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Android后台杀死系列之一:FragmentActivity及PhoneWindow后台杀死处理机制(上篇)

猪小花1号2018-08-29 13:30

作者:李尚

App在后台久置后,再次从桌面或最近的任务列表唤醒时经常会发生崩溃,这往往是App在后台被系统杀死,再次恢复的时候遇到了问题,而在使用FragmentActivity+Fragment的时候会更加频繁。比如,如果Fragment没有提供默认构造方法,就会在重建的时候因为反射创建Fragment失败而崩溃,再比如,在onCreate里面new 一个FragmentDialog,并且show,被后台杀死后,再次唤醒的时候,就会show两个对话框,这是为什么?其实这就涉及了后台杀死及恢复的机制,其中涉及的知识点主要是FragmentActivity、ActivityManagerService、LowMemoryKiller机制、ActivityStack、Binder等一系列知识点。放在一篇文章里面可能会有些长,因此,Android后台杀死系列写了三篇:

本篇是Android后台杀死系列的第一篇,主要讲解在开发过程中,由于后台杀死涉及的一些崩溃,以及如何避免这些崩溃,还有就是简单的介绍一下onSaveInstanceState与onRestoreInstanceState执行时机与原理,这两个函数也是Android面试时常问的两个点,是比简单的启动模式Activity声明周期稍微更深入细致一些的地方,也通过这个点引入后台杀死及恢复原理。

FragmentActivity被后台杀死后恢复逻辑

当App被后台异常杀死后,再次点击icon,或者从最近任务列表进入的时候,系统会帮助恢复当时的场景,重新创建Activity,对于FragmentActivity,由于其中有Framgent,逻辑会相对再复杂一些,系统会首先重建被销毁的Fragment。

举个栗子

我们创建一个Activity,并且在onCreate函数中新建并show一个DialogFragment,之后通过某种方式将APP异常杀死(RogueKiller模拟后台杀死工具),再次从最近的任务唤起App的时候,会发现显示了两个DialogFragment,代码如下:

public class DialogFragmentActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        DialogFragment dialogFragment = new FragmentDlg();
        dialogFragment.show(getSupportFragmentManager(), "");
    }

这不仅让我们奇怪,为什么呢?虽然被杀死了,但是onCreate函数在执行的时候还是只执行了一次啊,为什么会出现两个DialogFragment,这里其实就有一个DialogFragment是通过Android自身的恢复重建机制重建出来,在异常杀死的情况下onCreate(Bundle savedInstanceState)函数的savedInstanceState参数也不是null,而是包含了被杀死时所保存的场景信息。再来看个崩溃的例子,新建一个CrashFragment,并且丢弃默认无参构造方法:

public class CrashFragment extends Fragment {

    public CrashFragment(String tag) {
        super();
    }
}

之后再Activity中Add或replace添加这个CrashFragment,在CrashFragment显示后,通过RogueKiller模拟后台杀死工具模拟后台杀死,再次从最近任务列表里唤起App的时候,就会遇到崩溃,

Caused by: android.support.v4.app.Fragment$InstantiationException: 
  Unable to instantiate fragment xxx.CrashFragment: 
  make sure class name exists, is public, and has an empty constructor that is public
        at android.support.v4.app.Fragment.instantiate(Fragment.java:431)
        at android.support.v4.app.FragmentState.instantiate(Fragment.java:102)
        at android.support.v4.app.FragmentManagerImpl.restoreAllState(FragmentManager.java:1952)
        at android.support.v4.app.FragmentController.restoreAllState(FragmentController.java:144)
        at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:307)
        at android.support.v7.app.AppCompatActivity.onCreate(AppCompatActivity.java:81)

上面的这两个问题主要涉及后台杀死后FragmentActivity自身的恢复机制,其实super.onCreate(savedInstanceState)在恢复时做了很多我们没有看到的事情,先看一下崩溃:

为什么Fragment没有无参构造方法会引发崩溃

看一下support-V4中FragmentActivity中onCreate代码如下:

protected void onCreate(@Nullable Bundle savedInstanceState) {
    mFragments.attachHost(null /*parent*/);

    super.onCreate(savedInstanceState);
                    ...
    if (savedInstanceState != null) {
        Parcelable p = savedInstanceState.getParcelable(FRAGMENTS_TAG);
        mFragments.restoreAllState(p, nc != null ? nc.fragments : null);
    }
    mFragments.dispatchCreate();
}

可以看到如果savedInstanceState != null,就会执行mFragments.restoreAllState逻辑,其实这里就牵扯到恢复时重建逻辑,再被后台异常杀死前,或者说在Activity的onStop执行前,Activity的现场以及Fragment的现场都是已经被保存过的,其实是被保存早ActivityManagerService中,保存的格式FragmentState,重建的时候,会采用反射机制重新创Fragment

void restoreAllState(Parcelable state, List<Fragment> nonConfig) {

      ...
         for (int i=0; i<fms.mActive.length; i++) {
        FragmentState fs = fms.mActive[i];
        if (fs != null) {
            Fragment f = fs.instantiate(mHost, mParent);
            mActive.add(f);
    ...

