ViewPager中使用Fragment时防止数据预加载，安卓app开发教程
作者：互联网
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
final long itemId = getItemId(position);
// Do we already have this fragment?
String name = makeFragmentName(container.getId(), itemId);
Fragment fragment = mFragmentManager.findFragmentByTag(name);
if (fragment != null) {
if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
mCurTransaction.attach(fragment);
} else {
fragment = getItem(position);
if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
mCurTransaction.add(container.getId(), fragment,
makeFragmentName(container.getId(), itemId));
}
if (fragment != mCurrentPrimaryItem) {
fragment.setMenuVisibility(false);
fragment.setUserVisibleHint(false);
}
return fragment;
}
在instantiateItem方法的最后我们发现了调用的地方：
if (fragment != mCurrentPrimaryItem) {
fragment.setMenuVisibility(false);
fragment.setUserVisibleHint(false);
}
这里有个判断`fragment != mCurrentPrimaryItem`这个时候会把Fragment的Visible设为false, 那这个`mCurrentPrimaryItem`又是什么呢，继续搜索源码：
@SuppressWarnings(“ReferenceEquality”)
@Override
public void setPrimaryItem(ViewGroup container, int position, Object object) {
Fragment fragment = (Fragment)object;
if (fragment != mCurrentPrimaryItem) {
if (mCurrentPrimaryItem != null) {
mCurrentPrimaryItem.setMenuVisibility(false);
mCurrentPrimaryItem.setUserVisibleHint(false);
}
if (fragment != null) {
fragment.setMenuVisibility(true);
fragment.setUserVisibleHint(true);
}
mCurrentPrimaryItem = fragment;
}
}
在一个`setPrimaryItem`的方法中找到这个变量的赋值，这个方法将传进来的fragment的visible设置为true, 同时会更新mCurrentPrimaryItem变量的值。我们再继续搜索`setPrimaryItem`这个方法的调用，结果在当前FragmentPagerAdapter的源码中没有找到，但是在它的父类PagerAdapter的源码中找到了它的定义：
/**
* Called to inform the adapter of which item is currently considered to
* be the "primary", that is the one show to the user as the current page.
*
* @param container The containing View from which the page will be removed.
* @param position The page position that is now the primary.
* @param object The same object that was returned by
* {@link #instantiateItem(View, int)}.
*/
public void setPrimaryItem(ViewGroup container, int position, Object object) {
setPrimaryItem((View) container, position, object);
}
看注释大概明白了这个方法的含义是设置为ViewPager中当前展示给用户的那一页。
继续到ViewPager的源码中搜索，找到了调用它的地方：
是在一个`populate()`的方法中调用的，搜索发现好多地方调用了它，但是发现了有两个关键的地方：
/**
* Set a PagerAdapter that will supply views for this pager as needed.
*
* @param adapter Adapter to use
*/
public void setAdapter(PagerAdapter adapter) {
//...忽略部分源码
if (mAdapter != null) {
//...忽略部分源码
if (mRestoredCurItem >= 0) {
mAdapter.restoreState(mRestoredAdapterState, mRestoredClassLoader);
setCurrentItemInternal(mRestoredCurItem, false, true);
mRestoredCurItem = -1;
mRestoredAdapterState = null;
mRestoredClassLoader = null;
} else if (!wasFirstLayout) {
populate();
} else {
requestLayout();
}
}
//...忽略部分源码
}
/**
* Set the currently selected page.
*
* @param item Item index to select
* @param smoothScroll True to smoothly scroll to the new item, false to transition immediately
*/
public void setCurrentItem(int item, boolean smoothScroll) {
mPopulatePending = false;
setCurrentItemInternal(item, smoothScroll, false);
}
void setCurrentItemInternal(int item, boolean smoothScroll, boolean always) {
setCurrentItemInternal(item, smoothScroll, always, 0);
}
void setCurrentItemInternal(int item, boolean smoothScroll, boolean always, int velocity) {
//...忽略部分源码
if (mFirstLayout) {
// We don't have any idea how big we are yet and shouldn't have any pages either.
// Just set things up and let the pending layout handle things.
mCurItem = item;
if (dispatchSelected) {
dispatchOnPageSelected(item);
}
requestLayout();
} else {
populate(item);
scrollToItem(item, smoothScroll, velocity, dispatchSelected);
}
}
```
标签：false,Fragment,ViewPager,安卓,item,源码,mCurrentPrimaryItem,fragment,container 来源： https://blog.csdn.net/m0_61418295/article/details/120147419