你永远都追不上比你优秀的人，因为他们比你更努力~ 【今日份丧】
最近日常的需求量激增，写代码写的石乐志。上周在实现一个ViewPager+Fragment的时候在Fragment里面的onCreateView写下了下面这行代码
override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?,
savedInstanceState: Bundle?): View? {
mRootView = inflater?.inflate(R.layout.fragment_doemstic_city, container)
initView()
initIndexBar()
initData()
return mRootView
}
正常看也没觉得有毛病，但是一旦run起来，你会看到如下报错（移除部分信息）：
java.lang.StackOverflowError: stack size 8MB
at android.view.ViewGroup.jumpDrawablesToCurrentState(ViewGroup.java:6958)
at android.view.View.onAttachedToWindow(View.java:16862)
at android.view.ViewGroup.onAttachedToWindow(ViewGroup.java:4837)
at android.support.v4.view.ViewPager.onAttachedToWindow(ViewPager.java:1538)
at android.view.View.dispatchAttachedToWindow(View.java:17377)
at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:3319)
at android.view.ViewGroup.addViewInner(ViewGroup.java:4955)
at android.view.ViewGroup.addViewInLayout(ViewGroup.java:4891)
at android.view.ViewGroup.addViewInLayout(ViewGroup.java:4869)
at android.support.v4.view.ViewPager.addView(ViewPager.java:1477)
at android.view.ViewGroup.addView(ViewGroup.java:4686)
at android.view.ViewGroup.addView(ViewGroup.java:4659)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1427)
at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1752)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1821)
at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:797)
at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2595)
at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2382)
at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2337)
at android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:2214)
at android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:649)
at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:145)
at android.support.v4.view.ViewPager.populate(ViewPager.java:1238)
at android.support.v4.view.ViewPager.populate(ViewPager.java:1086)
at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1616)
at android.view.View.measure(View.java:21998)
黑人问号
怎么办？？？ 谷歌一下
网上给出的说法和解法
Inflate里面把ViewGroup传进去了，因为每一个View只能有一个父view即parentView。当container不为空时，比如此fragment所待在的activity的layout。而onCreateView中返回的view是给ViewPager使用的，所以就会出现这个view有两个parentView－即activity的layout和viewPager，所以会报出异常。解法如下：
//错误示范
mRootView = inflater?.inflate(R.layout.fragment_doemstic_city, container)
//正确写法
mRootView = inflater?.inflate(R.layout.fragment_doemstic_city, null)
mRootView = inflater?.inflate(R.layout.fragment_doemstic_city, container,false)
根据inflate的源码我们知道当传入的container不为null时，container会成为R.layout.XX布局的RootView
但是难道不应该报错是The specified child already has a parent. You must call removeView() on the child’s parent first......为什么会是StackOverflowError
不生气 不生气 Let's read the fucking source code
首先根据错误信息可以知道这次事件发生的简单经过是:
ViewPager(onMeasure -> populate -> addView)
ViewGroup(addViewInLayout -> addViewInner)
View(dispatchAttachedToWindow -> onAttachedToWindow -> jumpDrawablesToCurrentState)
日常经验可知在onMeasure和populate时是得不到有用的信息，我们从addView的时候开始追吧！！
ViewPager:addView
@Override
public void addView(View child, int index, ViewGroup.LayoutParams params) {
if (!checkLayoutParams(params)) {
params = generateLayoutParams(params);
}
final LayoutParams lp = (LayoutParams) params;
// Any views added via inflation should be classed as part of the decor
lp.isDecor |= isDecorView(child);
if (mInLayout) {
if (lp != null && lp.isDecor) {
throw new IllegalStateException("Cannot add pager decor view during layout");
}
lp.needsMeasure = true;
addViewInLayout(child, index, params);
} else {
super.addView(child, index, params);
}
if (USE_CACHE) {
if (child.getVisibility() != GONE) {
child.setDrawingCacheEnabled(mScrollingCacheEnabled);
} else {
child.setDrawingCacheEnabled(false);
}
}
}
没有有用的信息，接着看addViewInLayout
ViewPager:addViewInLayout
protected boolean addViewInLayout(View child, int index, LayoutParams params) {
return addViewInLayout(child, index, params, false);
}
protected boolean addViewInLayout(View child, int index, LayoutParams params,
boolean preventRequestLayout) {
if (child == null) {
throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
}
//注意看这里,强行将child的parent置空，难怪没有出现The specified child already has a parent. You must call removeView() on the child’s parent first
child.mParent = null;
addViewInner(child, index, params, preventRequestLayout);
child.mPrivateFlags = (child.mPrivateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
return true;
}
有了一点点眉目
ViewGroup:addViewInner
if (mTransition != null) {
// Don't prevent other add transitions from completing, but cancel remove
// transitions to let them complete the process before we add to the container
mTransition.cancel(LayoutTransition.DISAPPEARING);
}
//在ViewPager里面child.parent = null 所以这里才没有出发这个异常 为后面的StackOverflowError埋下了伏笔
if (child.getParent() != null) {
throw new IllegalStateException("The specified child already has a parent. " +
"You must call removeView() on the child's parent first.");
}
//省略无用信息
AttachInfo ai = mAttachInfo;
if (ai != null && (mGroupFlags & FLAG_PREVENT_DISPATCH_ATTACHED_TO_WINDOW) == 0) {
boolean lastKeepOn = ai.mKeepScreenOn;
ai.mKeepScreenOn = false;
child.dispatchAttachedToWindow(mAttachInfo, (mViewFlags&VISIBILITY_MASK));
if (ai.mKeepScreenOn) {
needGlobalAttributesUpdate(true);
}
ai.mKeepScreenOn = lastKeepOn;
}
//省略无用信息
}
好了到这里原因已经很明显了，因为ViewPager里面强行将childView的parentView置空，导致在ViewGroup里面没有检测出来，而实际情况确是因为错误的写法导致childView以及attach到了container上，而之后又被add到了ViewPager上，这种情况导致了整个布局树中存在循环引用，所以在最后会无限制的调用jumpDrawablesToCurrentState，递归无法结束....