Android LayoutInflater和findViewById 源码详解

LayoutInflater大家很熟悉，简单点说就是布局文件XML解析器,setContentView函数也是调用了LayoutInflater

用法:
View view = LayoutInflater.from(this).inflate(R.layout.activity_main, null);
LayoutInflater lyInflater = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
上面的用法本质上都是一样.

控制台打印 重复调用LayoutInflater.from(this)  同一个进程下面 实例是单例
com.android.internal.policy.impl.PhoneLayoutInflater@420dcf80
com.android.internal.policy.impl.PhoneLayoutInflater@420dcf80
com.android.internal.policy.impl.PhoneLayoutInflater@420dcf80

   首先 LayoutInflater 是一个抽象类,实例化对象是从Service取出的。但是并没有到底层Binder去实例化对象。最终实例对象是PhoneLayoutInflater类

    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;
    }

　　

    从Activity父类可以找到 getSystemService方法
    public Object getSystemService(String name) {
        if (LAYOUT_INFLATER_SERVICE.equals(name)) {
            if (mInflater == null) {
                mInflater = LayoutInflater.from(mBase).cloneInContext(this); //在这个地方实例化的,主要关联到mBase Context对象实例
            }
            return mInflater;
        }
        return mBase.getSystemService(name);
    }

    重点是Context的实例化对象 有个子类 ContextImpl.java 里面注册了相关Service
    public Object getSystemService(String name) {
        ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
        return fetcher == null ? null : fetcher.getService(this);
    }

    SYSTEM_SERVICE_MAP 是个hashMap  注册的服务都存在这个地方，对象被缓存。
    private static final HashMap<String, ServiceFetcher> SYSTEM_SERVICE_MAP = new HashMap<String, ServiceFetcher>();

    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(); //这个用于创建static服务
    }

    registerService(LAYOUT_INFLATER_SERVICE, new ServiceFetcher() { //创建独立的对象 用于LayoutInflater
        public Object createService(ContextImpl ctx) {
           return PolicyManager.makeNewLayoutInflater(ctx.getOuterContext());
    }});

    /*package*/ static class ServiceFetcher {
        int mContextCacheIndex = -1;

        /**
         * Main entrypoint; only override if you don't need caching.
         */
        public Object getService(ContextImpl ctx) {// ContextImpl实例。
            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);//获取缓存是否存在
                    if (service != null) {
                        return service;
                    }
                }
                service = createService(ctx); //用来缓存服务实例
                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");
        }
    }

    com.android.internal.policy.PolicyManager
    找到了LayoutInflater的实现类

    这个类算是个代理类 本质上是
    private static final String POLICY_IMPL_CLASS_NAME =
        "com.android.internal.policy.impl.Policy";

    policy\src\com\android\internal\policy\impl 目录下

    public LayoutInflater makeNewLayoutInflater(Context context) {
        return new PhoneLayoutInflater(context); //实例化
    }

    policy\src\com\android\internal\policy\impl\PhoneLayoutInflater

    回到LayoutInflater 递归解析XML 使用的是XMl Pull Parser解析器

    public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {

    }

    rInflate开始递归 都是XML解析

    void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,
            boolean finishInflate) throws XmlPullParserException, IOException {

        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;
            }

            final String name = parser.getName();

            if (TAG_REQUEST_FOCUS.equals(name)) {
                parseRequestFocus(parser, parent);
            } else if (TAG_INCLUDE.equals(name)) {
                if (parser.getDepth() == 0) {
                    throw new InflateException("<include /> cannot be the root element");
                }
                parseInclude(parser, parent, attrs);
            } else if (TAG_MERGE.equals(name)) {
                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 {
                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();//解析完了，这个方法我们自定义可以复写 可以会去掉 子View的信息
    }

基本上布局都是ViewGroup的子类 里面都有child view, 我们常用的findViewById 就是遍历去找对应ID的View
    public final View findViewById(int id) {
        if (id < 0) {
            return null;
        }
        return findViewTraversal(id);
    }
    直接看ViewGroup中的findViewTraversal

    @Override
    protected View findViewTraversal(int id) {
        if (id == mID) { //等于自己
            return this;
        }

        final View[] where = mChildren;
        final int len = mChildrenCount;

        for (int i = 0; i < len; i++) {
            View v = where[i];

            if ((v.mPrivateFlags & IS_ROOT_NAMESPACE) == 0) { //IS_ROOT_NAMESPACE 可能是查询到最后一层了（猜的）  然后再遍历 查询
                v = v.findViewById(id);

                if (v != null) {
                    return v;
                }
            }
        }

        return null;
    }

如果布局层级太深 解析XML和查找View都是耗费CPU的，所以做项目过程中 有必要进行布局优化。

本文源码: 4.1