其实就是调用FragmentState的instantiate,进而调用Fragment的instantiate,最后通过反射,构建Fragment,也就是,被加到FragmentActivity的Fragment在恢复的时候,会被自动创建,并且采用Fragment的默认无参构造方法,如果没哟这个方法,就会抛出InstantiationException异常,这也是为什么第二个例子中会出现崩溃的原因。

 */
public static Fragment instantiate(Context context, String fname, @Nullable Bundle args) {
    try {
        Class<?> clazz = sClassMap.get(fname);
        if (clazz == null) {
            // Class not found in the cache, see if it's real, and try to add it
            clazz = context.getClassLoader().loadClass(fname);
            sClassMap.put(fname, clazz);
        }
        Fragment f = (Fragment)clazz.newInstance();
        if (args != null) {
            args.setClassLoader(f.getClass().getClassLoader());
            f.mArguments = args;
        }
        return f;
    } catch (ClassNotFoundException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (java.lang.InstantiationException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (IllegalAccessException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    }
}

可以看到场景二提示的errormsg跟抛出的异常是可以对应上的,其实Fragment源码里面也说得很清楚:

 /**
 * Default constructor.  <strong>Every</strong> fragment must have an
 * empty constructor, so it can be instantiated when restoring its
 * activity's state.  It is strongly recommended that subclasses do not
 * have other constructors with parameters, since these constructors
 * will not be called when the fragment is re-instantiated; instead,
 * arguments can be supplied by the caller with {@link #setArguments}
 * and later retrieved by the Fragment with {@link #getArguments}.
 * 
 * <p>Applications should generally not implement a constructor.  The
 * first place application code an run where the fragment is ready to
 * be used is in {@link #onAttach(Activity)}, the point where the fragment
 * is actually associated with its activity.  Some applications may also
 * want to implement {@link #onInflate} to retrieve attributes from a
 * layout resource, though should take care here because this happens for
 * the fragment is attached to its activity.
 */

public Fragment() {
}

大意就是,Fragment必须有一个空构造方法,这样才能保证重建流程,并且,Fragment的子类也不推荐有带参数的构造方法,最好采用setArguments来保存参数。下面再来看下为什么会出现两个DialogFragment。

为什么出现两个DialogFragment

Fragment在被创建之后,如果不通过add或者replace添加到Activity的布局中是不会显示的,在保存现场的时候,也是保存了add的这个状态的,来看一下Fragment的add逻辑:此时被后台杀死,或旋转屏幕,被恢复的DialogFragmentActivity时会出现两个FragmentDialog,一个被系统恢复的,一个新建的。

Add一个Fragment,并显示的原理--所谓Fragment生命周期

通常我们FragmentActivity使用Fragment的方法如下:假设是在oncreate函数中:

@Override
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Fragment fr = Fragment.instance("")
        getSupportFragmentManager().beginTransaction()
        .add(R.id.container,fr).commit();

其中getSupportFragmentManager返回的是FragmentManager的子类FragmentManagerImpl,FragmentManagerImpl是FragmentActivity的一个内部类,其Fragment的管理逻辑都是由FragmentManagerImpl来处理的,本文是基于4.3,后面的高版本引入了FragmentController其实也只是多了一层封装,原理差别不是太大,有兴趣可以自己分析:

public class FragmentActivity extends Activity{
    ...
    final FragmentManagerImpl mFragments = new FragmentManagerImpl();
   ...
    final FragmentContainer mContainer = new FragmentContainer() {
        @Override
        @Nullable
        public View findViewById(int id) {
            return FragmentActivity.this.findViewById(id);
        }

        @Override
        public boolean hasView() {
            Window window = FragmentActivity.this.getWindow();
            return (window != null && window.peekDecorView() != null);
        }
    };

FragmentManagerImpl的beginTransaction()函数返回的是一个BackStackRecord()

@Override
public FragmentTransaction beginTransaction() {
    return new (this);
}

从名字就可以看出,beginTransaction是为FragmentActivity生成一条Transaction(事务),可以执行,也可以反向,作为退栈的一个依据,FragmentTransaction的add函数实现如下,

public FragmentTransaction add(Fragment fragment, String tag) {
    doAddOp(0, fragment, tag, OP_ADD);//异步操作的,跟Hander类似
    return this;
}


private void doAddOp(int containerViewId, Fragment fragment, String tag, int opcmd) {
    fragment.mFragmentManager = mManager;
     ...
    Op op = new Op();
    op.cmd = opcmd;
    op.fragment = fragment;
    addOp(op);
}

