Android中将xml布局文件转化为View树的过程分析（下）-- LayoutInflater源码分析

    // 1. get a instance of LayoutInflater, then do whatever you want
    LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        
    // 2. you‘re in some View class, then just call View‘s static inflate method 
    View.inflate(context, R.layout.xxx_xml, someViewGroup/null);

    <!-- View.java -->
    /**
     * 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, int resource, ViewGroup root) {
        LayoutInflater factory = LayoutInflater.from(context);
        return factory.inflate(resource, root);
    }

    <!-- LayoutInflater.java -->
    /**
     * Obtains the LayoutInflater from the given context.
     */
    public static LayoutInflater from(Context context) {
        LayoutInflater LayoutInflater =
                (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        if (LayoutInflater == null) {
            throw new AssertionError("LayoutInflater not found.");
        }
        return LayoutInflater;
    }

    /**
     * Override this class when the system service constructor needs a
     * ContextImpl.  Else, use StaticServiceFetcher below.
     */
    /*package*/ static class ServiceFetcher {
        int mContextCacheIndex = -1;

        /**
         * Main entrypoint; only override if you don‘t need caching.
         */
        public Object getService(ContextImpl ctx) {
            ArrayList<Object> cache = ctx.mServiceCache;
            Object service;
            synchronized (cache) {
                if (cache.size() == 0) {
                    // Initialize the cache vector on first access.
                    // At this point sNextPerContextServiceCacheIndex
                    // is the number of potential services that are
                    // cached per-Context.
                    for (int i = 0; i < sNextPerContextServiceCacheIndex; i++) {
                        cache.add(null);
                    }
                } else {
                    service = cache.get(mContextCacheIndex); // 先从cache中找，
                    if (service != null) { // 如果已经存在了直接返回
                        return service;
                    }
                }
                service = createService(ctx); // 否则创建并加入到cache中，只会调用1次
                cache.set(mContextCacheIndex, service);
                return service;
            }
        }

        /**
         * Override this to create a new per-Context instance of the
         * service.  getService() will handle locking and caching.
         */
        public Object createService(ContextImpl ctx) {
            throw new RuntimeException("Not implemented");
        }
    }

    /**
     * Override this class for services to be cached process-wide.
     */
    abstract static class StaticServiceFetcher extends ServiceFetcher {
        private Object mCachedInstance;

        @Override
        public final Object getService(ContextImpl unused) {
            synchronized (StaticServiceFetcher.this) {
                Object service = mCachedInstance;
                if (service != null) {
                    return service;
                }
                return mCachedInstance = createStaticService();
            }
        }

        public abstract Object createStaticService(); // 它不需要ContextImpl参数
    }

    private static final HashMap<String, ServiceFetcher> SYSTEM_SERVICE_MAP =
            new HashMap<String, ServiceFetcher>(); // 全局system service的map

    private static int sNextPerContextServiceCacheIndex = 0;
    private static void registerService(String serviceName, ServiceFetcher fetcher) {
        if (!(fetcher instanceof StaticServiceFetcher)) {
            fetcher.mContextCacheIndex = sNextPerContextServiceCacheIndex++;
        }
        SYSTEM_SERVICE_MAP.put(serviceName, fetcher); // 放到全局的静态map中
    }

    // 还有很多registerService的调用，这里都省略了，我们现在只关心LAYOUT_INFLATER_SERVICE
    registerService(LAYOUT_INFLATER_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) { // 我们前一篇文章中提到过会new一个
                    return PolicyManager.makeNewLayoutInflater(ctx.getOuterContext()); // PhoneLayoutInflater的对象返回 
                }});

    /**
     * Obtains the LayoutInflater from the given context.
     */
    public static LayoutInflater from(Context context) {
        LayoutInflater LayoutInflater =
                (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        if (LayoutInflater == null) {
            throw new AssertionError("LayoutInflater not found.");
        }
        return LayoutInflater;
    }

    /**
     * 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.
     * @return The root View of the inflated hierarchy. If root was supplied,
     *         this is the root View; otherwise it is the root of the inflated
     *         XML file.
     */
    public View inflate(int resource, ViewGroup root) { // 实际上调用3个参数的版本，从这里我们可以看出客户端代码
        return inflate(resource, root, root != null); // 没必要这样写(root!= null)：inflate(resource, root, true);
    }

