红橙Darren视频笔记换肤框架1 获取其他apk中的资源

1.Android xml属性资源的加载

以ImageView的src为例看看Android是如何找到图片资源的
我们通常使用image的代码如下

    <ImageViewandroid:layout_width="wrap_content"android:layout_height="wrap_content"android:src="@drawable/btn_back"/>

Android是如何加载到这个资源的呢
点进src的源码这是Android源码的定义的文件显然这种写法和我们使用

    <declare-styleable name="ImageView"><!-- Sets a drawable as the content of this ImageView. --><attr name="src" format="reference|color" />...</declare-styleable>

很明显这里使用了下xml自定义属性查看ImageView源码进一步证实这一点
我们在imageview的构造方法中发现了如下代码

        final TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.ImageView, defStyleAttr, defStyleRes);final Drawable d = a.getDrawable(R.styleable.ImageView_src);//重点跟踪if (d != null) {setImageDrawable(d);}

这是经典的读取自定义属性的代码我们接着跟踪看drawable是如何获取到的
TypeArray部分代码：

    @Nullablepublic Drawable getDrawable(@StyleableRes int index) {return getDrawableForDensity(index, 0);}public Drawable getDrawableForDensity(@StyleableRes int index, int density) {if (mRecycled) {throw new RuntimeException("Cannot make calls to a recycled instance!");}final TypedValue value = mValue;if (getValueAt(index * STYLE_NUM_ENTRIES, value)) {if (value.type == TypedValue.TYPE_ATTRIBUTE) {throw new UnsupportedOperationException("Failed to resolve attribute at index " + index + ": " + value);}if (density > 0) {// If the density is overridden, the value in the TypedArray will not reflect this.// Do a separate lookup of the resourceId with the density override.mResources.getValueForDensity(value.resourceId, density, value, true);}return mResources.loadDrawable(value, value.resourceId, density, mTheme);//重点}return null;}

最终我们发现图片资源由mResources进行解析
那么mResources是如何进行初始化的呢？

2.mResources的初始化

TypeArray中的mResources初始化不是很容易看懂，我们从其他方面入手，比如我们经常使用context.getResources()来获取string 颜色图片等那么这里的getResources中的Resource是如何初始化的呢（API 28）

