换肤，顾名思义，就是对应用中的视觉元素进行更新，呈现新的显示效果。一般来说，换肤的时候只是更新UI上使用的资源，如颜色，图片，字体等等。本文介绍一种笔者自己使用的基于布局的Android换肤方案，不仅可以更换所有的UI资源，而且可以更换主题样式（style）和布局样式。代码已托管到github：SkinFramework
换肤当然得有相应的皮肤包，不管是内置在应用内，还是做成可安装的皮肤应用包。但是这两种都有弊端：
1.内置在应用内会增加应用包的体积。
2.皮肤安装包需要安装过程，会占用更多的设备内置存储，用户会介意安装过多应用。而且为了是应用能够访问安装包内的资源，必须与应用使用相同的shareUserId。
鉴于此，本文推荐使用无需安装的外置皮肤包，优点在于：
1.无需安装，也无关乎shareUserId，不会引起用户反感。
2.按需下载使用，用户需要使用时自行下载，下载即可使用。
3.可放置于任何可访问的位置，SD卡或内置存储，可随时删除和添加，不会增加应用体积。
先来看一下效果图：
可以看到，图中有三种皮肤，默认皮肤，plain皮肤和vivid皮肤，都是更换了布局和资源的，其中还使用了AdapterView和Fragment作测试。可以看到，不同的皮肤有不同的布局样式，布局样式的不同也带来了很多可能，如隐藏或移动了功能入口。
所以说这是一个有很多可能的换肤框架，下面介绍一下核心 实现。
一、皮肤包
皮肤包就是一个不包含代码文件的Apk包，无需安装，可以新建工程，删除掉代码文件，复制应用里面需要修改的资源到新工程中修改，打成新包即可作为皮肤包使用，皮肤包后缀名可以改为任意。示例中使用了.skin作为后缀名。
二、皮肤包加载
皮肤包中包含的资源文件，需要加载到AssetManager中并创建Resources才能提供使用，关于Android的资源管理机制书上或网上已经有很多介绍，可以参考：Android中资源管理机制详细分析。所以我们的第一件事也是来加载皮肤包：
AssetManager assetManager = AssetManager.class.newInstance(); Method addAssetPath = assetManager.getClass().getMethod("addAssetPath", String.class); //Return 0 on failure. Object ret = addAssetPath.invoke(assetManager, skinPath); if (Integer.parseInt(ret.toString()) == 0) { throw new IllegalStateException("Add asset fail"); } Resources localRes = context.getResources(); return new SkinResource(context, assetManager, localRes.getDisplayMetrics(), localRes.getConfiguration(), packageName);
三、资源管理器
加载了皮肤包，我们就有了两套可共使用的皮肤资源，应用默认资源和皮肤包资源，何时使用默认，何时使用皮肤，需要有一个管理器来决定，所以我们实现一个名为ComposedResources的类来扮演ResourcesManager：
/** * Created by ARES on 2016/5/20 * This is a resources class consists of App default skin and external skin resources if exists. We will find resource in external skin resources first,then the default. * Assume all resources ids are original so that we should find corresponding resources ids in skin . */ public class ComposedResources extends BaseResources { static int LAYOUT_TAG_ID = -1; private Context mContext; private BaseSkinResources mSkinResources; public ComposedResources(Context context) { this(context, null); } public ComposedResources(Context context, BaseSkinResources skinResources) { super(context.getResources()); mContext = context; mSkinResources = skinResources; } public ComposedResources setSkinResources(BaseSkinResources resources) { mSkinResources = resources; return this; } public BaseSkinResources getSkinResources() { return mSkinResources; } @NonNull @Override public CharSequence getText(@StringRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { try { return mSkinResources.getText(realId); } catch (Exception e) { } } return super.getText(id); } @NonNull @Override public CharSequence getQuantityText(@PluralsRes int id, int quantity) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getQuantityText(realId, quantity); } return super.getQuantityText(id, quantity); } @NonNull @Override public String getQuantityString(@PluralsRes int id, int quantity, Object... formatArgs) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getQuantityString(realId, quantity, formatArgs); } return super.getQuantityString(id, quantity, formatArgs); } @NonNull @Override public String getQuantityString(@PluralsRes int id, int quantity) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getQuantityString(realId, quantity); } return super.getQuantityString(id, quantity); } @Override public CharSequence getText(@StringRes int id, CharSequence def) { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getText(realId, def); } return super.getText(id, def); } @NonNull @Override public CharSequence[] getTextArray(@ArrayRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getTextArray(id); } return super.getTextArray(id); } @NonNull @Override public String[] getStringArray(@ArrayRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getStringArray(realId); } return super.getStringArray(id); } @NonNull @Override public int[] getIntArray(@ArrayRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getIntArray(realId); } return super.getIntArray(id); } @NonNull @Override public TypedArray obtainTypedArray(@ArrayRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.obtainTypedArray(realId); } return super.obtainTypedArray(id); } @Override public float getDimension(@DimenRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getDimension(realId); } return super.getDimension(id); } @Override public int getDimensionPixelOffset(@DimenRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getDimensionPixelOffset(realId); } return super.getDimensionPixelOffset(id); } @Override