Android RRO动态加载资源

在 Android 8.0 的时候，Android 项目可以动态的加载资源而可以不用再overlay 中去静态处理，这样就可以方便的对 Android 资源进行处理了。
例如：

1. -- Android.mkLOCAL_PATH := $(call my-dir)
$(warning mike LOCAL_PATH = $(LOCAL_PATH))include $(CLEAR_VARS)LOCAL_RESOURCE_DIR := \$(LOCAL_PATH)/res \$(warning mike LOCAL_RESOURCE_DIR = $(LOCAL_RESOURCE_DIR))
LOCAL_MODULE_PATH := $(TARGET_OUT_VENDOR)/overlay
LOCAL_MODULE_TAGS := optional
LOCAL_SRC_FILES := $(call all-subdir-java-files)
LOCAL_SDK_VERSION := current
# 用于当前目录的命名
LOCAL_PACKAGE_NAME := LauncherOverlay
LOCAL_AAPT_FLAGS := --auto-add-overlay
LOCAL_MANIFEST_FILE := AndroidManifest.xml
include $(BUILD_PACKAGE)2. -- AndroidManifest.xml<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"package="com.android.launcher3overlay"><overlay android:targetPackage="com.android.launcher3" android:priority="1" android:isStatic="true"/>
</manifest>3. 直接在 res 中添加对应的资源。编译完成后，会生成一个 apk。

动态加载资源 RRO 具体实现在 com.android.server.om，看看官方给的表述

用于管理 asset overlay 的服务。Asset overlays 来自于系统和 app apks 加载的额外资源。这个服务（OMS），跟踪已经被安装了要使用的 overlay，提供了更改此资源的方法。在 Activity 的运行生命周期中，对正在运行的应用去更改它的资源。允许应用重新读取 overlay 的资源。OMS本身不会改变 overlay。相反，它只负责确保在技术和安全的角度，overlays 可以在外部的请求下被激活。在组件中这种打开或关闭 overlay 的方式能在不同的用例中被使用，如主题或者是动态定制。

OMS从三个来源接收信息：

从 SystemService 类回调，特别是当 Android 框架正在启动以及最终用户切换 Android 用户时。
来至 PMS 的 Intent。Overlay 是一个常规的 apk，无论这个应用被安装，卸载等操作时，PMS 都会通过发送广播意图。当 OMS 接收到这些意图时，OMS 更新可用 overlay 的内部表示。如果这是一个可见的改变，将在受影响的 apk 中重新触发 asset 的更新。
通过 @link ioverlaymanager aidl 接口的外部请求。这个接口允许客户机去读取关于当前可用 overlay 的信息，更新一个 overlay 或者是相关的 order。如果请求的目标是与调用者运行相同的Android用户，或者调用者拥有“跨用户交互”的完全权限，则授予读取访问权限。如果调用方被授予读访问权限，并且另外还拥有更改覆盖包权限，则授予写访问权限。

英语不好，附上英语表述

/*** Service to manage asset overlays.** <p>Asset overlays are additional resources that come from apks loaded* alongside the system and app apks. This service, the OverlayManagerService* (OMS), tracks which installed overlays to use and provides methods to change* this. Changes propagate to running applications as part of the Activity* lifecycle. This allows Activities to reread their resources at a well* defined point.</p>** <p>By itself, the OMS will not change what overlays should be active.* Instead, it is only responsible for making sure that overlays *can* be used* from a technical and security point of view and to activate overlays in* response to external requests. The responsibility to toggle overlays on and* off lies within components that implement different use-cases such as themes* or dynamic customization.</p>** <p>The OMS receives input from three sources:</p>** <ul>*     <li>Callbacks from the SystemService class, specifically when the*     Android framework is booting and when the end user switches Android*     users.</li>**     <li>Intents from the PackageManagerService (PMS). Overlays are regular*     apks, and whenever a package is installed (or removed, or has a*     component enabled or disabled), the PMS broadcasts this as an intent.*     When the OMS receives one of these intents, it updates its internal*     representation of the available overlays and, if there was a visible*     change, triggers an asset refresh in the affected apps.</li>**     <li>External requests via the {@link IOverlayManager AIDL interface}.*     The interface allows clients to read information about the currently*     available overlays, change whether an overlay should be used or not, and*     change the relative order in which overlay packages are loaded.*     Read-access is granted if the request targets the same Android user as*     the caller runs as, or if the caller holds the*     INTERACT_ACROSS_USERS_FULL permission. Write-access is granted if the*     caller is granted read-access and additionaly holds the*     CHANGE_OVERLAY_PACKAGES permission.</li>* </ul>** <p>The AIDL interface works with String package names, int user IDs, and* {@link OverlayInfo} objects. OverlayInfo instances are used to track a* specific pair of target and overlay packages and include information such as* the current state of the overlay. OverlayInfo objects are immutable.</p>** <p>Internally, OverlayInfo objects are maintained by the* OverlayManagerSettings class. The OMS and its helper classes are notified of* changes to the settings by the OverlayManagerSettings.ChangeListener* callback interface. The file /data/system/overlays.xml is used to persist* the settings.</p>** <p>Creation and deletion of idmap files are handled by the IdmapManager* class.</p>** <p>The following is an overview of OMS and its related classes. Note how box* (2) does the heavy lifting, box (1) interacts with the Android framework,* and box (3) replaces box (1) during unit testing.</p>** <pre>*         Android framework*            |         ^*      . . . | . . . . | . . . .*     .      |         |       .*     .    AIDL,   broadcasts  .*     .   intents      |       .*     .      |         |       . . . . . . . . . . . .*     .      v         |       .                     .*     .  OverlayManagerService . OverlayManagerTests .*     .                  \     .     /               .*     . (1)               \    .    /            (3) .*      . . . . . . . . . . \ . . . / . . . . . . . . .*     .                     \     /              .*     . (2)                  \   /               .*     .           OverlayManagerServiceImpl      .*     .                  |            |          .*     .                  |            |          .*     . OverlayManagerSettings     IdmapManager  .*     .                                          .*     . . . .  . . . . . . . . . . . . . . . . . .* </pre>** <p>Finally, here is a list of keywords used in the OMS context.</p>** <ul>*     <li><b>target [package]</b> -- A regular apk that may have its resource*     pool extended  by zero or more overlay packages.</li>**     <li><b>overlay [package]</b> -- An apk that provides additional*     resources to another apk.</li>**     <li><b>OMS</b> -- The OverlayManagerService, i.e. this class.</li>**     <li><b>approved</b> -- An overlay is approved if the OMS has verified*     that it can be used technically speaking (its target package is*     installed, at least one resource name in both packages match, the*     idmap was created, etc) and that it is secure to do so. External*     clients can not change this state.</li>**     <li><b>not approved</b> -- The opposite of approved.</li>**     <li><b>enabled</b> -- An overlay currently in active use and thus part*     of resource lookups. This requires the overlay to be approved. Only*     external clients can change this state.</li>**     <li><b>disabled</b> -- The opposite of enabled.</li>**     <li><b>idmap</b> -- A mapping of resource IDs between target and overlay*     used during resource lookup. Also the name of the binary that creates*     the mapping.</li>* </ul>*/

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