 getResources().getDrawable(-1,null);//ContextThemeWrapper@Overridepublic Resources getResources() {return getResourcesInternal();}private Resources getResourcesInternal() {if (mResources == null) {if (mOverrideConfiguration == null) {mResources = super.getResources();//走这里} else {final Context resContext = createConfigurationContext(mOverrideConfiguration);mResources = resContext.getResources();}}return mResources;}//ContextWrapper@Overridepublic Resources getResources() {return mBase.getResources();}// Contextpublic abstract Resources getResources();// 追踪到实现类 ContextImpl@Overridepublic Resources getResources() {return mResources;}void setResources(Resources r) {if (r instanceof CompatResources) {((CompatResources) r).setContext(this);}mResources = r;}c.setResources(createResources(mActivityToken, pi, null, displayId, null,getDisplayAdjustments(displayId).getCompatibilityInfo()));private static Resources createResources(IBinder activityToken, LoadedApk pi, String splitName,int displayId, Configuration overrideConfig, CompatibilityInfo compatInfo) {final String[] splitResDirs;final ClassLoader classLoader;try {splitResDirs = pi.getSplitPaths(splitName);classLoader = pi.getSplitClassLoader(splitName);} catch (NameNotFoundException e) {throw new RuntimeException(e);}return ResourcesManager.getInstance().getResources(activityToken,//关键pi.getResDir(),splitResDirs,pi.getOverlayDirs(),pi.getApplicationInfo().sharedLibraryFiles,displayId,overrideConfig,compatInfo,classLoader);}// ResourcesManagerpublic @Nullable Resources getResources(@Nullable IBinder activityToken,@Nullable String resDir,@Nullable String[] splitResDirs,@Nullable String[] overlayDirs,@Nullable String[] libDirs,int displayId,@Nullable Configuration overrideConfig,@NonNull CompatibilityInfo compatInfo,@Nullable ClassLoader classLoader) {try {Trace.traceBegin(Trace.TRACE_TAG_RESOURCES, "ResourcesManager#getResources");final ResourcesKey key = new ResourcesKey(resDir,splitResDirs,overlayDirs,libDirs,displayId,overrideConfig != null ? new Configuration(overrideConfig) : null, // CopycompatInfo);classLoader = classLoader != null ? classLoader : ClassLoader.getSystemClassLoader();return getOrCreateResources(activityToken, key, classLoader);// 关键} finally {Trace.traceEnd(Trace.TRACE_TAG_RESOURCES);}}// ResourcesMananger// 创建Resources的有两个方法 getOrCreateResourcesForActivityLocked以及getOrCreateResourcesLockedprivate @Nullable Resources getOrCreateResources(@Nullable IBinder activityToken,@NonNull ResourcesKey key, @NonNull ClassLoader classLoader) {synchronized (this) {if (activityToken != null) {...} else {// Clean up any dead references so they don't pile up.ArrayUtils.unstableRemoveIf(mResourceReferences, sEmptyReferencePredicate);// Not tied to an Activity, find a shared Resources that has the right ResourcesImplResourcesImpl resourcesImpl = findResourcesImplForKeyLocked(key);if (resourcesImpl != null) {if (DEBUG) {Slog.d(TAG, "- using existing impl=" + resourcesImpl);}return getOrCreateResourcesLocked(classLoader, resourcesImpl, key.mCompatInfo);}// We will create the ResourcesImpl object outside of holding this lock.}// If we're here, we didn't find a suitable ResourcesImpl to use, so create one now.ResourcesImpl resourcesImpl = createResourcesImpl(key);if (resourcesImpl == null) {return null;}// Add this ResourcesImpl to the cache.mResourceImpls.put(key, new WeakReference<>(resourcesImpl));final Resources resources;//重点 看他如何初始化if (activityToken != null) {resources = getOrCreateResourcesForActivityLocked(activityToken, classLoader,resourcesImpl, key.mCompatInfo);} else {resources = getOrCreateResourcesLocked(classLoader, resourcesImpl, key.mCompatInfo);}return resources;}}//1 getOrCreateResourcesLockedprivate @NonNull Resources getOrCreateResourcesLocked(@NonNull ClassLoader classLoader,@NonNull ResourcesImpl impl, @NonNull CompatibilityInfo compatInfo) {// Find an existing Resources that has this ResourcesImpl set.final int refCount = mResourceReferences.size();for (int i = 0; i < refCount; i++) {WeakReference<Resources> weakResourceRef = mResourceReferences.get(i);Resources resources = weakResourceRef.get();if (resources != null &&Objects.equals(resources.getClassLoader(), classLoader) &&resources.getImpl() == impl) {if (DEBUG) {Slog.d(TAG, "- using existing ref=" + resources);}return resources;}}// Create a new Resources reference and use the existing ResourcesImpl object.Resources resources = compatInfo.needsCompatResources() ? new CompatResources(classLoader): new Resources(classLoader);//重点resources.setImpl(impl);mResourceReferences.add(new WeakReference<>(resources));if (DEBUG) {Slog.d(TAG, "- creating new ref=" + resources);Slog.d(TAG, "- setting ref=" + resources + " with impl=" + impl);}return resources;}//2 getOrCreateResourcesForActivityLockedprivate @NonNull Resources getOrCreateResourcesForActivityLocked(@NonNull IBinder activityToken,@NonNull ClassLoader classLoader, @NonNull ResourcesImpl impl,@NonNull CompatibilityInfo compatInfo) {final ActivityResources activityResources = getOrCreateActivityResourcesStructLocked(activityToken);final int refCount = activityResources.activityResources.size();for (int i = 0; i < refCount; i++) {WeakReference<Resources> weakResourceRef = activityResources.activityResources.get(i);Resources resources = weakResourceRef.get();if (resources != null&& Objects.equals(resources.getClassLoader(), classLoader)&& resources.getImpl() == impl) {if (DEBUG) {Slog.d(TAG, "- using existing ref=" + resources);}return resources;}}Resources resources = compatInfo.needsCompatResources() ? new CompatResources(classLoader): new Resources(classLoader);//重点resources.setImpl(impl);activityResources.activityResources.add(new WeakReference<>(resources));if (DEBUG) {Slog.d(TAG, "- creating new ref=" + resources);Slog.d(TAG, "- setting ref=" + resources + " with impl=" + impl);}return resources;}//两个创建resources的方法都类似 都使用了缓存机制 都是通过如下代码创建resources的Resources resources = compatInfo.needsCompatResources() ? new CompatResources(classLoader): new Resources(classLoader);//而通过CompatResources创建的其实和直接用Resources创建的差不多// CompatResources.javapublic CompatResources(ClassLoader cls) {super(cls);//super是ResourcesmContext = new WeakReference<>(null);}//  Resources.javapublic Resources(@Nullable ClassLoader classLoader) {mClassLoader = classLoader == null ? ClassLoader.getSystemClassLoader() : classLoader;}// 也就是无论如何 都是通过 Resources resources = new Resources(classLoader)创建的Resources// 而实际上 Resources内部包含了ResourcesImpl对象 Resources利用ResourcesImpl来获取系统的一些资源// 如1.获取图片public Drawable getDrawable(@DrawableRes int id, @Nullable Theme theme)throws NotFoundException {return getDrawableForDensity(id, 0, theme);}public Drawable getDrawableForDensity(@DrawableRes int id, int density, @Nullable Theme theme) {final TypedValue value = obtainTempTypedValue();try {final ResourcesImpl impl = mResourcesImpl;impl.getValueForDensity(id, density, value, true);return impl.loadDrawable(this, value, id, density, theme);} finally {releaseTempTypedValue(value);}}// 2.获取Dimensionpublic float getDimension(@DimenRes int id) throws NotFoundException {final TypedValue value = obtainTempTypedValue();try {final ResourcesImpl impl = mResourcesImpl;impl.getValue(id, value, true);if (value.type == TypedValue.TYPE_DIMENSION) {return TypedValue.complexToDimension(value.data, impl.getDisplayMetrics());}throw new NotFoundException("Resource ID #0x" + Integer.toHexString(id)+ " type #0x" + Integer.toHexString(value.type) + " is not valid");} finally {releaseTempTypedValue(value);}}// 3.获取colorpublic int getColor(@ColorRes int id, @Nullable Theme theme) throws NotFoundException {final TypedValue value = obtainTempTypedValue();try {final ResourcesImpl impl = mResourcesImpl;impl.getValue(id, value, true);if (value.type >= TypedValue.TYPE_FIRST_INT&& value.type <= TypedValue.TYPE_LAST_INT) {return value.data;} else if (value.type != TypedValue.TYPE_STRING) {throw new NotFoundException("Resource ID #0x" + Integer.toHexString(id)+ " type #0x" + Integer.toHexString(value.type) + " is not valid");}final ColorStateList csl = impl.loadColorStateList(this, value, id, theme);return csl.getDefaultColor();} finally {releaseTempTypedValue(value);}}// 4.获取Stringpublic String getString(@StringRes int id) throws NotFoundException {return getText(id).toString();}@NonNull public CharSequence getText(@StringRes int id) throws NotFoundException {CharSequence res = mResourcesImpl.getAssets().getResourceText(id);if (res != null) {return res;}throw new NotFoundException("String resource ID #0x"+ Integer.toHexString(id));}// 我们可以直接创建ResourcesImpl或者创建Resources后通过其内部的ResourcesImpl来访问资源 这里我像视频一样选择后者// 其中要使得Resources和mResourcesImpl都进行初始化 最简单的调用时调用Resources的三个参数的构造方法public Resources(AssetManager assets, DisplayMetrics metrics, Configuration config) {this(null);mResourcesImpl = new ResourcesImpl(assets, metrics, config, new DisplayAdjustments());}// 需要看一下assets metrics config是如何初始化的/*** Only for creating the System resources.*/private Resources() {this(null);final DisplayMetrics metrics = new DisplayMetrics();metrics.setToDefaults();final Configuration config = new Configuration();config.setToDefaults();mResourcesImpl = new ResourcesImpl(AssetManager.getSystem(), metrics, config,new DisplayAdjustments());}