    /**
     * 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.
     * @return The root View of the inflated hierarchy. If root was supplied,
     *         this is the root View; otherwise it is the root of the inflated
     *         XML file.
     */
    public View inflate(XmlPullParser parser, ViewGroup root) { // 不太常用，我们一般使用layout文件的版本，但实质都一样
        return inflate(parser, root, root != null);             // 下面的代码中inflate一个include tag时调用了此版本
    }

    /**
     * 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(int resource, ViewGroup root, boolean attachToRoot) {
        if (DEBUG) System.out.println("INFLATING from resource: " + resource);
        XmlResourceParser parser = getContext().getResources().getLayout(resource);
        try {
            return inflate(parser, root, attachToRoot);
        } finally {
            parser.close();
        }
    }

    /**
     * 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, ViewGroup root, boolean attachToRoot) { // 这是最终调用的版本
        synchronized (mConstructorArgs) { // 进入同步块
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");

            final AttributeSet attrs = Xml.asAttributeSet(parser);
            Context lastContext = (Context)mConstructorArgs[0];
            mConstructorArgs[0] = mContext;
            View result = root; // 此方法最后的返回值，初始化为传入的root

            try {
                // Look for the root node.
                int type;
                while ((type = parser.next()) != XmlPullParser.START_TAG &&
                        type != XmlPullParser.END_DOCUMENT) {
                    // Empty
                }
                // 能走到这里，说明type是START_TAG 或 END_DOCUMENT
                if (type != XmlPullParser.START_TAG) { // 如果一开始就是END_DOCUMENT，那说明xml文件有问题
                    throw new InflateException(parser.getPositionDescription()
                            + ": No start tag found!");
                }
                // 能到这里，那type一定是START_TAG，也就是xml文件里的root node
                final String name = parser.getName(); // 获得当前start tag的name
                
                if (DEBUG) {
                    System.out.println("**************************");
                    System.out.println("Creating root view: "
                            + name);
                    System.out.println("**************************");
                }

                if (TAG_MERGE.equals(name)) { // 处理merge tag的情况
                    if (root == null || !attachToRoot) { // root必须非空且attachToRoot为true，否则抛异常结束
                        throw new InflateException("<merge /> can be used only with a valid "
                                + "ViewGroup root and attachToRoot=true"); // 因为merge的xml并不代表某个具体的view，只是将它
                    }                                                 // 包起来的其他xml的内容加到某个上层ViewGroup中

                    rInflate(parser, root, attrs, false); // 递归的inflate
                } else {
                    // Temp is the root view that was found in the xml
                    View temp; // xml文件中的root view
                    if (TAG_1995.equals(name)) {
                        temp = new BlinkLayout(mContext, attrs);
                    } else {
                        temp = createViewFromTag(root, name, attrs); // 根据tag节点创建view对象
                    }

                    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); // 根据root生成合适的LayoutParams实例
                        if (!attachToRoot) { // 如果不attach的话就调用view的setLayoutParams方法
                            // Set the layout params for temp if we are not
                            // attaching. (If we are, we use addView, below)
                            temp.setLayoutParams(params);
                        }
                    }

                    if (DEBUG) {
                        System.out.println("-----> start inflating children");
                    }
                    // Inflate all children under temp
                    rInflate(parser, temp, attrs, true); // 递归inflate剩下的所有children
                    if (DEBUG) {
                        System.out.println("-----> done inflating children");
                    }

                    // We are supposed to attach all the views we found (int temp)
                    // to root. Do that now.
                    if (root != null && attachToRoot) { // root非空且指定了要attachToRoot
                        root.addView(temp, params); // 将xml文件的root view 加到用户提供的root里
                    }

                    // Decide whether to return the root that was passed in or the
                    // top view found in xml.
                    if (root == null || !attachToRoot) {
                        result = temp; // 否则我们将返回xml里发现的root view：temp，而不是方法中传递进来的root对象
                    }
                }

