Activitty状态保存onSaveInstanceState和恢复onRestoreInstanceState
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标签(空格分隔):Actiivty 状态保存和恢复
当我们在前台和后台切换,或者横竖屏切换的时候,Activity会被重新创建,Android系统默认是帮我们自动保存和恢复了和这个Activity有关的一些状态,涉及到ActivityThread Ams的调度机制,这里暂时不要去case,我们主要看包括界面ui上view的状态,如何保证能够恢复回来。我们都知道,保存的时候会调用onSaveInstanceState保存一些数据到bundle中,恢复的时候会调用onRestoreInstanceState来恢复。那么这套机制又是什么样子的?保存的是哪些信息?又是以一个什么流程去恢复的。
1.Activity.onSaveInstanceState
protected void onSaveInstanceState(Bundle outState) { outState.putBundle(WINDOW_HIERARCHY_TAG, mWindow.saveHierarchyState()); Parcelable p = mFragments.saveAllState(); if (p != null) { outState.putParcelable(FRAGMENTS_TAG, p); } getApplication().dispatchActivitySaveInstanceState(this, outState);}
这个方法中,outState是在ActiivtyThread中新出来的,此时将mWindow的state保存到了bundle中。然后获取了当前应用的Application对象,分发交给ActivityLifecycleCallbacks去处理,等于Application预留的接口,这里不用去关注,除非有主动去注册。下面分析mWindow.saveHierarchyState的具体实现,mWindow对象是一个PhoneWindow对象。
2.PhoneWindow.saveHierarchyState
@Overridepublic Bundle saveHierarchyState() { Bundle outState = new Bundle(); if (mContentParent == null) { return outState; } SparseArray<Parcelable> states = new SparseArray<Parcelable>(); mContentParent.saveHierarchyState(states); outState.putSparseParcelableArray(VIEWS_TAG, states); // save the focused view id View focusedView = mContentParent.findFocus(); if (focusedView != null) { if (focusedView.getId() != View.NO_ID) { outState.putInt(FOCUSED_ID_TAG, focusedView.getId()); } else { if (false) { Log.d(TAG, "couldn't save which view has focus because the focused view " + focusedView + " has no id."); } } } // save the panels SparseArray<Parcelable> panelStates = new SparseArray<Parcelable>(); savePanelState(panelStates); if (panelStates.size() > 0) { outState.putSparseParcelableArray(PANELS_TAG, panelStates); } if (mDecorContentParent != null) { SparseArray<Parcelable> actionBarStates = new SparseArray<Parcelable>(); mDecorContentParent.saveToolbarHierarchyState(actionBarStates); outState.putSparseParcelableArray(ACTION_BAR_TAG, actionBarStates); } return outState;}这个方法中,mContentParent对应的是顶级视图DecorView下的child(id为 com.android.internal.R.id.content),如果它为空,直接返回。接下来分别是创建了三个SparseArray<Parcelable>,它是android提供的一个类似HashMap的类,效率比Map高,不过key必须为Interger,分别用来保存view ,panels和actionbar state的状态,构建了整个window对象的state。这里我们着重分析view层的state。因为其他两个也是类似的效果。分析mContentParent.saveHierarchyState的实现。mContentParent是一个ViewGroup对象.