public int getDimensionPixelSize(@DimenRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getDimensionPixelSize(realId); } return super.getDimensionPixelSize(id); } @Override public float getFraction(@FractionRes int id, int base, int pbase) { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getFraction(id, base, pbase); } return super.getFraction(id, base, pbase); } @Override public Drawable getDrawable(@DrawableRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getDrawable(realId); } return super.getDrawable(id); } @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP) @Override public Drawable getDrawable(@DrawableRes int id, @Nullable Theme theme) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getDrawable(realId, theme); } return super.getDrawable(id, theme); } @Override public Drawable getDrawableForDensity(@DrawableRes int id, int density) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getDrawableForDensity(realId, density); } return super.getDrawableForDensity(id, density); } @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP) @Override public Drawable getDrawableForDensity(@DrawableRes int id, int density, @Nullable Theme theme) { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getDrawableForDensity(realId, density, theme); } return super.getDrawableForDensity(id, density, theme); } @Override public Movie getMovie(@RawRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getMovie(realId); } return super.getMovie(id); } @Override public int getColor(@ColorRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getColor(realId); } return super.getColor(id); } @RequiresApi(api = Build.VERSION_CODES.M) @Override public int getColor(@ColorRes int id, @Nullable Theme theme) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getColor(realId, theme); } return super.getColor(id, theme); } @Nullable @Override public ColorStateList getColorStateList(@ColorRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getColorStateList(realId); } return super.getColorStateList(id); } @RequiresApi(api = Build.VERSION_CODES.M) @Nullable @Override public ColorStateList getColorStateList(@ColorRes int id, @Nullable Theme theme) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getColorStateList(realId, theme); } return super.getColorStateList(id, theme); } @Override public boolean getBoolean(@BoolRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getBoolean(realId); } return super.getBoolean(id); } @Override public int getInteger(@IntegerRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.getInteger(realId); } return super.getInteger(id); } @Override public XmlResourceParser getLayout(@LayoutRes int id) throws NotFoundException { int realId = getCorrespondResIdStrictly(id); if (realId > 0) { return mSkinResources.getLayout(realId); } return super.getLayout(id); } @Override public XmlResourceParser getAnimation(@AnimRes int id) throws NotFoundException { int realId = getCorrespondResIdStrictly(id); if (realId > 0) { return mSkinResources.getAnimation(realId); } return super.getAnimation(id); } @Override public XmlResourceParser getXml(@XmlRes int id) throws NotFoundException { int realId = getCorrespondResIdStrictly(id); if (realId > 0) { return mSkinResources.getXml(realId); } return super.getXml(id); } @Override public InputStream openRawResource(@RawRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.openRawResource(realId); } return super.openRawResource(id); } @Override public InputStream openRawResource(@RawRes int id, TypedValue value) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.openRawResource(realId, value); } return super.openRawResource(id, value); } @Override public AssetFileDescriptor openRawResourceFd(@RawRes int id) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { return mSkinResources.openRawResourceFd(realId); } return super.openRawResourceFd(id); } @Override public void getValue(@AnyRes int id, TypedValue outValue, boolean resolveRefs) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { mSkinResources.getValue(realId, outValue, resolveRefs); return; } super.getValue(id, outValue, resolveRefs); } @Override public void getValueForDensity(@AnyRes int id, int density, TypedValue outValue, boolean resolveRefs) throws NotFoundException { int realId = getCorrespondResId(id); if (realId > 0) { mSkinResources.getValueForDensity(realId, density, outValue, resolveRefs); return; } super.getValueForDensity(id, density, outValue, resolveRefs); } @Override