热修复之资源更新的实现
热修复之资源更新的实现
本文分析了InstantRun的部分源码，并由此得出资源热修复的一种实现方法。
资源热修复
所谓资源热修复就是在不重新安装APP的情况下修改其资源，包括音频、图片，也包括布局、文字等资源。本文最终实现的效果如下:
在第一次退出应用时，在后台下发补丁，应用重启后会根据补丁加载新的布局
InstantRun分析
InstantRun已经在前面做过介绍，不再赘述。需注意重点不是InstantRun的实现代码，而是Android加载资源的源码，只有明白了Android是如何加载资源的，才能体会InstantRun的意图。
从Android系统的角度来看，每个应用的资源管理都是通过一个AssetManager(源码./frameworks/base/core/java/android/content/res/AssetManager.java)对象实现，如同Qzone对代码修复时修改pathList一般，InstantRun正是重新构建了一个AssetManager，使其应用补丁包中的资源，并替换原来的AssetManager来实现的:
public static void monkeyPatchExistingResources(@Nullable Context context,
@Nullable String externalResourceFile,
@Nullable Collection<Activity> activities) {
if (externalResourceFile == null) {
return;
}
/*
(Note: the resource directory is *also* inserted into the loadedApk in
monkeyPatchApplication)
The code seems to perform this:
File externalResourceFile = <path to resources.ap_ or extracted directory>
AssetManager newAssetManager = new AssetManager();
newAssetManager.addAssetPath(externalResourceFile)
// Kitkat needs this method call, Lollipop doesn't. However, it doesn't seem to cause any harm
// in L, so we do it unconditionally.
newAssetManager.ensureStringBlocks();
// Find the singleton instance of ResourcesManager
ResourcesManager resourcesManager = ResourcesManager.getInstance();
// Iterate over all known Resources objects
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
for (WeakReference<Resources> wr : resourcesManager.mActiveResources.values()) {
Resources resources = wr.get();
// Set the AssetManager of the Resources instance to our brand new one
resources.mAssets = newAssetManager;
resources.updateConfiguration(resources.getConfiguration(), resources.getDisplayMetrics());
}
}
// Also, for each context, call getTheme() to get the current theme; null out its
// mTheme field, then invoke initializeTheme() to force it to be recreated (with the
// new asset manager!)
*/
try {
// Create a new AssetManager instance and point it to the resources installed under
// /sdcard
AssetManager newAssetManager = AssetManager.class.getConstructor().newInstance();
Method mAddAssetPath = AssetManager.class.getDeclaredMethod("addAssetPath", String.class);
mAddAssetPath.setAccessible(true);
if (((Integer) mAddAssetPath.invoke(newAssetManager, externalResourceFile)) == 0) {
throw new IllegalStateException("Could not create new AssetManager");
}
// Kitkat needs this method call, Lollipop doesn't. However, it doesn't seem to cause any harm
// in L, so we do it unconditionally.
Method mEnsureStringBlocks = AssetManager.class.getDeclaredMethod("ensureStringBlocks");
mEnsureStringBlocks.setAccessible(true);
mEnsureStringBlocks.invoke(newAssetManager);
if (activities != null) {
for (Activity activity : activities) {
Resources resources = activity.getResources();
try {
Field mAssets = Resources.class.getDeclaredField("mAssets");
mAssets.setAccessible(true);
mAssets.set(resources, newAssetManager);
} catch (Throwable ignore) {
Field mResourcesImpl = Resources.class.getDeclaredField("mResourcesImpl");
mResourcesImpl.setAccessible(true);
Object resourceImpl = mResourcesImpl.get(resources);
Field implAssets = resourceImpl.getClass().getDeclaredField("mAssets");
implAssets.setAccessible(true);
implAssets.set(resourceImpl, newAssetManager);
}
Resources.Theme theme = activity.getTheme();
try {
try {
Field ma = Resources.Theme.class.getDeclaredField("mAssets");
ma.setAccessible(true);
ma.set(theme, newAssetManager);
} catch (NoSuchFieldException ignore) {
Field themeField = Resources.Theme.class.getDeclaredField("mThemeImpl");
themeField.setAccessible(true);
