Android XML佈局文件解析過程源碼解析

時間 2019-11-19

原文原文鏈接

咱們知道，在Activity#setContentView()中會調用PhoneWindow#setContentView()。而在PhoneWindow#setContentView()中有這麼一句mLayoutInflater.inflate(layoutResID, mContentParent)。這行代碼的做用是將咱們的activity_main.xml填充到mContentParent中去。詳見：setContentView源碼解析。在寫adapter的時候，也常常寫mInflater.inflate(layoutResID, parent,false)。那麼，這行代碼怎麼就將xml文件轉換成了View或者ViewGroup了呢？java

獲取LayoutInflater對象無非如下兩種方式：android

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

其實這倆是同一種方式，首先看下LayoutInflater#from()緩存

源碼位置：frameworks/base/core/Java/Android/view/LayoutInflater.javaapp

LayoutInflater#from()ide

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;
    }

第一種獲取LayoutInflater對象的方式，不過就是對第二種方式的一個簡單封裝。實際上仍是一回事。Context的實現類是ContextImpl，跟進。佈局

源碼位置：frameworks/base/core/java/android/app/ContextImpl.javafetch

ContextImpl#getSystemService()this

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

跟進。.net

源碼位置：frameworks/base/core/java/android/app/SystemServiceRegistry.javacode

SystemServiceRegistry#getSystemService()

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

直接從全局變量SYSTEM_SERVICE_FETCHERS中依據名字就get到了fetcher，以後依據fetcher直接get到了LayoutInflater對象。大寫的懵B~原來啊，在SystemServiceRegistry中有個靜態代碼塊，先看下這部分。

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);
    }

這裏連續貼了兩個方法和一個抽象內部類CachedServiceFetcher。因爲在抽象方法CachedServiceFetcher#createService()的具體實現中返回的是PhoneLayoutInflater，因此後文中使用的一直是PhoneLayoutInflater的對象。獲取LayoutInflater對象(實際上是其子類PhoneLayoutInflater對象)以後，調用LayoutInflater#inflate()。跟進。

源碼位置：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);
    }

這裏以setContentView中的mLayoutInflater.inflate(layoutResID, mContentParent)爲例，順帶也會講解adapter中mInflater.inflate(layoutResID,null)這種狀況。也就是root參數爲null和不爲null兩種狀況。root==null，則第三個參數爲false.root!=null，則第三個參數爲true。跟進。

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();
        }
    }

跟進。

LayoutInflater#inflate()

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            ...
            View result = root;
            try {
                ...
                // 獲取根節點的字符串，例如LinearLayout
                final String name = parser.getName();
                // 根節點merge開頭
                if (TAG_MERGE.equals(name)) {
                    ...
                } else {
                    // 建立根視圖View
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);
                    ViewGroup.LayoutParams params = null;
                    if (root != null) {
                        // 獲取LayoutParams
                        params = root.generateLayoutParams(attrs);
                        if (!attachToRoot) {
                            // 應用LayoutParams到根節點View
                            temp.setLayoutParams(params);
                        }
                    }
                    // 遍歷解析子View，並添加到根節點temp中
                    rInflateChildren(parser, temp, attrs, true);
                    // root不爲空，直接將根節點View添加到root中
                    if (root != null && attachToRoot) {
                        root.addView(temp, params);
                    }
                    // root等於null,直接返回根節點temp
                    if (root == null || !attachToRoot) {
                        result = temp;
                    }
                }
            }catch (Exception e) {
                ...
            }    
            return result;
        }
    }

上面每一步都有註釋，下面重點看下生成根節點View的createViewFromTag()和遍歷生成子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;
    }

跟進。

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是個HashMap<String, Constructor<? extends View>>對象。首先依據根節點的名字，例如LinearLayout去查找緩存的構造器，若是是第一次執行，確定返回null。若是返回爲null，則經過反射出構造方法，並強制設置可訪問，以後存進sConstructorMap中。若是緩存中有構造器，那麼直接取出。最後調用newInstance反射出根節點View實例。獲得根節點View實例以後，接着設置屬性，最後調用rInflateChildren()遍歷建立子View。跟進。

LayoutInflater#rInflateChildren()

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

parent參數是根節點View。這裏只是簡單轉發給rInflate()方法處理。跟進。

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();
        }
    }

遍歷體如今While循環上，name爲子節點View的名稱，例如：TextView，RelativeLayout等。幾個以tag、include等開頭的子節點走最上面幾個if的邏輯，咱們的重點在於尋常View走的else邏輯。能夠看到：首先，和建立根節點View調用同一個方法createViewFromTag()建立子View，緊接着設置子View的參數，而後調用遞歸調用rInflateChildren()方法再去測量子節點的全部View，最後纔將子節點添加到父佈局，這個父佈局多是根節點，也多是某個子節點。遍歷結束以後，全部子View也添加到佈局當中並設置好相應的佈局參數。

至此，LayoutInflater.from().inflate()源碼解析結束~