Android Fragment+ViewPager 懒加载

摘要: Android Fragment+ViewPager 实现懒加载

public abstract class BaseLazyFragment extends BaseFragment {

private boolean isFragmentVisible;
private boolean isFirstVisible = true;
private View rootView;
@Override
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
//如果setUserVisibleHint()在rootView创建前调用时，那么
//就等到rootView创建完后才回调onFragmentVisibleChange(true)
//保证onFragmentVisibleChange()的回调发生在rootView创建完成之后，以便支持ui操作
if (rootView == null) {
rootView = view;
if (getUserVisibleHint()) {
if (isFirstVisible) {
onLoadData();
isFirstVisible = false;
}
onFragmentVisibleChange(true);
isFragmentVisible = true;
}
}
super.onViewCreated(rootView, savedInstanceState);
}

@Override
public void onDestroy() {
super.onDestroy();
initVariable();
}

private void initVariable() {
isFirstVisible = true;
isFragmentVisible = false;
rootView = null;
}

/**
* 去除setUserVisibleHint()多余的回调场景，保证只有当fragment可见状态发生变化时才回调
* 回调时机在view创建完后，所以支持ui操作，解决在setUserVisibleHint()里进行ui操作有可能报null异常的问题
* <p>
* 可在该回调方法里进行一些ui显示与隐藏，比如加载框的显示和隐藏
*
* @param isVisible true  不可见 -> 可见
*                  false 可见  -> 不可见
*/
protected void onFragmentVisibleChange(boolean isVisible) {

}

/**
* 在fragment首次可见时回调，可在这里进行加载数据，保证只在第一次打开Fragment时才会加载数据，
* 这样就可以防止每次进入都重复加载数据
* 该方法会在 onFragmentVisibleChange() 之前调用，所以第一次打开时，可以用一个全局变量表示数据下载状态，
* 然后在该方法内将状态设置为下载状态，接着去执行下载的任务
* 最后在 onFragmentVisibleChange() 里根据数据下载状态来控制下载进度ui控件的显示与隐藏
*/
protected void onLoadData() {
}

public boolean isFragmentVisible() {
return isFragmentVisible;
}

/**
* setUserVisibleHint()在Fragment创建时会先被调用一次，传入isVisibleToUser = false
* 如果当前Fragment可见，那么setUserVisibleHint()会再次被调用一次，传入isVisibleToUser = true
* 如果Fragment从可见->不可见，那么setUserVisibleHint()也会被调用，传入isVisibleToUser = false
* 总结：setUserVisibleHint()除了Fragment的可见状态发生变化时会被回调外，在new Fragment()时也会被回调
* 如果我们需要在 Fragment 可见与不可见时干点事，用这个的话就会有多余的回调了，那么就需要重新封装一个
*
* @param isVisibleToUser
*/
@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) {
onLoadData();
isFirstVisible = false;
}

if (isVisibleToUser) {
onFragmentVisibleChange(true);
isFragmentVisible = true;
return;
}

if (isFragmentVisible) {
isFragmentVisible = false;
onFragmentVisibleChange(false);
}
}
}

/**
* Called immediately after {@link #onCreateView(LayoutInflater, ViewGroup, Bundle)}
* has returned, but before any saved state has been restored in to the view.
* This gives subclasses a chance to initialize themselves once
* they know their view hierarchy has been completely created.  The fragment's
* view hierarchy is not however attached to its parent at this point.
* @param view The View returned by {@link #onCreateView(LayoutInflater, ViewGroup, Bundle)}.
* @param savedInstanceState If non-null, this fragment is being re-constructed
* from a previous saved state as given here.
*/
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
}

onViewCreated(View view, @Nullable Bundle savedInstanceState) 在onCreateView(LayoutInflater, ViewGroup, Bundle) 之后调用