <a href="http://blog.csdn.net/feixiangdexin123087/article/details/42059975">ViewPager中切换界面Fragment被销毁的问题分析</a></p>
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