之后commit这个Transaction, 将Transaction插入到Transaction队列中去,最终会回调FragmentManager的addFragment方法,将Fragment添加FragmentManagerImpl到维护Fragment列表中去,并且根据当前的Activity状态,将Fragment调整到合适的状态,代码如下:

public void addFragment(Fragment fragment, boolean moveToStateNow) {

    if (mAdded == null) {
        mAdded = new ArrayList<Fragment>();
    }

    makeActive(fragment);

    if (!fragment.mDetached) {
        if (mAdded.contains(fragment)) {
            throw new IllegalStateException("Fragment already added: " + fragment);
        }
        mAdded.add(fragment);
        fragment.mAdded = true;
        fragment.mRemoving = false;
        if (fragment.mHasMenu && fragment.mMenuVisible) {
            mNeedMenuInvalidate = true;
        }
        if (moveToStateNow) {
            moveToState(fragment);
        }
    }
}    

为什么说FragmentManager是FragmentActivity管理Fragment的核心呢,请看下面:

final class FragmentManagerImpl extends FragmentManager implements LayoutInflaterFactory {
    ...

    ArrayList<Runnable> mPendingActions;
    Runnable[] mTmpActions;
    boolean mExecutingActions;

    ArrayList<Fragment> mActive;
    ArrayList<Fragment> mAdded;
    ArrayList<Integer> mAvailIndices;
    ArrayList<BackStackRecord> mBackStack;

可以看出FragmentManagerImpl帮FragmentActivity维护着所有管理Fragment的列表,FragmentManagerImpl的State是和Activity的State一致的,这是管理Fragment的关键。其实Fragment自身是没有什么生命周期的,它只是一个View的封装,完全依靠FragmentManagerImpl来进行同步模拟生命周期,比如在onCreate函数中创建Fragment,add后,在执行的到Activity自身的onCreateView之前,Fragment的onCreateView是不会执行的,也就是Fragment是被动式的跟FragmentActivity保持一致。既然Fragment只是个View的封装,那么它是如何转换成View,并添加到Container中去的呢?关键是moveToState函数,这个函数强制将新add的Fragment的生命周期与Activity同步:

 void moveToState(Fragment f, int newState, int transit, int transitionStyle,
        boolean keepActive) {
        ...        
     if (f.mState < newState) { //低于当前Activity的状态
        switch (f.mState) {
            case Fragment.INITIALIZING:
                    ...
                f.mActivity = mActivity;
                f.mParentFragment = mParent;
                f.mFragmentManager = mParent != null
                        ? mParent.mChildFragmentManager : mActivity.mFragments;
                f.mCalled = false;
                f.onAttach(mActivity);
               ...
                if (!f.mRetaining) {
                    f.performCreate(f.mSavedFragmentState);
                } 
            case Fragment.CREATED:
                if (newState > Fragment.CREATED) {

                      f.mView = f.performCreateView(f.getLayoutInflater(
                      f.mSavedFragmentState), container, f.mSavedFragmentState);
                      f.onViewCreated(f.mView, f.mSavedFragmentState);

                    f.performActivityCreated(f.mSavedFragmentState);
                    if (f.mView != null) {
                        f.restoreViewState(f.mSavedFragmentState);
                    }
                    f.mSavedFragmentState = null;
                }
            case Fragment.ACTIVITY_CREATED:
            case Fragment.STOPPED:
                    if (newState > Fragment.STOPPED) {
                        f.performStart();
                    }
            case Fragment.STARTED:
                if (newState > Fragment.STARTED) {
                      f.mResumed = true;
                    f.performResume();

可以看出,add Fragment之后,需要让Fragment跟当前Activity的State保持一致。现在回归正题,对于后台杀死状态下,为什么会show两个DialogFragment呢,我们需要接着看就要Fragment的异常处理的流程,在Fragment没有无参构造方法会引发崩溃里面,分析只是走到了Fragment的构建,现在接着往下走。提供无参构造函数后,Fragment可以正确的新建出来,之后呢?之后就是一些恢复逻辑,接着看restoreAllState

void restoreAllState(Parcelable state, ArrayList<Fragment> nonConfig) {

    if (state == null) return;
    FragmentManagerState fms = (FragmentManagerState)state;
    mActive = new ArrayList<Fragment>(fms.mActive.length);
     for (int i=0; i<fms.mActive.length; i++) {
        FragmentState fs = fms.mActive[i];
        if (fs != null) {
            Fragment f = fs.instantiate(mActivity, mParent);

            mActive.add(f);
            fs.mInstance = null;