            } catch (XmlPullParserException e) {
                InflateException ex = new InflateException(e.getMessage());
                ex.initCause(e);
                throw ex;
            } catch (IOException e) {
                InflateException ex = new InflateException(
                        parser.getPositionDescription()
                        + ": " + e.getMessage());
                ex.initCause(e);
                throw ex;
            } finally {
                // Don‘t retain static reference on context.
                mConstructorArgs[0] = lastContext;
                mConstructorArgs[1] = null;
            }

            Trace.traceEnd(Trace.TRACE_TAG_VIEW);

            return result; // 返回参数root或xml文件里的root view
        }
    }

    /**
     * Recursive method used to descend down the xml hierarchy and instantiate
     * views, instantiate their children, and then call onFinishInflate().
     */
    void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,
            boolean finishInflate) throws XmlPullParserException, IOException { // 深度优先inflate，所有才能保证你在onFinish
                                                                 // Inflate()里可以通过findViewById找到已经创建完毕的孩子view
        final int depth = parser.getDepth();
        int type;

        while (((type = parser.next()) != XmlPullParser.END_TAG ||
                parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {

            if (type != XmlPullParser.START_TAG) {
                continue;
            }
            // 确保是一个START_TAG node
            final String name = parser.getName(); // 拿到tagName
            
            if (TAG_REQUEST_FOCUS.equals(name)) { // 处理REQUEST_FOCUS tag
                parseRequestFocus(parser, parent);
            } else if (TAG_INCLUDE.equals(name)) { // 处理include tag
                if (parser.getDepth() == 0) { // include节点不能是根节点，否则就抛异常了。。。
                    throw new InflateException("<include /> cannot be the root element");
                }
                parseInclude(parser, parent, attrs);
            } else if (TAG_MERGE.equals(name)) { // merge节点必须是xml文件里的根节点，也就是说到这里的时候不应该再出现merge节点了
                throw new InflateException("<merge /> must be the root element");
            } else if (TAG_1995.equals(name)) {
                final View view = new BlinkLayout(mContext, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                rInflate(parser, view, attrs, true);
                viewGroup.addView(view, params);                
            } else { // 一般情况，各种Android view、widget或用户自定义的view节点
                final View view = createViewFromTag(parent, name, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                rInflate(parser, view, attrs, true);
                viewGroup.addView(view, params);
            }
        }

        if (finishInflate) parent.onFinishInflate(); // parent的所有孩子节点都inflate完毕的时候，调用onFinishInflate回调
    }

    private void parseRequestFocus(XmlPullParser parser, View parent)
            throws XmlPullParserException, IOException {
        int type;
        parent.requestFocus();
        final int currentDepth = parser.getDepth();
        while (((type = parser.next()) != XmlPullParser.END_TAG || // 忽略此节点剩下的所有内容，直到下一个新的START_TAG
                parser.getDepth() > currentDepth) && type != XmlPullParser.END_DOCUMENT) {
            // Empty
        }
    }

    private void parseInclude(XmlPullParser parser, View parent, AttributeSet attrs)
            throws XmlPullParserException, IOException {

        int type;

        if (parent instanceof ViewGroup) {
            final int layout = attrs.getAttributeResourceValue(null, "layout", 0); // include节点中必须指定layout属性的值
            if (layout == 0) {
                final String value = attrs.getAttributeValue(null, "layout");
                if (value == null) {
                    throw new InflateException("You must specifiy a layout in the"
                            + " include tag: <include layout=\"@layout/layoutID\" />");
                } else {
                    throw new InflateException("You must specifiy a valid layout "
                            + "reference. The layout ID " + value + " is not valid.");
                }
            } else {
                final XmlResourceParser childParser =
                        getContext().getResources().getLayout(layout);

                try {
                    final AttributeSet childAttrs = Xml.asAttributeSet(childParser);

                    while ((type = childParser.next()) != XmlPullParser.START_TAG &&
                            type != XmlPullParser.END_DOCUMENT) {
                        // Empty.
                    }

                    if (type != XmlPullParser.START_TAG) {
                        throw new InflateException(childParser.getPositionDescription() +
                                ": No start tag found!");
                    }

                    final String childName = childParser.getName();

                    if (TAG_MERGE.equals(childName)) { // 处理include xml里包含merge的情况
                        // Inflate all children.
                        rInflate(childParser, parent, childAttrs, false);
                    } else { // 处理一般的include layout文件，创建此xml文件的root view
                        final View view = createViewFromTag(parent, childName, childAttrs);
                        final ViewGroup group = (ViewGroup) parent;