其中最重要的是参数AssetManager 上述例子使用的是系统的AssetManager 我们自己创建的时候需要使用自己path来创建AssetManager，具体灵感来自API 26的源码
AssetManager assets = new AssetManager();
assets.addAssetPath(resDir)// resDir apk的目录
API 28的相关代码如下

    /*** @deprecated Use {@link #setApkAssets(ApkAssets[], boolean)}* @hide*/@Deprecatedpublic int addAssetPath(String path) {return addAssetPathInternal(path, false /*overlay*/, false /*appAsLib*/);}private int addAssetPathInternal(String path, boolean overlay, boolean appAsLib) {Preconditions.checkNotNull(path, "path");synchronized (this) {ensureOpenLocked();final int count = mApkAssets.length;// See if we already have it loaded.for (int i = 0; i < count; i++) {if (mApkAssets[i].getAssetPath().equals(path)) {return i + 1;}}final ApkAssets assets;try {if (overlay) {// TODO(b/70343104): This hardcoded path will be removed once// addAssetPathInternal is deleted.final String idmapPath = "/data/resource-cache/"+ path.substring(1).replace('/', '@')+ "@idmap";assets = ApkAssets.loadOverlayFromPath(idmapPath, false /*system*/);} else {assets = ApkAssets.loadFromPath(path, false /*system*/, appAsLib);}} catch (IOException e) {return 0;}mApkAssets = Arrays.copyOf(mApkAssets, count + 1);mApkAssets[count] = assets;nativeSetApkAssets(mObject, mApkAssets, true);invalidateCachesLocked(-1);return count + 1;}}