    // Build the list of currently added fragments.
    if (fms.mAdded != null) {
        mAdded = new ArrayList<Fragment>(fms.mAdded.length);
        for (int i=0; i<fms.mAdded.length; i++) {
            Fragment f = mActive.get(fms.mAdded[i]);
            if (f == null) {
                throwException(new IllegalStateException(
                        "No instantiated fragment for index #" + fms.mAdded[i]));
            }
            f.mAdded = true;
            if (DEBUG) Log.v(TAG, "restoreAllState: added #" + i + ": " + f);
            if (mAdded.contains(f)) {
                throw new IllegalStateException("Already added!");
            }
            mAdded.add(f);
        }

    // Build the back stack.
    if (fms.mBackStack != null) {
        mBackStack = new ArrayList<BackStackRecord>(fms.mBackStack.length);
        for (int i=0; i<fms.mBackStack.length; i++) {
            BackStackRecord bse = fms.mBackStack[i].instantiate(this);

            mBackStack.add(bse);
            if (bse.mIndex >= 0) {
                setBackStackIndex(bse.mIndex, bse);
}

其实到现在现在Fragment相关的信息已经恢复成功了,之后随着FragmentActivity周期显示或者更新了,这些都是被杀死后,在FragmentActiivyt的onCreate函数处理的,也就是默认已经将之前的Fragment添加到mAdded列表中去了,但是,在场景一,我们有手动新建了一个Fragment,并添加进去,所以,mAdded函数中就有连个两个Fragment。这样,在FragmentActivity调用onStart函数之后,会新建mAdded列表中Fragment的视图,将其添加到相应的container中去,并在Activity调用onReusume的时候,显示出来做的,这个时候,就会显示两份,其实如果,在这个时候,你再杀死一次,恢复,就会显示三分,在杀死,重启,就是四份。。。。

@Override
protected void onStart() {
    super.onStart();

    mStopped = false;
    mReallyStopped = false;
    mHandler.removeMessages(MSG_REALLY_STOPPED);

    if (!mCreated) {
        mCreated = true;
        mFragments.dispatchActivityCreated();
    }

    mFragments.noteStateNotSaved();
    mFragments.execPendingActions();

    mFragments.doLoaderStart();

    // NOTE: HC onStart goes here.

    mFragments.dispatchStart();
    mFragments.reportLoaderStart();
}

以上就是针对两个场景,对FramgentActivity的一些分析,主要是回复时候,对于Framgent的一些处理。

onSaveInstanceState与OnRestoreInstance的调用时机

在在点击home键,或者跳转其他界面的时候,都会回调用onSaveInstanceState,但是再次唤醒却不一定调用OnRestoreInstance,这是为什么呢?onSaveInstanceState与OnRestoreInstance难道不是配对使用的?在Android中,onSaveInstanceState是为了预防Activity被后台杀死的情况做的预处理,如果Activity没有被后台杀死,那么自然也就不需要进行现场的恢复,也就不会调用OnRestoreInstance,而大多数情况下,Activity不会那么快被杀死。

onSaveInstanceState的调用时机

onSaveInstanceState函数是Android针对可能被后台杀死的Activity做的一种预防,它的执行时机在2.3之前是在onPause之前,2.3之后,放在了onStop函数之前,也就说Activity失去焦点后,可能会由于内存不足,被回收的情况下,都会去执行onSaveInstanceState。对于startActivity函数的调用很多文章都有介绍,可以简单参考下老罗的博客Android应用程序内部启动Activity过程(startActivity)的源代码分析,比如在Activity A 调用startActivity启动Activity B的时候,会首先通过AMS pause Activity A,之后唤起B,在B显示,再stop A,在stop A的时候,需要保存A的现场,因为不可见的Activity都是可能被后台杀死的,比如,在开发者选项中打开不保留活动,就会达到这种效果,在启动另一个Activity时,上一个Activity的保存流程大概如下,这里先简单描述,在下一篇原理篇的时候,会详细讲解下流程:

在2.3之后,onSaveInstanceState的时机都放在了onStop之前,看一下FragmentActivity的onSaveInstanceState源码:

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    Parcelable p = mFragments.saveAllState();
    if (p != null) {
        outState.putParcelable(FRAGMENTS_TAG, p);
    }
}

可以看出,首先就是父类的onSaveInstanceState,主要是保存一些窗口及View的信息,比如ViewPager当前显示的是第几个View等。之后,就是就是通过FragmentManager的saveAllState,来保存FragmentActivity自身的现场-Fragment的一些状态,这些数据是FragmentActivity恢复Framgent所必须的数据,处理不好就会出现上面的那种异常。

相关阅读:Android后台杀死系列之一:FragmentActivity及PhoneWindow后台杀死处理机制(下篇)

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