Android

记一次踩坑：onSaveInstanceState & onRestoreInstanceState过程分析

摘要

app灰度中，发现如下crash一枚。而且全部集中在Android 4.0和5.0的手机上。

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Caused by java.lang.IllegalArgumentException
Wrong state class, expecting View State but received class android.widget.HorizontalScrollView$SavedState instead. This usually happens when two views of different type have the same id in the same hierarchy. This view's id is id/0x0. Make sure other views do not use the same id.

分析步骤

1.查看异常大意

异常大意：同一个层级中存在两个不同类型的HorizontalScrollView，但却使用了同样的id。

1.看到HorizontalScrollView，第一个是联想到商详页多件装使用HorizontalScrollView控件，查看代码，

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mMultiViewStub = new ViewStub(getContext(), R.layout.goods_detail_multi_set);
<HorizontalScrollView
	xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/horizontal_scroll_mult_layout"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    >
...
</HorizontalScrollView>
if (mMultiViewStub != null) {
	mHorizontalScrollView = (HorizontalScrollView) mMultiViewStub.inflate()；
    mMultiViewStub = null;
}

在特定条件下mMultiViewStub被inflate，通过AS自带的Tools-Layout Inspector工具查看此处mID内容为0x0;

2.发现通用组件标题栏中使用SmartTabLayout，它继承自HorizontalScrollView，通过AS自带的Tools-Layout Inspector工具查看此处mID内容为NO_ID,NO_ID是一个常量，值为-1。

很明显，这两个id并不相等，所以不是因为使用了同样的id导致此异常。

2.搜索异常关键字

搜索异常关键字when two views of different type have the same id in the same hierarchy，发现抛出异常的地方和保存&恢复数据相关。

3.尝试复现crash

找了一台5.1的测试机，打开开发者选项-不保留活动，进入一个包含多件装的商品详情页，从多件装A切换到多件装B后，将app切到后台，再切回前台，即crash。

分析原因

那么onSaveInstanceState & onRestoreInstanceState 到底做了什么。来扒一扒源码。

以下基于5.1.1源码分析：

1.1 Activity#onSaveInstanceState

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protected void onSaveInstanceState(Bundle outState) {
	// 1.注意mWindow#saveHierarchyState的调用
       outState.putBundle(WINDOW_HIERARCHY_TAG, mWindow.saveHierarchyState());
       // 2.mFragments.saveAllState主要保存Fragment相关的数据：
       // 2.1 执行一些未完成的事务、退出动画等；
       // 2.2 把所有active状态的fragment转化为可序列化的FragmentState对象保存下来
       // 2.3 把所有添加到FragmentManager的fragment的index记录下来
       // 2.4 把回退栈的信息记录下来
       // 3.在Activity#onCreate中恢复数据
       Parcelable p = mFragments.saveAllState();
       if (p != null) {
           outState.putParcelable(FRAGMENTS_TAG, p);
       }
       getApplication().dispatchActivitySaveInstanceState(this, outState);
   }

1.2 Activity#onRestoreInstanceState

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protected void onRestoreInstanceState(Bundle savedInstanceState) {
       if (mWindow != null) {
           Bundle windowState = savedInstanceState.getBundle(WINDOW_HIERARCHY_TAG);
           if (windowState != null) {
             // 1.注意mWindow#restoreHierarchyState的调用  
             mWindow.restoreHierarchyState(windowState);
           }
       }
   }

2.1 PhoneWindow#saveHierarchyState

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  public Bundle saveHierarchyState() {
  	// 1.new Bundle
      Bundle outState = new Bundle();
      // 2.mContentParent为id=ID_ANDROID_CONTENT的ViewGroup
      if (mContentParent == null) {
          return outState;
      }
// 3.new SparseArray
      SparseArray<Parcelable> states = new SparseArray<Parcelable>();
      // 4.调用View#saveHierarchyState保存状态
      mContentParent.saveHierarchyState(states);
      outState.putSparseParcelableArray(VIEWS_TAG, states);
      ...
      return outState;
  }

2.2 PhoneWindow#restoreHierarchyState

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public void restoreHierarchyState(Bundle savedInstanceState) {
    if (mContentParent == null) {
        return;
    }
    SparseArray<Parcelable> savedStates
            = savedInstanceState.getSparseParcelableArray(VIEWS_TAG);
    if (savedStates != null) {
        // 1.调用View#restoreHierarchyState恢复状态
    	mContentParent.restoreHierarchyState(savedStates);
    }
    ...
}