public void getValue(String name, TypedValue outValue, boolean resolveRefs) throws NotFoundException { if (mSkinResources != null) { try { mSkinResources.getValue(name, outValue, resolveRefs); return; } catch (Exception e) { } } super.getValue(name, outValue, resolveRefs); } @Override public void updateConfiguration(Configuration config, DisplayMetrics metrics) { if (mSkinResources != null) { mSkinResources.updateConfiguration(config, metrics); } super.updateConfiguration(config, metrics); } /** * Get correspond resources id with app package. See also {@link #getCorrespondResId(int)} * * @param resId * @return 0 if not exist */ public int getCorrespondResIdStrictly(int resId) { if (mSkinResources == null) { return 0; } String resName = getResourceName(resId); return mSkinResources.getIdentifier(resName, null, null); } /** * Get correspond resources id with skin package. See also {@link #getCorrespondResId(int)} * * @param resId * @return */ public int getCorrespondResId(int resId) { if (mSkinResources == null) { return 0; } return mSkinResources.getCorrespondResId(resId); } @Override public View getView(Context context, @LayoutRes int resId) { //Take a resource id as the tag key. if (LAYOUT_TAG_ID < 1) { LAYOUT_TAG_ID = resId; } View view; if (mSkinResources != null) { int realId = getCorrespondResId(resId); if (realId > 0) { view = mSkinResources.getView(context, realId); if (view != null) { view.setTag(LAYOUT_TAG_ID, mSkinResources.getPackageName()); SkinUtils.showIds(view); return view; } } } view = LayoutInflater.from(context).inflate(resId, null); view.setTag(LAYOUT_TAG_ID, getPackageName()); SkinUtils.showIds(view); return view; } @Override public String getPackageName() { return mContext.getPackageName(); } }
可以看到这个ResourceManager本身也是一个Resources，它继承自BaseResources，BaseResources继承自android.content.res.Resources。所以它可以直接作为应用的Resources来使用。其中有几点需要注意：
1.查找资源时，资源管理器应优先查找皮肤包中的资源，若皮肤包中没有相应资源，才使用应用默认资源。
2.每个应用包中的资源id是不同的，查找资源时，我们传入Resources的id都是应用中的id，而非皮肤包中的id，所以我们需要转换为皮肤包中相应的资源id，再获取具体的资源（此代码实现在SkinResources中，ComposedResources调用了此方法）：
/** * Get correspond resource id in skin archive. * @param resId Resource id in app. * @return 0 if not exist */ public int getCorrespondResId(int resId) { Resources appResources = getAppResources(); String resName = appResources.getResourceName(resId); if (!TextUtils.isEmpty(resName)) { String skinName = resName.replace(mAppPackageName, getPackageName()); int id = getIdentifier(skinName, null, null); return id; } return 0; }
3.在获取XmlResourceParser时，需要使用应用对于资源的描述，而非皮肤包中的资源描述，所以有了getCorrespondResIdStrictly：
/** * Get correspond resources id with app package. See also {@link #getCorrespondResId(int)} * * @param resId * @return 0 if not exist */ public int getCorrespondResIdStrictly(int resId) { if (mSkinResources == null) { return 0; } String resName = getResourceName(resId); return mSkinResources.getIdentifier(resName, null, null); }
4.我们使用了LAYOUT_TAG_ID来记录了Layout所属的皮肤包，以便可以动态的判断是否需要更换布局（此方法可以用在动态换肤的时候，详情参考Demo）：
@Override public View getView(Context context, @LayoutRes int resId) { //Take a resource id as the tag key. if (LAYOUT_TAG_ID < 1) { LAYOUT_TAG_ID = resId; } View view; if (mSkinResources != null) { int realId = getCorrespondResId(resId); if (realId > 0) { view = mSkinResources.getView(context, realId); if (view != null) { view.setTag(LAYOUT_TAG_ID, mSkinResources.getPackageName()); return view; } } } view = LayoutInflater.from(context).inflate(resId, null); view.setTag(LAYOUT_TAG_ID, getPackageName()); return view; }
四、布局更换处理
通过上述的代码，我们就已经能够完成常见资源的换肤了。但是对于布局资源，我们还需要做额外的处理。
1.Context与LayoutInflater
渲染View时，我们需要使用皮肤对应的Context和LayoutInflater，这样才能在View中使用正确的资源，所以我们为外置皮肤包创建相应的Context：
/** * Context implementation for skin package. */ private class SkinThemeContext extends ContextThemeWrapper { private WeakReference<Context> mContextRef; public SkinThemeContext(Context base) { super(); if (base instanceof ContextThemeWrapper) { attachBaseContext(((ContextThemeWrapper) base).getBaseContext()); mContextRef = new WeakReference<Context>(base); } else { attachBaseContext(base); } int themeRes = getThemeRes(); if (themeRes <= 0) { themeRes = android.R.style.Theme_Light; } setTheme(themeRes); } /** * This implementation will support <code>onClick</code> attribute of view in xml. * @param v */ public void onClick(View v) { Context context = mContextRef == null ? null : mContextRef.get(); if (context == null) { return; } if (context instanceof View.OnClickListener) { ((View.OnClickListener) context).onClick(v); } else { Class cls = context.getClass(); try { Method m = cls.getDeclaredMethod("onClick", View.class); if (m != null) { m.invoke(context, v); } } catch (NoSuchMethodException e) { e.printStackTrace(); } catch (InvocationTargetException e) { e.printStackTrace(); } catch (IllegalAccessException e) { e.printStackTrace(); } } } @Override public AssetManager getAssets() { return getAssets(); } @Override public Resources getResources() { return SkinResource.this; } private int getThemeRes() { try { Method m = Context.class.getMethod("getThemeResId"); return (int) m.invoke(getBaseContext()); } catch (Exception e) { e.printStackTrace(); } return -1; } }
其中我们对xml布局文件中的onClick属性作了支持，同时通过提供了皮肤包对应的资源，然后我们使用这个Context的实例创建LayoutInflater并渲染View：
@Override public View getView(Context context, @LayoutRes int resId) { try { Context skinContext = new SkinThemeContext(context); View v = LayoutInflater.from(skinContext).inflate(resId, null); handleView(skinContext, v); return v; } catch (Exception e) { } return null; }
其中注意到handleView方法，正如前文所说，每个应用包生成的资源id是不一样的，这里View中生成的id是皮肤包中的id，需要转换为应用中的id方可使用：
/** * Handle view to support used by app. * * @param v View resource from skin package. */ public void handleView(Context context, View v) { resetID(); //Id map: Key as skin id and Value as local id. SparseIntArray array = new SparseIntArray(); buildIdRules(context, v, array); int size = array.size(); // Map ids to which app can recognize locally. for (int i = 0; i < size; i++) { //Map id defined in skin package into real id in app. v.findViewById(array.keyAt(i)).setId(array.valueAt(i)); } } /** * Extract id from view , build id rules and inflate rules if needed. * * @param v * @param array */ protected void buildIdRules(Context context, View v, SparseIntArray array) { if (v.getId() != View.NO_ID) { //Get mapped id by id name. String idName = getResourceEntryName(v.getId()); int mappedId = getAppResources().getIdentifier(idName, "id", context.getPackageName()); //Add custom id to avoid id conflict when mapped id not exist. //Key as skin id and value as mapped id. array.put(v.getId(), mappedId > 0 ? mappedId : generateId()); } if (v instanceof ViewGroup) { ViewGroup vp = (ViewGroup) v; int childCount = vp.getChildCount(); for (int i = 0; i < childCount; i++) { buildIdRules(context, vp.getChildAt(i), array); } } buildInflateRules(v, array); } /** * Build inflate rules. * * @param v * @param array ID map of which Key as skin id and value as mapped id. */ protected void buildInflateRules(View v, SparseIntArray array) { ViewGroup.LayoutParams lp = v.getLayoutParams(); if (lp == null) { return; } if (lp instanceof RelativeLayout.LayoutParams) { int[] rules = ((RelativeLayout.LayoutParams) lp).getRules(); if (rules == null) { return; } int size = rules.length; int mapRule = -1; for (int i = 0; i < size; i++) { //Key as skin id and value as mapped id. if (rules[i] > 0 && (mapRule = array.get(rules[i])) > 0) { // Log.i(TAG, "Rules[" + i + "]: Mapped from: " + rules[i] + " to " +mapRule); rules[i] = mapRule; } } } }
五、使用
下载源码，集成skin module到工程中，然后使用SkinManager提供的接口：
public SkinManager initialize(Context context);//初始化皮肤管理器 /** * Register an observer to be informed of skin changed for ui interface such as activity,fragment, dialog etc. * @param observer */ public void register(ISkinObserver observer)；//注册换肤监听器，用于需要动态换肤的场景。 /** * Get resources. * @return */ public BaseResources getResources()；//获取资源 /** * Change skin. * @param skinPath Path of skin archive. * @param pkgName Package name of skin archive. * @param cb Callback to be informed of skin-changing event. */ public void changeSkin(String skinPath, String pkgName, ISkinCallback cb)；//更换皮肤 /** * Restore skin to app default skin. * * @param cb */ public void restoreSkin(ISkinCallback cb) ；//恢复应用默认皮肤 /** * Resume skin.Call it on application started. * * @param cb */ public void resumeSkin(ISkinCallback cb) ；//恢复当前使用的皮肤，应在应用启动界面调用。
框架支持两种换肤方式：
1.静态换肤（推荐）
换肤完成后，关闭掉所有的Activity，然后重新启动主界面。简单方便。
2.动态换肤
需要换肤的Activity、Fragment、Dialog实现ISkinObserver, 并通过register(ISkinObserver observer)注册到SkinManager,动态更换布局，详情见Sample代码。
这种方式需要重新渲染View，绑定数据，在使用Fragment时，还需要在换肤期间detach/attach fragment,使用起来比较麻烦。优点是换肤后可以停留在原来界面。
两种方式都需要使用SkinManager提供的Resource来获取布局或其他资源。推荐写自己的BaseActivity，重写getResources()返回SkinManager提供的Resources方便使用。小伙伴们根据自己的实际情况来选择具体使用何种方式。
好了，到这里换肤框架就介绍完了，欢迎关注SkinFramework的最新动态，若有任何建议和意见，欢迎指出！