android 换肤无非就是改变字体的颜色，控件的背景等显示效果。

下面看看SkinActivity的实现：

先上布局：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="40dp"
        android:background="@color/colorPrimary"
        android:gravity="center_vertical"
        android:paddingLeft="16dp"
        android:text="要换肤的地方"
        android:textColor="@color/white" />

    <Button
        android:id="@+id/btn_change_skin"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:onClick="changeSkin"
        android:text="换肤" />

</LinearLayout>

原来的样式：

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="colorPrimary">#3F51B5</color>
    <color name="white">#ffffff</color>
</resources>

image.png

创建个皮肤包项目，仅颜色变一下，生成单独的apk放到sd卡中：

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="colorPrimary">#ff0000</color>
    <color name="white">#000000</color>
</resources>

换肤后的效果：

image.png

设计到的技术：

apk 动态加载
View的解析监听

// SkinActivity 的代码
private var skinInflateFactory = SkinInflateFactory()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        LayoutInflaterCompat.setFactory(layoutInflater, skinInflateFactory)
        setContentView(R.layout.activity_skin)
    }

    fun changeSkin(v: View) {
        SkinManager.getInstance().let {
            it.context = this
            it.loadSkin("skin01")
            skinInflateFactory.apply()
        }
    }

监听View的解析过程，判断哪些控件需要更换样式：

class SkinInflateFactory : LayoutInflaterFactory {

    val TAG = SkinInflateFactory::class.java.simpleName!!

    val PREFIX_LIST = arrayListOf("android.widget.", "android.view.", "android.webkit.")

    val skinViewList = ArrayList<SkinView>()

    override fun onCreateView(parent: View?, name: String, context: Context, attrs: AttributeSet): View? {
        var view: View? = null
        if (!name.contains(".")) {
            // 系统控件
            PREFIX_LIST.forEach {
                val v = createView(it + name, context, attrs)
                if (v != null) {
                    view = v!!
                    return@forEach
                }
            }
        } else {
            // 兼容包或自定义控件
            view = createView(name, context, attrs)!!
        }
        val styleList = ArrayList<SkinStyle>()
        (0..attrs.attributeCount - 1).forEach {
            val attrName = attrs.getAttributeName(it)
            val attrVal = attrs.getAttributeValue(it)
            if (attrName.equals("textColor") || attrName.equals("background")) {
                val refId = Integer.parseInt(attrVal.substring(1))
                styleList.add(SkinStyle(attrName, refId))
            }
        }
        if (styleList.isNotEmpty()) {
            skinViewList.add(SkinView(view, styleList))
        }
        return view
    }

    companion object {
        class SkinStyle(val attrName: String, val attrRefId: Int)

        class SkinView(val view: View?, val styleList: List<SkinStyle>) {
            fun apply() {
                for (style in styleList) {
                    if (style.attrName.equals("textColor")) {
                        if (view is TextView) {
                            view.setTextColor(SkinManager.getInstance().getColor(style.attrRefId))
                        }
                    } else if (style.attrName.equals("background")) {
                        view?.setBackgroundDrawable(SkinManager.getInstance().getDrawable(style.attrRefId))
                    }
                }
            }
        }
    }

    fun apply() {
        for (view in skinViewList) {
            view.apply()
        }
    }

    private fun createView(name: String, context: Context, attrs: AttributeSet): View? {
        try {
            var c = Class.forName(name).getConstructor(Context::class.java, AttributeSet::class.java)
            return c.newInstance(context, attrs) as View
        } catch (e: Exception) {
            return null
        }
    }
}

由于皮肤包是apk的方式实现，所以需要动态加载皮肤：

class SkinManager {

    lateinit var context: Context

    var resources: Resources? = null

    private constructor()

    private object INNER {
        val INSTANCE = SkinManager()
    }

    companion object {

        val SKIN_ROOT: String = Environment.getExternalStorageDirectory().absolutePath

        @JvmStatic fun getInstance(): SkinManager {
            return INNER.INSTANCE
        }
    }

    fun loadSkin(skinName: String) {
        try {
            val assetManager = AssetManager::class.java.newInstance()
            val addAssetPath = AssetManager::class.java.getMethod("addAssetPath", String::class.java)
            addAssetPath.invoke(assetManager, SKIN_ROOT + File.separator + skinName + ".apk")
            resources = Resources(assetManager, context.resources.displayMetrics, context.resources.configuration)
        } catch (e: Exception) {
            e.printStackTrace()
        }
    }

    fun getColor(refId: Int): Int {
        val entryName = context.resources.getResourceEntryName(refId)
        val typeName = context.resources.getResourceTypeName(refId)
        val resId = resources?.getIdentifier(entryName, typeName, context.packageName) ?: 0
        if (resId != 0) {
            return resources!!.getColor(resId)
        }
        return context.resources.getColor(refId)
    }

    fun getDrawable(refId: Int): Drawable {
        val entryName = context.resources.getResourceEntryName(refId)
        val typeName = context.resources.getResourceTypeName(refId)
        val resId = resources?.getIdentifier(entryName, typeName, context.packageName) ?: 0
        if (resId != 0) {
            return resources!!.getDrawable(resId)
        }
        return context.resources.getDrawable(refId)
    }
}

至此Android换肤的效果就实现了。