有了以上的调查我们就可以创建自己的Resource并取得图片颜色 String等资源了

3.Demo 获取其他APK的资源

准备工作：

3.1.创建皮肤包

创建一个Android项目里面仅仅放一个图片abc.png 生成一个APK 重命名为test.skin 将apk放置到手机内存根目录
/storage/emulated/0/test.skin
首先需要申请内存访问权限，之前就是因为没有申请权限导致获取的id一直是0 结果调试了半天才发现是没有内存的访问权限

3.2.申请内存访问权限

public class Util {public static void checkAndRequestReadSDCardPermission(Activity activity) {if (activity == null) {return;}if (activity.checkSelfPermission(Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {activity.requestPermissions(new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, 0);}}
}<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

3.3.创建Resource和AssertManager

    protected void initView() {Util.checkAndRequestReadSDCardPermission(this);loadImg();}private void loadImg() {try {ImageView img = findViewById(R.id.emptyImg);// 读取本地的一个 .skin里面的资源Resources superRes = getResources();// 创建AssetManagerAssetManager assetManager = AssetManager.class.newInstance();// 寻找hide的方法Method method = AssetManager.class.getDeclaredMethod("addAssetPath",String.class);// method.setAccessible(true); 如果是私有的Log.e(TAG, "loadImg from : "+Environment.getExternalStorageDirectory().getAbsolutePath()+File.separator + "test.skin");// 反射执行方法 assetManager指向指定的皮肤包 /storage/emulated/0/test.skinmethod.invoke(assetManager, Environment.getExternalStorageDirectory().getAbsolutePath()+File.separator + "test.skin");Resources resource = new Resources(assetManager,superRes.getDisplayMetrics(),superRes.getConfiguration());// 获取指定路径apk的packageNameString packageName = "";if (this.getApplication() != null) {Log.e(TAG, "loadImage: getApplication!=null");if (this.getApplication().getPackageManager() != null) {String myPath = Environment.getExternalStorageDirectory().getAbsolutePath() +File.separator + "test.skin";Log.e(TAG, "loadImage: myPath ==="+myPath);packageName = this.getApplication().getPackageManager().getPackageArchiveInfo(Environment.getExternalStorageDirectory().getAbsolutePath() +File.separator + "test.skin", PackageManager.GET_ACTIVITIES).packageName;} else {Log.e(TAG, "loadImage: getPackageManager==null");}}// 获取资源 idint drawableId = resource.getIdentifier("abc","drawable",packageName);Drawable drawable = resource.getDrawable(drawableId);img.setImageDrawable(drawable);} catch (Exception e) {Log.e(TAG, "loadImg: "+e.getStackTrace().toString());e.printStackTrace();}}

4.AssetManager的Android部分源码（API28）

我们之前已经知道资源的加载是通过Resources类来加载的，而实际上执行方法的是Resources的内部成员ResourcesImpl，这里我们继续往下深入
以Resources的getDrawableForDensity方法为例其内部调用了ResourcesImpl的方法
loadDrawable(this, value, id, density, theme);