3.1 View#dispatchSaveInstanceState

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protected void dispatchSaveInstanceState(SparseArray<Parcelable> container) {
	// 1.一个View必须有id，SAVE_DISABLED_MASK标志没被打开才保存状态
       if (mID != NO_ID && (mViewFlags & SAVE_DISABLED_MASK) == 0) {
           mPrivateFlags &= ~PFLAG_SAVE_STATE_CALLED;
           // 2.提供让子类重写的方法，以实现自己的保存逻辑
           Parcelable state = onSaveInstanceState();
           // 3.检测，子类必须要调用父类的onSaveInstanceState()方法，否则抛异常
           if ((mPrivateFlags & PFLAG_SAVE_STATE_CALLED) == 0) {
               throw new IllegalStateException(
                       "Derived class did not call super.onSaveInstanceState()");
           }
           if (state != null) {
               // Log.i("View", "Freezing #" + Integer.toHexString(mID)
               // + ": " + state);
               // 4.将state放进SparseArray中，以view自身的id为key，当key相同的情况下，后面的put会覆盖掉前面put的结果
               container.put(mID, state);
           }
       }
   }
	protected Parcelable onSaveInstanceState() {
   	// 1.设置标志位，在dispatchSaveInstanceState会检测
       mPrivateFlags |= PFLAG_SAVE_STATE_CALLED;
       // 2.默认不保存任何东西
       return BaseSavedState.EMPTY_STATE;
   }

3.2 View#dispatchRestoreInstanceState

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protected void dispatchRestoreInstanceState(SparseArray<Parcelable> container) {
       if (mID != NO_ID) {
       	// 1.通过id拿到state
           Parcelable state = container.get(mID);
           if (state != null) {
               // Log.i("View", "Restoreing #" + Integer.toHexString(mID)
               // + ": " + state);
               mPrivateFlags &= ~PFLAG_SAVE_STATE_CALLED;
               onRestoreInstanceState(state);
               if ((mPrivateFlags & PFLAG_SAVE_STATE_CALLED) == 0) {
                   throw new IllegalStateException(
                           "Derived class did not call super.onRestoreInstanceState()");
               }
           }
       }
   }
   
   protected void onRestoreInstanceState(Parcelable state) {
       mPrivateFlags |= PFLAG_SAVE_STATE_CALLED;
       if (state != BaseSavedState.EMPTY_STATE && state != null) {
           throw new IllegalArgumentException("Wrong state class, expecting View State but "
                   + "received " + state.getClass().toString() + " instead. This usually happens "
                   + "when two views of different type have the same id in the same hierarchy. "
                   + "This view's id is " + ViewDebug.resolveId(mContext, getId()) + ". Make sure "
                   + "other views do not use the same id.");
       }
   }

4.1 HorizontalScrollView#onSaveInstanceState

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@Override
    protected Parcelable onSaveInstanceState() {
        if (mContext.getApplicationInfo().targetSdkVersion <= Build.VERSION_CODES.JELLY_BEAN_MR2) {
            // Some old apps reused IDs in ways they shouldn't have.
            // Don't break them, but they don't get scroll state restoration.
            return super.onSaveInstanceState();
        }
        Parcelable superState = super.onSaveInstanceState();
        SavedState ss = new SavedState(superState);
        ss.scrollPosition = mScrollX;
        ss.isLayoutRtl = isLayoutRtl();
        return ss;
    }

可以看到，HorizontalScrollView复写了onSaveInstanceState方法，所以符合条件state != BaseSavedState.EMPTY_STATE && state != null，所以在恢复数据的时候抛出异常。

6.0View#onRestoreInstanceState对比

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protected void onRestoreInstanceState(Parcelable state) {
        mPrivateFlags |= PFLAG_SAVE_STATE_CALLED;
        if (state != null && !(state instanceof AbsSavedState)) {
            throw new IllegalArgumentException("Wrong state class, expecting View State but "
                    + "received " + state.getClass().toString() + " instead. This usually happens "
                    + "when two views of different type have the same id in the same hierarchy. "
                    + "This view's id is " + ViewDebug.resolveId(mContext, getId()) + ". Make sure "
                    + "other views do not use the same id.");
        }
        if (state != null && state instanceof BaseSavedState) {
            mStartActivityRequestWho = ((BaseSavedState) state).mStartActivityRequestWhoSaved;
        }
    }

对比发现，条件从state != BaseSavedState.EMPTY_STATE && state != null变成了state != null && !(state instanceof AbsSavedState)，在6.0中HorizontalScrollView#onSaveInstanceState对比5.0并没有太大变化，SavedState继承自AbsSavedState，所以state instanceof AbsSavedState为true，在6.0机型上并不会抛出该异常。

解决办法

复写Activity#onSaveInstanceState和Activity#onRestoreInstanceState方法，在6.0(不包含)及以下的版本，不调用super#onSaveInstanceState和super#onRestoreInstanceState方法。

总结

当View确定要保存&恢复数据的时候，请确保它们有唯一id。因为Android内部用id作为保存&恢复数据时使用的Key(SparseArray的key)，不指定唯一id会造成数据被覆盖。

参考

View的onSaveInstanceState和onRestoreInstanceState过程分析