                        // We try to load the layout params set in the <include /> tag. If
                        // they don‘t exist, we will rely on the layout params set in the
                        // included XML file.
                        // During a layoutparams generation, a runtime exception is thrown
                        // if either layout_width or layout_height is missing. We catch
                        // this exception and set localParams accordingly: true means we
                        // successfully loaded layout params from the <include /> tag,
                        // false means we need to rely on the included layout params.
                        ViewGroup.LayoutParams params = null;
                        try {
                            params = group.generateLayoutParams(attrs);
                        } catch (RuntimeException e) {
                            params = group.generateLayoutParams(childAttrs);
                        } finally {
                            if (params != null) {
                                view.setLayoutParams(params); // 设置其layoutParams
                            }
                        }

                        // Inflate all children.
                        rInflate(childParser, view, childAttrs, true); // 递归inflate剩下的节点

                        // Attempt to override the included layout‘s android:id with the
                        // one set on the <include /> tag itself.
                        TypedArray a = mContext.obtainStyledAttributes(attrs,
                            com.android.internal.R.styleable.View, 0, 0);
                        int id = a.getResourceId(com.android.internal.R.styleable.View_id, View.NO_ID);
                        // While we‘re at it, let‘s try to override android:visibility.
                        int visibility = a.getInt(com.android.internal.R.styleable.View_visibility, -1);
                        a.recycle();

                        if (id != View.NO_ID) {
                            view.setId(id); // override id，如果include节点提供了
                        }

                        switch (visibility) { // 同样的，override visibility，如果include节点提供了
                            case 0:
                                view.setVisibility(View.VISIBLE);
                                break;
                            case 1:
                                view.setVisibility(View.INVISIBLE);
                                break;
                            case 2:
                                view.setVisibility(View.GONE);
                                break;
                        }

                        group.addView(view); // 将include的xml文件里的root view加到上层group中 
                    }
                } finally {
                    childParser.close();
                }
            }
        } else { // include节点必须是某个ViewGroup的子节点
            throw new InflateException("<include /> can only be used inside of a ViewGroup");
        }

        final int currentDepth = parser.getDepth();
        while (((type = parser.next()) != XmlPullParser.END_TAG || // skip掉include节点剩下的内容
                parser.getDepth() > currentDepth) && type != XmlPullParser.END_DOCUMENT) {
            // Empty
        }
    }

/*
     * default visibility so the BridgeInflater can override it.
     */
    View createViewFromTag(View parent, String name, AttributeSet attrs) {
        if (name.equals("view")) {
            name = attrs.getAttributeValue(null, "class");
        }

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

        try {
            View view;
            // 这里我们忽略掉了各种factory的onCreateView，有兴趣的读者可自行研究
            if (mFactory2 != null) view = mFactory2.onCreateView(parent, name, mContext, attrs);
            else if (mFactory != null) view = mFactory.onCreateView(name, mContext, attrs);
            else view = null;

            if (view == null && mPrivateFactory != null) {
                view = mPrivateFactory.onCreateView(parent, name, mContext, attrs);
            }
            
            if (view == null) {
                if (-1 == name.indexOf(‘.‘)) { // 创建android.view.*里的任何view，如TextView，ImageView等等
                    view = onCreateView(parent, name, attrs); // 其子类PhoneLayoutInflater override了此方法用来
                } else {                                      // 创建android.widget.*/android.webkit.*里的任何对象
                    view = createView(name, null, attrs); // 创建用户自定义的各种View(如com.xiaoweiz.browser.MyCustomView)
                }
            }

            if (DEBUG) System.out.println("Created view is: " + view);
            return view;

        } catch (InflateException e) {
            throw e;