3.View.saveHierarchyState(SparseArray)
public void saveHierarchyState(SparseArray<Parcelable> container) { dispatchSaveInstanceState(container);}
View和ViewGroup均由实现这个dispatchSaveInstanceState方法,类似组合模式我们见的比较多,可以推测ViewGroup中会继续分发给的child去保存状态,下面分别看看view和viewgroup中的实现,看是否是这样一回事。
View中的dispatchSaveInstanceState实现:
protected void dispatchSaveInstanceState(SparseArray<Parcelable> container) { if (mID != NO_ID && (mViewFlags & SAVE_DISABLED_MASK) == 0) { mPrivateFlags &= ~PFLAG_SAVE_STATE_CALLED; Parcelable state = onSaveInstanceState(); 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); container.put(mID, state); } }}
View中的作用在于保存自己的状态,首先会判断id号是否有设置,以及根据mViewFlags来判断是否需要保存,一般来说如果这个view没有被设置id,系统会认为你根本就没有进行操作,会认为不需要去保存状态,当然这里只是系统给出的一个规则,不用去纠结好还是不好,实际使用过程中我们需要去遵守就好,接下来回调onSaveInstanceState去保存状态,这个接口是给用户根据不同的情况去客制化保存的一些信息,每一个view都有自己的实现方式,最后将保存的Parcelable对象存储到container中,以mId–Parcelable键值对的形式存储。
ViewGroup中的dispatchSaveInstanceState实现:
@Overrideprotected void dispatchRestoreInstanceState(SparseArray<Parcelable> container) { super.dispatchRestoreInstanceState(container); final int count = mChildrenCount; final View[] children = mChildren; for (int i = 0; i < count; i++) { View c = children[i]; if ((c.mViewFlags & PARENT_SAVE_DISABLED_MASK) != PARENT_SAVE_DISABLED) { c.dispatchRestoreInstanceState(container); } }}
看代码实现,和我们推测的一致,分为两步,首先是super.dispatchRestoreInstanceState先保存自己的状态,而后是遍历去保存子child的state。以递归view树的形式去执行。通过在ViewGroup以及android的主要容器LinearLayout,RelativeLayout中发现,有些类似的容器也是不需要保存的,只需要它的子child保存了一些信息即可在重新layout的时候去恢复ui显示,因此对于容器来说,一般也就不需要保存状态了。当然如果有特殊需要,比如动态改变了容器的颜色,padding值等等,那么要想恢复也必须手动去处理保存和恢复了。
由于View是一个所有控件的父类,一般也极少直接去使用,我们来看看典型的TextView究竟是保存了哪些state
TextView.onSaveInstanceState
@Overridepublic Parcelable onSaveInstanceState() { Parcelable superState = super.onSaveInstanceState(); // Save state if we are forced to boolean save = mFreezesText; int start = 0; int end = 0; if (mText != null) { start = getSelectionStart(); end = getSelectionEnd(); if (start >= 0 || end >= 0) { // Or save state if there is a selection save = true; } } if (save) { SavedState ss = new SavedState(superState); // XXX Should also save the current scroll position! ss.selStart = start; ss.selEnd = end; if (mText instanceof Spanned) { Spannable sp = new SpannableStringBuilder(mText); if (mEditor != null) { removeMisspelledSpans(sp); sp.removeSpan(mEditor.mSuggestionRangeSpan); } ss.text = sp; } else { ss.text = mText.toString(); } if (isFocused() && start >= 0 && end >= 0) { ss.frozenWithFocus = true; } ss.error = getError(); return ss; } return superState;}
从这段保存的code来看,对于TextView主要是保存了它的mText信息和selection信息,一般来说TextView是不满足条件的,对于它的子类EditText可以满足条件,主要工作就是保存了mText信息到了SavedState的text字段中。有了这个初步的印象,下面来看看恢复的流程。
4.Activity.onRestoreInstanceState
protected void onRestoreInstanceState(Bundle savedInstanceState) { if (mWindow != null) { Bundle windowState = savedInstanceState.getBundle(WINDOW_HIERARCHY_TAG); if (windowState != null) { mWindow.restoreHierarchyState(windowState); } }}
与保存类似,onRestoreInstanceState首先会通过Bundle拿到前面onSaveInstanceState存储在WINDOW_HIERARCHY_TAG key中的bundle对象,那么这两个bundle对象是一个吗?这里我可以解释下,这两个bundle对象是同一个,每一个Activity创建的时候会同时新建一个ActivityClientRecord的binder对象,用以和远程的ams进行通信,调度Actiivty的生命周期,同时在ActivityThread的ArrayMap<>[IBinder, ActivityClientRecord] mActivities对象中会保存这些ActivityClientRecord的信息。在ActivityClientRecord中的字段token就是远程ams中标识一个Activity的标示符。mActivities中的key即是对应的这些token。这里说了这么多,就需要明白一点,当一个Activity恢复的时候,也就是relaunch的时候,会根据token去mActivities查找ActivityClientRecord,所以是会公用上一次的ActivityClientRecord对象的。这里继续跟踪可以发现,onSaveInstanceState的Bunlde对象就是ActivityClientRecord对象中的state字段,恢复的时候也是取的state字段,一步步传给onRestoreInstanceState去恢复。到这里为止,我们已经知道onSaveInstanceState和onRestoreInstanceState操作的是同一个Bundle对象,并且对应的就是ActivityClientRecord对象中的state字段。
取到onSaveInstanceState保存的信息之后,接下来分析mWindow.restoreHierarchyState的实现。
