Android LayoutInflater.from().inflate()源码解析

我们知道，在Activity#setContentView()中会调用PhoneWindow#setContentView()。而在PhoneWindow#setContentView()中有这么一句mLayoutInflater.inflate(layoutResID, mContentParent)。这行代码的作用是将我们的activity_main.xml填充到mContentParent中去。详见：setContentView源码解析。在写adapter的时候，也经常写mInflater.inflate(layoutResID, null)。那么，这行代码怎么就将xml文件转换成了View或者ViewGroup了呢？

获取LayoutInflater对象无非以下两种方式：

1. LayoutInflater.from(Context context);
2. LayoutInflater LayoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

其实这俩是同一种方式，首先看下LayoutInflater#from()

源码位置：frameworks/base/core/java/android/view/LayoutInflater.java

LayoutInflater#from()

    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;
    }

第一种获取LayoutInflater对象的方式，不过就是对第二种方式的一个简单封装。实际上还是一回事。Context的实现类是ContextImpl，跟进。

源码位置：frameworks/base/core/java/android/app/ContextImpl.java

ContextImpl#getSystemService()

    @Override
    public Object getSystemService(String name) {
        return SystemServiceRegistry.getSystemService(this, name);
    }

跟进。

源码位置：frameworks/base/core/java/android/app/SystemServiceRegistry.java

SystemServiceRegistry#getSystemService()

    public static Object getSystemService(ContextImpl ctx, String name) {
        ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
        return fetcher != null ? fetcher.getService(ctx) : null;
    }

直接从全局变量SYSTEM_SERVICE_FETCHERS中依据名字就get到了fetcher，之后依据fetcher直接get到了LayoutInflater对象。大写的懵B~原来啊，在SystemServiceRegistry中有个静态代码块，先看下这部分。

    static {
        ...
        registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,
                new CachedServiceFetcher<LayoutInflater>() {
            @Override
            public LayoutInflater createService(ContextImpl ctx) {
                return new PhoneLayoutInflater(ctx.getOuterContext());
        }});
        ...
    }

    private static <T> void registerService(String serviceName, Class<T> serviceClass,
            ServiceFetcher<T> serviceFetcher) {
        SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);
        SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);
    }

    static abstract class CachedServiceFetcher<T> implements ServiceFetcher<T> {
        private final int mCacheIndex;

        public CachedServiceFetcher() {
            mCacheIndex = sServiceCacheSize++;
        }

        @Override
        @SuppressWarnings("unchecked")
        public final T getService(ContextImpl ctx) {
            final Object[] cache = ctx.mServiceCache;
            synchronized (cache) {
                // Fetch or create the service.
                Object service = cache[mCacheIndex];
                if (service == null) {
                    service = createService(ctx);
                    cache[mCacheIndex] = service;
                }
                return (T)service;
            }
        }

        public abstract T createService(ContextImpl ctx);
    }

这里连续贴了两个方法和一个抽象内部类CachedServiceFetcher。由于在抽象方法CachedServiceFetcher#createService()的具体实现中返回的是PhoneLayoutInflater，所以后文中使用的一直是PhoneLayoutInflater的对象。获取LayoutInflater对象(其实是其子类PhoneLayoutInflater对象)之后，调用LayoutInflater#inflate()。跟进。

源码位置：frameworks/base/core/java/android/view/LayoutInflater.java

LayoutInflater#inflate()

    public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
        return inflate(resource, root, root != null);
    }

这里以setContentView中的mLayoutInflater.inflate(layoutResID, mContentParent)为例，顺带也会讲解adapter中mInflater.inflate(layoutResID,null)这种情况。也就是root参数为null和不为null两种情况。root==null，则第三个参数为false.root!=null，则第三个参数为true。跟进。

LayoutInflater#inflate()

    public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
        final Resources res = getContext().getResources();
        final XmlResourceParser parser = res.getLayout(resource);
        try {
            return inflate(parser, root, attachToRoot);
        } finally {
            parser.close();
        }
    }