Object impl = themeField.get(theme);
Field ma = impl.getClass().getDeclaredField("mAssets");
ma.setAccessible(true);
ma.set(impl, newAssetManager);
}
Field mt = ContextThemeWrapper.class.getDeclaredField("mTheme");
mt.setAccessible(true);
mt.set(activity, null);
Method mtm = ContextThemeWrapper.class.getDeclaredMethod("initializeTheme");
mtm.setAccessible(true);
mtm.invoke(activity);
Method mCreateTheme = AssetManager.class.getDeclaredMethod("createTheme");
mCreateTheme.setAccessible(true);
Object internalTheme = mCreateTheme.invoke(newAssetManager);
Field mTheme = Resources.Theme.class.getDeclaredField("mTheme");
mTheme.setAccessible(true);
mTheme.set(theme, internalTheme);
} catch (Throwable e) {
Log.e(LOG_TAG, "Failed to update existing theme for activity " + activity,
e);
}
pruneResourceCaches(resources);
}
}
// Iterate over all known Resources objects
Collection<WeakReference<Resources>> references;
if (SDK_INT >= KITKAT) {
// Find the singleton instance of ResourcesManager
Class<?> resourcesManagerClass = Class.forName("android.app.ResourcesManager");
Method mGetInstance = resourcesManagerClass.getDeclaredMethod("getInstance");
mGetInstance.setAccessible(true);
Object resourcesManager = mGetInstance.invoke(null);
try {
Field fMActiveResources = resourcesManagerClass.getDeclaredField("mActiveResources");
fMActiveResources.setAccessible(true);
@SuppressWarnings("unchecked")
ArrayMap<?, WeakReference<Resources>> arrayMap =
(ArrayMap<?, WeakReference<Resources>>) fMActiveResources.get(resourcesManager);
references = arrayMap.values();
} catch (NoSuchFieldException ignore) {
Field mResourceReferences = resourcesManagerClass.getDeclaredField("mResourceReferences");
mResourceReferences.setAccessible(true);
//noinspection unchecked
references = (Collection<WeakReference<Resources>>) mResourceReferences.get(resourcesManager);
}
} else {
Class<?> activityThread = Class.forName("android.app.ActivityThread");
Field fMActiveResources = activityThread.getDeclaredField("mActiveResources");
fMActiveResources.setAccessible(true);
Object thread = getActivityThread(context, activityThread);
@SuppressWarnings("unchecked")
HashMap<?, WeakReference<Resources>> map =
(HashMap<?, WeakReference<Resources>>) fMActiveResources.get(thread);
references = map.values();
}
for (WeakReference<Resources> wr : references) {
Resources resources = wr.get();
if (resources != null) {
// Set the AssetManager of the Resources instance to our brand new one
try {
Field mAssets = Resources.class.getDeclaredField("mAssets");
mAssets.setAccessible(true);
mAssets.set(resources, newAssetManager);
} catch (Throwable ignore) {
Field mResourcesImpl = Resources.class.getDeclaredField("mResourcesImpl");
mResourcesImpl.setAccessible(true);
Object resourceImpl = mResourcesImpl.get(resources);
Field implAssets = resourceImpl.getClass().getDeclaredField("mAssets");
implAssets.setAccessible(true);
implAssets.set(resourceImpl, newAssetManager);
}
resources.updateConfiguration(resources.getConfiguration(), resources.getDisplayMetrics());
}
}
} catch (Throwable e) {
throw new IllegalStateException(e);
}
}
除去打日志和处理版本兼容的代码，主要就做了两件事：
- 构建一个新的AssetManager，并通过反射调用addAssetPath，把这个完整的新资源包加入到AssetManager中，这样就得到一个含有所有新资源的AssetManager。
- 找到所有之前引用到原AssetManager的地方，通过反射，吧引用处替换为新的AssetManager。
实现
自定义一个Application并在其onCreate方法中调用monkeyPatchExistingResources即可，而补丁包则是更新资源后打包出来的apk文件，可以直接安装，也可以作为资源修复的补丁包。
改进之处
这种策略最大的问题在于补丁包的大小，可以参考Tinker的做法，根据新旧补丁包生成diff文件，下发diff文件再在客户端合成完整的apk文件，但这样的问题是牺牲了一部分性能。对于InstantRun来说，通过USB传输数据并不是瓶颈，在客户端合成反而会影响体验，所以并没有采用Tinker的做法。 那么是否可以深入挖掘源码，找到更好的办法？