5.PhoneWindow.restoreHierarchyState(Bundle)
@Overridepublic void restoreHierarchyState(Bundle savedInstanceState) { if (mContentParent == null) { return; } SparseArray<Parcelable> savedStates = savedInstanceState.getSparseParcelableArray(VIEWS_TAG); if (savedStates != null) { mContentParent.restoreHierarchyState(savedStates); } // restore the focused view int focusedViewId = savedInstanceState.getInt(FOCUSED_ID_TAG, View.NO_ID); if (focusedViewId != View.NO_ID) { View needsFocus = mContentParent.findViewById(focusedViewId); if (needsFocus != null) { needsFocus.requestFocus(); } else { Log.w(TAG, "Previously focused view reported id " + focusedViewId + " during save, but can't be found during restore."); } } // restore the panels SparseArray<Parcelable> panelStates = savedInstanceState.getSparseParcelableArray(PANELS_TAG); if (panelStates != null) { restorePanelState(panelStates); } if (mDecorContentParent != null) { SparseArray<Parcelable> actionBarStates = savedInstanceState.getSparseParcelableArray(ACTION_BAR_TAG); if (actionBarStates != null) { doPendingInvalidatePanelMenu(); mDecorContentParent.restoreToolbarHierarchyState(actionBarStates); } else { Log.w(TAG, "Missing saved instance states for action bar views! " + "State will not be restored."); } }}
这里我们可以看到,和PhoneWindow.saveHierarchyState就是一个一一对应的关系,也是一个逆向的过程。首先是取到保存在bunlde对象VIEWS_TAG中的SparseArray对象,然后交给mContentParent.restoreHierarchyState(savedStates)去处理。
6.View.restoreHierarchyState(SparseArray)
public void restoreHierarchyState(SparseArray<Parcelable> container) { dispatchRestoreInstanceState(container);}
和上面类似,View中dispatchRestoreInstanceState实现如下:
protected void dispatchRestoreInstanceState(SparseArray<Parcelable> container) { if (mID != NO_ID) { 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()"); } } }}
ViewGroup中dispatchRestoreInstanceState实现如下:
@Overrideprotected void dispatchRestoreInstanceState(SparseArray<Parcelable> container) { super.dispatchRestoreInstanceState(container); final int count = mChildrenCount; final View[] children = mChildren; for (int i = 0; i < count; i++) { View c = children[i]; if ((c.mViewFlags & PARENT_SAVE_DISABLED_MASK) != PARENT_SAVE_DISABLED) { c.dispatchRestoreInstanceState(container); } }}
由此可以看到,在一个恢复case中,也会以递归view树的形式去恢复,如果是ViewGroup首先会恢复自己的state,然后递归去恢复它的child。在View中dispatchRestoreInstanceState方法中,获取保存在mID key中的Parcelable对象,回调onRestoreInstanceState接口,会去真正的执行restore state操作。这个接口中我们可以实现自己的业务逻辑。
下面具体来看我们上面TextView保存的逻辑和恢复的过程
TextView.onRestoreInstanceState
@Overridepublic void onRestoreInstanceState(Parcelable state) { if (!(state instanceof SavedState)) { super.onRestoreInstanceState(state); return; } SavedState ss = (SavedState)state; super.onRestoreInstanceState(ss.getSuperState()); // XXX restore buffer type too, as well as lots of other stuff if (ss.text != null) { setText(ss.text); } if (ss.selStart >= 0 && ss.selEnd >= 0) { if (mText instanceof Spannable) { int len = mText.length(); if (ss.selStart > len || ss.selEnd > len) { String restored = ""; if (ss.text != null) { restored = "(restored) "; } Log.e(LOG_TAG, "Saved cursor position " + ss.selStart + "/" + ss.selEnd + " out of range for " + restored + "text " + mText); } else { Selection.setSelection((Spannable) mText, ss.selStart, ss.selEnd); if (ss.frozenWithFocus) { createEditorIfNeeded(); mEditor.mFrozenWithFocus = true; } } } } if (ss.error != null) { final CharSequence error = ss.error; // Display the error later, after the first layout pass post(new Runnable() { public void run() { setError(error); } }); }}
可以看到保存和恢复就是一个写入和读取的过程,整个过程也是一一对应的关系。是一个可逆的过程。将这些数据恢复之后,系统重新布局,也就达到了恢复ui界面的目的。
至此,关于Activitty状态保存onSaveInstanceState和恢复onRestoreInstanceState也就分析完了,主要是一个流程问题,整个过程中保存和恢复都是共享的同一份bundle实例。对于特殊情况,系统并未handle的case,我们需要自己去实现onSaveInstanceState和onRestoreInstanceState方法。
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