Android-skin-support换肤原理

导言：项目最近用到换肤功能，要匹配四套皮肤，在github上对比了几个换肤库，发现Android-skin-support的star量比较高，（github地址）用起来效果也不错。当时项目比较紧，留下几个疑问，现在把源码看了看，搞明白了到底是怎么回事。
先说心中的疑问：
1：换肤库是怎么实现对原有的系统控件View的换肤支持？
2：是怎么根据不同的皮肤来找到对应的资源文件的？
3：切换皮肤后，是怎么更新现有View的皮肤的？

带着这三个疑问，我们来从源码看起（本文只讲解androidx的，android包的其实也是一样道理,也可以跳过源码直接看总结结果）：

一、xml布局文件是怎么变成对象的？

xml布局文件要想变成一个实实在在的view对象，都会调用LayoutInflater.inflate()方法。

下图是AppcompatActivity的setContentView()的加载layout的实现处。（此处略过AppCompatDelegate代理AppCompatActivity的全生命周期）

看到LayoutInflater.inflate()是不是很熟悉。比如View通过xml布局创建，也是通过View.inflate(Context context, @LayoutRes int resource, ViewGroup root)方法来加载的。

/**
 * Inflate a view from an XML resource.  This convenience method wraps the {@link
 * LayoutInflater} class, which provides a full range of options for view inflation.
 *
 * @param context The Context object for your activity or application.
 * @param resource The resource ID to inflate
 * @param root A view group that will be the parent.  Used to properly inflate the
 * layout_* parameters.
 * @see LayoutInflater
 */
public static View inflate(Context context, @LayoutRes int resource, ViewGroup root) {
   
   
    LayoutInflater factory = LayoutInflater.from(context);
    return factory.inflate(resource, root);
}

方法的本质还是通过LayoutInflater.inflate() 来实现的。下面我们来看下LayoutInflater.inflate()的具体实现。

  /**
     * Inflate a new view hierarchy from the specified xml resource. Throws
     * {@link InflateException} if there is an error.
     *
     * @param resource ID for an XML layout resource to load (e.g.,
     *        <code>R.layout.main_page</code>)
     * @param root Optional view to be the parent of the generated hierarchy (if
     *        <em>attachToRoot</em> is true), or else simply an object that
     *        provides a set of LayoutParams values for root of the returned
     *        hierarchy (if <em>attachToRoot</em> is false.)
     * @param attachToRoot Whether the inflated hierarchy should be attached to
     *        the root parameter? If false, root is only used to create the
     *        correct subclass of LayoutParams for the root view in the XML.
     * @return The root View of the inflated hierarchy. If root was supplied and
     *         attachToRoot is true, this is root; otherwise it is the root of
     *         the inflated XML file.
     */
    public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
   
   
        final Resources res = getContext().getResources();
        if (DEBUG) {
   
   
            Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
                  + Integer.toHexString(resource) + ")");
        }

        View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
        if (view != null) {
   
   
            return view;
        }
        XmlResourceParser parser = res.getLayout(resource);
        try {
   
   
            return inflate(parser, root, attachToRoot);
        } finally {
   
   
            parser.close();
        }
    }

注意到先是tryInflatePrecompiled(resource, res, root, attachToRoot) 此处不做过多解释，主要是先从预编译的dex包里面通过反射来加载View。而反射获取对应的View时可以直接获取到预编译的View对象，而不需要递归调用XML解析。
下面我们看return inflate(parser, root, attachToRoot);方法。

   /**
     * Inflate a new view hierarchy from the specified XML node. Throws
     * {@link InflateException} if there is an error.
     * <p>
     * <em><strong>Important</strong></em>&nbsp;&nbsp;&nbsp;For performance
     * reasons, view inflation relies heavily on pre-processing of XML files
     * that is done at build time. Therefore, it is not currently possible to
     * use LayoutInflater with an XmlPullParser over a plain XML file at runtime.
     *
     * @param parser XML dom node containing the description of the view
     *        hierarchy.
     * @param root Optional view to be the parent of the generated hierarchy (if
     *        <em>attachToRoot</em> is true), or else simply an object that
     *        provides a set of LayoutParams values for root of the returned
     *        hierarchy (if <em>attachToRoot</em> is false.)
     * @param attachToRoot Whether the inflated hierarchy should be attached to
     *        the root parameter? If false, root is only used to create the
     *        correct subclass of LayoutParams for the root view in the XML.
     * @return The root View of the inflated hierarchy. If root was supplied and
     *         attachToRoot is true, this is root; otherwise it is the root of
     *         the inflated XML file.
     */
    public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
   
   
        synchronized (mConstructorArgs) {
   
   
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");

            final Context inflaterContext = mContext;
            final AttributeSet attrs = Xml.asAttributeSet(parser);
            Context lastContext = (Context) mConstructorArgs[0];
            mConstructorArgs[0] = inflaterContext;
            View result = root;

            try {
   
   
                advanceToRootNode(parser);
                final String name = parser.getName();

                if (DEBUG) {
   
   
                    System.out.println("**************************");
                    System.out.println("Creating root view: "
                            + name);
                    System.out.println("**************************");
                }

                if (TAG_MERGE.equals(name)) {
   
   
                    if (root == null || !attachToRoot) {
   
   
                        throw new InflateException("<merge /> can be used only with a valid "
                                + "ViewGroup root and attachToRoot=true");
                    }

                    rInflate(parser, root, inflaterContext, attrs, false);
                } else {
   
   
                    // Temp is the root view that was found in the xml
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);

                    ViewGroup.LayoutParams params = null;

                    if (root != null) {
   
   
                        if (DEBUG) {
   
   
                            System.out.println("Creating params from root: " +
                                    root);
                        }
                        // Create layout params that match root, if supplied
                        params = root.generateLayoutParams(attrs);
                        if (!attachToRoot) {