基于api28

setter getter

    public boolean isSaveEnabled() {
        return (mViewFlags & SAVE_DISABLED_MASK) != SAVE_DISABLED;
    }

    public void setSaveEnabled(boolean enabled) {
        setFlags(enabled ? 0 : SAVE_DISABLED, SAVE_DISABLED_MASK);
    }

這里對應的標志位是SAVE_DISABLED

還有兩個類似的方法：

    public boolean isSaveFromParentEnabled() {
        return (mViewFlags & PARENT_SAVE_DISABLED_MASK) != PARENT_SAVE_DISABLED;
    }

    public void setSaveFromParentEnabled(boolean enabled) {
        setFlags(enabled ? 0 : PARENT_SAVE_DISABLED, PARENT_SAVE_DISABLED_MASK);
    }

這里對應的標志位是PARENT_SAVE_DISABLED

兩個標志位有什么區(qū)別呢？我們來看下整個事件的傳遞過程就知道了。這里只看saveInstanceState的傳遞，restoreInstanceState是一樣的。

事件傳遞

saveInstanceState事件最早由ActivityThread發(fā)起。
ActivityThread源碼不再貼了，清楚調用時機即可：

1. api < 11，onSaveInstance在onPause之前執(zhí)行
2. 11 <= api < 28，onSaveInstance在onPause之后，onStop之前執(zhí)行
3. api >= 28，onSaveInstance在onStop之后執(zhí)行

Activity:

    //由ActivityThread調用
    final void performSaveInstanceState(Bundle outState) {
        onSaveInstanceState(outState);
        ...
    }

    protected void onSaveInstanceState(Bundle outState) {
        outState.putBundle(WINDOW_HIERARCHY_TAG, mWindow.saveHierarchyState());
        ...
    }

這里的mWindow是PhoneWindow。

PhoneWindow:

@Override
    public Bundle saveHierarchyState() {
        Bundle outState = new Bundle();
        if (mContentParent == null) {
            return outState;
        }

        //當前窗口內的view的狀態(tài)都會保存在這個SparseArray中
        SparseArray<Parcelable> states = new SparseArray<Parcelable>();
        //調用view的方法，開始遍歷視圖樹
        mContentParent.saveHierarchyState(states);
        ...
        return outState;
    }

View:

    public void saveHierarchyState(SparseArray<Parcelable> container) {
        dispatchSaveInstanceState(container);
    }

    protected void dispatchSaveInstanceState(SparseArray<Parcelable> container) {
        //想要保存自身狀態(tài)，需要設置id，并且未設置SAVE_DISABLED_MASK標志位
        if (mID != NO_ID && (mViewFlags & SAVE_DISABLED_MASK) == 0) {
            mPrivateFlags &= ~PFLAG_SAVE_STATE_CALLED;
            //調用onSaveInstanceState，真正實現保存自身狀態(tài)
            Parcelable state = onSaveInstanceState();
            if ((mPrivateFlags & PFLAG_SAVE_STATE_CALLED) == 0) {
                throw new IllegalStateException(
                        "Derived class did not call super.onSaveInstanceState()");
            }
            if (state != null) {
                // 最終放到這里保存
                container.put(mID, state);
            }
        }
    }

    @CallSuper
    @Nullable protected Parcelable onSaveInstanceState() {
        ...
    }

ViewGroup:

    @Override
    protected void dispatchSaveInstanceState(SparseArray<Parcelable> container) {
        //先調用super.dispatchSaveInstanceState來保存自身的狀態(tài)
        super.dispatchSaveInstanceState(container);
        final int count = mChildrenCount;
        final View[] children = mChildren;
        //開始遍歷
        for (int i = 0; i < count; i++) {
            View c = children[i];
            if ((c.mViewFlags & PARENT_SAVE_DISABLED_MASK) != PARENT_SAVE_DISABLED) {
                //未設置PARENT_SAVE_DISABLED才會傳遞給c，
                //否則，c及其子View都不會收到saveInstanceState事件
                c.dispatchSaveInstanceState(container);
            }
        }
    }

結論如下：

1. setSaveEnabled(boolean enabled)僅僅影響自身，不會影響其子View
2. setSaveFromParentEnabled(boolean enabled)同時影響自身及其子View
3. 要真正實現保存自身狀態(tài)，還需要為View設置id
4. View的id作為SparseArray中的鍵值，還必須在視圖樹中保證唯一性，否則會造成混亂

關于第四點，特意試驗了一下，id相同的兩個view，在onSaveInstance中保存自己的hashCode，然后在onRestoreInstanceState讀取出來，結果如下：

2019-01-25 15:30:02.385 8499-8499/com.sollian.sourcestudy D/MyTextView: onSaveInstanceState: 73681134
2019-01-25 15:30:02.386 8499-8499/com.sollian.sourcestudy D/MyTextView: onSaveInstanceState: 50315663
2019-01-25 15:30:03.389 8499-8499/com.sollian.sourcestudy D/MyTextView: onRestoreInstanceState: 50315663
2019-01-25 15:30:03.389 8499-8499/com.sollian.sourcestudy D/MyTextView: onRestoreInstanceState: 50315663

可以看到，第一個view的數據被后面的覆蓋了。

最后，onRestoreInstanceState的調用時機在onStart之后，onResume之前。附上一段生命周期圖：

圖1