跟进。

LayoutInflater#inflate()

    public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            ...
            View result = root;
            try {
                ...
                // 获取根节点的字符串，例如LinearLayout
                final String name = parser.getName();
                // 根节点merge开头
                if (TAG_MERGE.equals(name)) {
                    ...
                } else {
                    // 创建根视图View
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);
                    ViewGroup.LayoutParams params = null;
                    if (root != null) {
                        // 获取LayoutParams
                        params = root.generateLayoutParams(attrs);
                        if (!attachToRoot) {
                            // 应用LayoutParams到根节点View
                            temp.setLayoutParams(params);
                        }
                    }
                    // 遍历解析子View，并添加到根节点temp中
                    rInflateChildren(parser, temp, attrs, true);
                    // root不为空，直接将根节点View添加到root中
                    if (root != null && attachToRoot) {
                        root.addView(temp, params);
                    }
                    // root等于null,直接返回根节点temp
                    if (root == null || !attachToRoot) {
                        result = temp;
                    }
                }
            }catch (Exception e) {
                ...
            }    
            return result;
        }
    }                       

上面每一步都有注释，下面重点看下生成根节点View的createViewFromTag()和遍历生成子View的rInflateChildren()方法。

LayoutInflater#createViewFromTag()

    private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
        return createViewFromTag(parent, name, context, attrs, false);
    }

    View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
            boolean ignoreThemeAttr) {
        ...
        if (-1 == name.indexOf(‘.‘)) {
            view = onCreateView(parent, name, attrs);
        } else {
            view = createView(name, null, attrs);
        }
        ...
        return view;
    }

跟进。

LayoutInflater#createView()

    public final View createView(String name, String prefix, AttributeSet attrs)
            throws ClassNotFoundException, InflateException {
        Constructor<? extends View> constructor = sConstructorMap.get(name);
        Class<? extends View> clazz = null;
        try {
            if (constructor == null) {
                clazz = mContext.getClassLoader().loadClass(
                        prefix != null ? (prefix + name) : name).asSubclass(View.class);
                ...
                constructor = clazz.getConstructor(mConstructorSignature);
                constructor.setAccessible(true);
                sConstructorMap.put(name, constructor);
            } else {
                ...
            }
            final View view = constructor.newInstance(args);
            return view;
        } catch (Exception e) {
            ...
        }   
    }    

sConstructorMap是个HashMap<String, Constructor<? extends View>>对象。首先依据根节点的名字，例如LinearLayout去查找缓存的构造器，如果是第一次执行，肯定返回null。如果返回为null，则通过反射出构造方法，并强制设置可访问，之后存进sConstructorMap中。如果缓存中有构造器，那么直接取出。最后调用newInstance反射出根节点View实例。得到根节点View实例之后，接着设置属性，最后调用rInflateChildren()遍历创建子View。跟进。

LayoutInflater#rInflateChildren()

    final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
            boolean finishInflate) throws XmlPullParserException, IOException {
        rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
    }

parent参数是根节点View。这里只是简单转发给rInflate()方法处理。跟进。

LayoutInflater#rInflateChildren()

    void rInflate(XmlPullParser parser, View parent, Context context,
            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)) {
                ...
            }
            ...
            } else {
                final View view = createViewFromTag(parent, name, context, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                rInflateChildren(parser, view, attrs, true);
                viewGroup.addView(view, params);
            }
        }
        if (finishInflate) {
            parent.onFinishInflate();
        }
    }

遍历体现在While循环上，name为子节点View的名称，例如：TextView，RelativeLayout等。几个以tag、include等开头的子节点走最上面几个if的逻辑，我们的重点在于寻常View走的else逻辑。可以看到：首先，和创建根节点View调用同一个方法createViewFromTag()创建子View，紧接着设置子View的参数，然后调用递归调用rInflateChildren()方法再去测量子节点的所有View，最后才将子节点添加到父布局，这个父布局可能是根节点，也可能是某个子节点。遍历结束之后，所有子View也添加到布局当中并设置好相应的布局参数。

至此，LayoutInflater.from().inflate()源码解析结束~

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