    @NullableDrawable loadDrawable(@NonNull Resources wrapper, @NonNull TypedValue value, int id,int density, @Nullable Resources.Theme theme)throws NotFoundException {...try {if (TRACE_FOR_PRELOAD) {// Log only framework resourcesif ((id >>> 24) == 0x1) {final String name = getResourceName(id);//获取预加载的Android原生的资源 内部调用的也是AssetManager的方法if (name != null) {Log.d("PreloadDrawable", name);}}}...// First, check whether we have a cached version of this drawable// that was inflated against the specified theme. Skip the cache if// we're currently preloading or we're not using the cache.if (!mPreloading && useCache) {// 缓存机制final Drawable cachedDrawable = caches.getInstance(key, wrapper, theme);if (cachedDrawable != null) {cachedDrawable.setChangingConfigurations(value.changingConfigurations);return cachedDrawable;}}// 预加载资源// Next, check preloaded drawables. Preloaded drawables may contain// unresolved theme attributes.final Drawable.ConstantState cs;if (isColorDrawable) {cs = sPreloadedColorDrawables.get(key);} else {cs = sPreloadedDrawables[mConfiguration.getLayoutDirection()].get(key);}Drawable dr;boolean needsNewDrawableAfterCache = false;if (cs != null) {//加载Android原生资源的caseif (TRACE_FOR_DETAILED_PRELOAD) {// Log only framework resources if (((id >>> 24) == 0x1) && (android.os.Process.myUid() != 0)) {final String name = getResourceName(id);if (name != null) {Log.d(TAG_PRELOAD, "Hit preloaded FW drawable #"+ Integer.toHexString(id) + " " + name);}}}dr = cs.newDrawable(wrapper);} else if (isColorDrawable) {//加载颜色类型的drawabledr = new ColorDrawable(value.data);} else {// 加载真正的drawable 重点！！！dr = loadDrawableForCookie(wrapper, value, id, density);}...}}private Drawable loadDrawableForCookie(@NonNull Resources wrapper, @NonNull TypedValue value,int id, int density) {...if (file.endsWith(".xml")) {//加在xml类型的drawablefinal XmlResourceParser rp = loadXmlResourceParser(file, id, value.assetCookie, "drawable");dr = Drawable.createFromXmlForDensity(wrapper, rp, density, null);rp.close();} else {// 加载png jpeg等类型的图片final InputStream is = mAssets.openNonAsset(value.assetCookie, file, AssetManager.ACCESS_STREAMING);AssetInputStream ais = (AssetInputStream) is;dr = decodeImageDrawable(ais, wrapper, value);}...}

我们知道加载是通过Resources类来加载的，而实际上执行方法的是Resources的内部成员，现在看了源码其实真正查找资源的是AssetManager
我们看看AssetManager是如何初始化的吧
查看AssetManager的构造方法有两个

    /*** Private constructor that doesn't call ensureSystemAssets.* Used for the creation of system assets.*/@SuppressWarnings("unused")private AssetManager(boolean sentinel) {mObject = nativeCreate();if (DEBUG_REFS) {mNumRefs = 0;incRefsLocked(hashCode());}}/*** Create a new AssetManager containing only the basic system assets.* Applications will not generally use this method, instead retrieving the* appropriate asset manager with {@link Resources#getAssets}.    Not for* use by applications.* @hide*/public AssetManager() {final ApkAssets[] assets;synchronized (sSync) {createSystemAssetsInZygoteLocked();assets = sSystemApkAssets;}mObject = nativeCreate();if (DEBUG_REFS) {mNumRefs = 0;incRefsLocked(hashCode());}// Always set the framework resources.setApkAssets(assets, false /*invalidateCaches*/);}

方法的注释已经说明了区别无参的构造方法是专门为了创建系统资源的通过源码我们也知道无参的构造方法在createSystemAssetsInZygoteLocked中调用了包含boolean作为参数的构造方法。
createSystemAssetsInZygoteLocked方法中有个值得关注的点

apkAssets.add(ApkAssets.loadFromPath(FRAMEWORK_APK_PATH, true /*system*/));
private static final String FRAMEWORK_APK_PATH = "/system/framework/framework-res.apk";

这里加载了Android系统的资源apk 我猜测小米的MIUI或者其他更改系统皮肤的手机制造商是可以更改这个apk中的资源来达到控制UI式样的目的的
由于Android9.0中 AssetManager的源码变动较大，我能力有限，没有看懂在9.0中AssetManager如何在framework层初始化的
猜测可能的创建方式如下

 nativeCreate()// frameworks/base/core/jni/android_util_AssetManager.cppstatic jlong NativeCreate(JNIEnv* /*env*/, jclass /*clazz*/) {// AssetManager2 needs to be protected by a lock. To avoid cache misses, we allocate the lock and// AssetManager2 in a contiguous block (GuardedAssetManager).return reinterpret_cast<jlong>(new GuardedAssetManager());}struct GuardedAssetManager : public ::AAssetManager {Guarded<AssetManager2> guarded_assetmanager;};//AssetManager2.cppAssetManager2::AssetManager2() {memset(&configuration_, 0, sizeof(configuration_));}

最终创建的可能是AssetManager2
因为不是很懂c++ 再往下就分析不动了。。。
罗升阳的博客讲的比较清楚，不过他的Android版本可能是6.0版本的 Android9.0系统源码已经大变不过还是有很大的参考价值
https://blog.csdn.net/luoshengyang/article/details/8791064
另外值得说一下的是 resources.arsc
这个是打包后的apk中的一个文件是各种资源和id的映射，assetManager管理资源应该和这个文件关系很大使用Android Studio打开apk并选择resources.arsc打开可以看到详细的映射我找到了系统的资源apk的resources.arsc 如图

打开我们之前使用皮肤包还能看到我们放入的资源的id与图片名称的映射