        } catch (ClassNotFoundException e) {
            InflateException ie = new InflateException(attrs.getPositionDescription()
                    + ": Error inflating class " + name);
            ie.initCause(e);
            throw ie;

        } catch (Exception e) {
            InflateException ie = new InflateException(attrs.getPositionDescription()
                    + ": Error inflating class " + name);
            ie.initCause(e);
            throw ie;
        }
    }

    /**
     * Low-level function for instantiating a view by name. This attempts to
     * instantiate a view class of the given <var>name</var> found in this
     * LayoutInflater‘s ClassLoader.
     * 
     * <p>
     * There are two things that can happen in an error case: either the
     * exception describing the error will be thrown, or a null will be
     * returned. You must deal with both possibilities -- the former will happen
     * the first time createView() is called for a class of a particular name,
     * the latter every time there-after for that class name.
     * 
     * @param name The full name of the class to be instantiated.
     * @param attrs The XML attributes supplied for this instance.
     * 
     * @return View The newly instantiated view, or null.
     */
    public final View createView(String name, String prefix, AttributeSet attrs) // 用户自定义的view不需要prefix，因为
            throws ClassNotFoundException, InflateException { // name中已经有所有需要的信息了；系统的prefix则是android.view.
        Constructor<? extends View> constructor = sConstructorMap.get(name); // 或android.widget. 或 android.webkit.
        Class<? extends View> clazz = null;

        try {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);

            if (constructor == null) {
                // Class not found in the cache, see if it‘s real, and try to add it
                clazz = mContext.getClassLoader().loadClass( // 加载class文件
                        prefix != null ? (prefix + name) : name).asSubclass(View.class);
                
                if (mFilter != null && clazz != null) {
                    boolean allowed = mFilter.onLoadClass(clazz);
                    if (!allowed) {
                        failNotAllowed(name, prefix, attrs);
                    }
                }
                constructor = clazz.getConstructor(mConstructorSignature); // 拿到此类型的ctor
                sConstructorMap.put(name, constructor);
            } else {
                // If we have a filter, apply it to cached constructor
                if (mFilter != null) {
                    // Have we seen this name before?
                    Boolean allowedState = mFilterMap.get(name);
                    if (allowedState == null) {
                        // New class -- remember whether it is allowed
                        clazz = mContext.getClassLoader().loadClass(
                                prefix != null ? (prefix + name) : name).asSubclass(View.class);
                        
                        boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
                        mFilterMap.put(name, allowed);
                        if (!allowed) {
                            failNotAllowed(name, prefix, attrs);
                        }
                    } else if (allowedState.equals(Boolean.FALSE)) {
                        failNotAllowed(name, prefix, attrs);
                    }
                }
            }

            Object[] args = mConstructorArgs; // 需要2个参数Context，AttributeSet的版本，所以如果你不打算动态inflate
            args[1] = attrs;                  // 你的view，则没必要提供此版本的ctor。

            final View view = constructor.newInstance(args); // new一个View(可能是其子类)的对象，可能为null
            if (view instanceof ViewStub) { // ViewStub的特殊处理
                // always use ourselves when inflating ViewStub later
                final ViewStub viewStub = (ViewStub) view;
                viewStub.setLayoutInflater(this);
            }
            return view;

        } catch (NoSuchMethodException e) {
            InflateException ie = new InflateException(attrs.getPositionDescription()
                    + ": Error inflating class "
                    + (prefix != null ? (prefix + name) : name));
            ie.initCause(e);
            throw ie;

        } catch (ClassCastException e) {
            // If loaded class is not a View subclass
            InflateException ie = new InflateException(attrs.getPositionDescription()
                    + ": Class is not a View "
                    + (prefix != null ? (prefix + name) : name));
            ie.initCause(e);
            throw ie;
        } catch (ClassNotFoundException e) {
            // If loadClass fails, we should propagate the exception.
            throw e;
        } catch (Exception e) {
            InflateException ie = new InflateException(attrs.getPositionDescription()
                    + ": Error inflating class "
                    + (clazz == null ? "<unknown>" : clazz.getName()));
            ie.initCause(e);
            throw ie;
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }