Android后台保活4.0~8.0系统（一）

一：前言
在谈保活之前，我们应该知道随着Android系统的不断发展，系统管控越来越严格，后台保活将是一个伪命题了，后台保活主要是和推送相关，随着技术的发展，泰尔终端实验室也制定了统一的推送标准，我想在后面Android会像Iphone一样，使用统一系统服务进行推送

二：关于Setting模块中关于保活的重要概念
1）开发者选项–不保留活动，即用户离开后清除活动
当一个activity启动的时候，他会关联一个栈，同时栈也有自己的亲和性的；大家可以搜索launch moder了解下大致信息；
这里的不保留活动的意思距离说明是从activity A跳转到activity B，则activity A被销毁，如果此时点击back键，activity A会被重建

在7.0源码的Settings源码目录里面，如果你想通过“不保留活动”关键字去搜索对应代码是搜索不到的，关键代码如下

res/xml/development_prefs.xml

        "immediately_destroy_activities"
            android:title="@string/immediately_destroy_activities"
            android:summary="@string/immediately_destroy_activities_summary"/>

相关类的位置 src/com/android/settings/DevelopmentSettings.java
选中打开“不保留活动”选项执行的代码如下

    private void writeImmediatelyDestroyActivitiesOptions() {
        try {
            ActivityManagerNative.getDefault().setAlwaysFinish(
                    mImmediatelyDestroyActivities.isChecked());
        } catch (RemoteException ex) {
        }
    }

查看源码得知，setAlwaysFinish如下

    public void setAlwaysFinish(boolean enabled) throws RemoteException
    {
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IActivityManager.descriptor);
        data.writeInt(enabled ? 1 : 0);
        mRemote.transact(SET_ALWAYS_FINISH_TRANSACTION, data, reply, 0);
        reply.readException();
        data.recycle();
        reply.recycle();
    }

可以看出这个功能是通过往Settings.Global.ALWAYS_FINISH_ACTIVITIES写入数据实现的；

在打开设置这个app的时候，通过读取相关的值来设置开关状态

    private void updateImmediatelyDestroyActivitiesOptions() {
        updateSwitchPreference(mImmediatelyDestroyActivities, Settings.Global.getInt(
                getActivity().getContentResolver(), Settings.Global.ALWAYS_FINISH_ACTIVITIES, 0) != 0);
    }

其中ALWAYS_FINISH_ACTIVITIES为\frameworks\base\core\java\android\provider\Settings.java

         /**
         * If not 0, the activity manager will aggressively finish activities and
         * processes as soon as they are no longer needed.  If 0, the normal
         * extended lifetime is used.
         */
        public static final String ALWAYS_FINISH_ACTIVITIES = "always_finish_activities";

总结：当用户勾选了“不保留活动”这个选项，则会将ALWAYS_FINISH_ACTIVITIES 设置为1，activity manager将在activity不在前台的时候finish掉他

2)限制后台进程
该功能可选的配置有：标准限制，无后台程序，最多1个程序，最多2个程序…
xml配置文件为：res/xml/development_prefs.xml

        "app_process_limit"
            android:title="@string/app_process_limit_title"
            android:entries="@array/app_process_limit_entries"
            android:entryValues="@array/app_process_limit_values" />

选择对应的条目之后，相关代码为：src/com/android/settings/DevelopmentSettings.java

    private void writeAppProcessLimitOptions(Object newValue) {
        try {
            int limit = newValue != null ? Integer.parseInt(newValue.toString()) : -1;
            ActivityManagerNative.getDefault().setProcessLimit(limit);
            updateAppProcessLimitOptions();
        } catch (RemoteException e) {
        }
    }

ActivityManagerNative.getDefault()实现如下：

    /**
     * Retrieve the system's default/global activity manager.
     */
    static public IActivityManager getDefault() {
        return gDefault.get();
    }

    private static final Singleton gDefault = new Singleton() {
        protected IActivityManager create() {
            IBinder b = ServiceManager.getService("activity");
            if (false) {
                Log.v("ActivityManager", "default service binder = " + b);
            }
            IActivityManager am = asInterface(b);
            if (false) {
                Log.v("ActivityManager", "default service = " + am);
            }
            return am;
        }
    };

其中Singleton是一个模板类，通过get（）方法返回IActivityManager；
那IActivityManager 的实现是谁尼，我们紧接着分析asInterface(b)

    static public IActivityManager asInterface(IBinder obj) {
        if (obj == null) {
            return null;
        }
        IActivityManager in =
            (IActivityManager)obj.queryLocalInterface(descriptor);
        if (in != null) {
            return in;
        }

        return new ActivityManagerProxy(obj);
    }

从这里可以看出这里才是真正的返回“in”，只有当in为null的时候，返回ActivityManagerProxy；

这里的IActivityManager的实现者为：ActivityManagerService，最上面的ActivityManagerNative.getDefault().setProcessLimit(limit)其实是调用ActivityManagerService.java的setProcessLimit方法，我们来看一下详细的方法

    @Override
    public void setProcessLimit(int max) {
        enforceCallingPermission(android.Manifest.permission.SET_PROCESS_LIMIT,
                "setProcessLimit()");
        synchronized (this) {
            mProcessLimit = max < 0 ? ProcessList.MAX_CACHED_APPS : max;
            mProcessLimitOverride = max;
        }
        trimApplications();
    }

这个方法的主要作用是杀死已经移除的进程，并动态调整oom adj for all processes；

综述：限制后台进程是强行获取android.permission.SET_PROCESS_LIMIT权限，杀死已经移除的进程并动态调整所有进程的oom adj

这里我们并没有解决标准显示到底是限制多少个App，其实答案就在setProcessLimit方法中

 mProcessLimit = max < 0 ? ProcessList.MAX_CACHED_APPS : max;

这里的ProcessList.MAX_CACHED_APPS的值为：

    // The maximum number of cached processes we will keep around before killing them.
    // NOTE: this constant is *only* a control to not let us go too crazy with
    // keeping around processes on devices with large amounts of RAM.  For devices that
    // are tighter on RAM, the out of memory killer is responsible for killing background
    // processes as RAM is needed, and we should *never* be relying on this limit to
    // kill them.  Also note that this limit only applies to cached background processes;
    // we have no limit on the number of service, visible, foreground, or other such
    // processes and the number of those processes does not count against the cached
    // process limit.
    static final int MAX_CACHED_APPS = 32;

也就是说标准显示其实是允许保留32个后台进程；