LayoutInflater源码解析-白红宇

LayoutInflater源码解析

LayoutInflater是开发过程中经常使用到的类，作用是将描述视图XML文件解析成View对象。

获取LayoutInflater对象有下面3种方式：

1.在Activity中调用getLayoutInflater()方法，

2.调用LayoutInflater.from(context)方法，

3.调用context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)方法。

下面我们分别分析这三种方式。

第一种方式，调用Activity的getLayoutInflater()方法。源码如下：

public LayoutInflater getLayoutInflater() {	return getWindow().getLayoutInflater();}复制代码

这里调用了getWindow()方法获取Window对象，再调用了window对象的getLayoutInflater()方法。我们知道Activity中的window是PhoneWindow的类的实例，找到PhoneWindow类的getLayoutInflater()方法如下：

public LayoutInflater getLayoutInflater() {    return mLayoutInflater;}复制代码

PhoneWindow中包含一个mLayoutInflater的实例，这个实例是在PhoneWindow的构造方法中创建的。

public PhoneWindow(Context context) {    super(context);    mLayoutInflater = LayoutInflater.from(context);}复制代码

最终调用了LayoutInflater.from(context)方法。

第二种方式，调用LayoutInflater.from(context)方法。源码如下：

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;}复制代码

在LayoutInflater.from(context)方法中调用了context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)方法。

第三种方式，调用context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)方法。前两种方式最终都是调用的这个方式。而context.getSystemService()方法的最终实现是在ContextImpl类中，我们找到ContextImpl类的getSystemService(name)方法

public Object getSystemService(String name) {    return SystemServiceRegistry.getSystemService(this, name);}复制代码

这里出现了SystemServiceRegistry类，这个类管理了所有的系统服务。通过context.getSystemService()方法最终都会访问SystemServiceRegistry的getSystemService()方法。该方法的实现如下：

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

SYSTEM_SERVICE_FETCHERS是SystemServiceRegistry类的一个静态成员，类型是HashMap<String, ServiceFetcher<?>>。这个方法通过名称获取到一个ServiceFetcher对象，再通过ServiceFetcher.getService(context)获取到系统服务对象。

在SystemServiceFetcher的静态代码块中会初始化一些系统服务，其中就包含LayoutInfalter。

static{    ~~~代码省略    registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,        new CachedServiceFetcher
    
     () {    	@Override    	public LayoutInflater createService(ContextImpl ctx) {    		return new PhoneLayoutInflater(ctx.getOuterContext());        }    });    ~~~代码省略}复制代码

我们可以看到系统注册的LAYOUT_INFLATER_SERVICE服务实际上是PhoneLayoutInflater类的对象。

值得一提的是，Activity的父类为ContextThemeWrapper，这个类的getSystemService()方法有些特殊处理，代码如下：

public Object getSystemService(String name) {    if (LAYOUT_INFLATER_SERVICE.equals(name)) {        if (mInflater == null) {            mInflater = LayoutInflater.from(getBaseContext()).cloneInContext(this);        }        return mInflater;    }    return getBaseContext().getSystemService(name);}复制代码

当在Activity中调用getSystemService(name)获取LayoutInflater对象时，会通过cloneInContext创建一个新的LayoutInflater对象，并且将Context关联为ContextThemeWrapper，这样在解析布局文件的时候就可以解析主题相关的属性了。

分析完创建过程，我们再看LayoutInflater的使用。一般会使用inflate方法解析xml文件，该方法有多个重载，分别如下:

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

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) + ")");    }    final XmlResourceParser parser = res.getLayout(resource);    try {        return inflate(parser, root, attachToRoot);    } finally {        parser.close();    }}复制代码

public View inflate(XmlPullParser parser, @Nullable ViewGroup root) {    return inflate(parser, root, root != null);}复制代码

前三个重载方法最终都会调用下面这个方法

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 {                // Look for the root node.                int type;                while ((type = parser.next()) != XmlPullParser.START_TAG &&                        type != XmlPullParser.END_DOCUMENT) {                    // Empty                }                if (type != XmlPullParser.START_TAG) {                    throw new InflateException(parser.getPositionDescription()                            + ": No start tag found!");                }                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("
     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) {                            // 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 against its context.                    rInflateChildren(parser, temp, attrs, true);                    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.addView(temp, params);                    }                    // Decide whether to return the root that was passed in or the                    // top view found in xml.                    if (root == null || !attachToRoot) {                        result = temp;                    }                }            } catch (XmlPullParserException e) {                final InflateException ie = new InflateException(e.getMessage(), e);                ie.setStackTrace(EMPTY_STACK_TRACE);                throw ie;            } catch (Exception e) {                final InflateException ie = new InflateException(parser.getPositionDescription()                        + ": " + e.getMessage(), e);                ie.setStackTrace(EMPTY_STACK_TRACE);                throw ie;            } finally {                // Don't retain static reference on context.                mConstructorArgs[0] = lastContext;                mConstructorArgs[1] = null;                Trace.traceEnd(Trace.TRACE_TAG_VIEW);            }            return result;        }    }复制代码

该方法结构很清晰，分为一下几个步骤：

找到布局文件根节点

如果根节点是merge标签，就通过rInflate()方法解析merge节点的所有子节点。

如果不是merge标签就通过createViewFromTag()方法根据节点名称创建一个View对象。

如果方法参数root不为空，则会通过root的generateLayoutParams(attr)方法为创建的View对象生成布局参数。如果方法参数attachToRoot为false，表示不会将新创建的View对象加入到root容器中，直接将布局参数设置到View中。

调用rInflateChildren(parser, temp, attrs, true)方法，解析根节点的所有子节点。

如果root参数不为空，并且attachToRoot参数为true，会调用root.addView(temp, params)将新创建的View添加到root容器中。

最后将创建的View对象作为结果返回。

在这个方法内有三个比较重要的方法，分别为rInflate(),createViewFromTag()和rInflateChildren()，而rInflateChildren又会直接调用rInflate()方法。rInflater方法会递归解析xml节点，我们来看rInflate()方法的源码

void rInflate(XmlPullParser parser, View parent, Context context,            AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {    final int depth = parser.getDepth();    int type;    boolean pendingRequestFocus = false;    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)) {            pendingRequestFocus = true;            consumeChildElements(parser);        } else if (TAG_TAG.equals(name)) {            parseViewTag(parser, parent, attrs);        } else if (TAG_INCLUDE.equals(name)) {            if (parser.getDepth() == 0) {                throw new InflateException("
     cannot be the root element");            }            parseInclude(parser, context, parent, attrs);        } else if (TAG_MERGE.equals(name)) {            throw new InflateException("
     must be the root element");        } 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 (pendingRequestFocus) {        parent.restoreDefaultFocus();    }    if (finishInflate) {        parent.onFinishInflate();    }}复制代码

rInflate()方法中对一些特殊标签进行处理，包括requestFocus标签，tag标签和include标签，这部分不做深入解析。我们看最后else代码段，首先会调用createViewFromTag(parent, name, context, attrs)创建View对象，再通过viewGroup.generateLayoutParams(attrs)生成view的布局参数，然后通过rInflateChildren(parser, view, attrs, true)递归解析该节点的子节点，最后调用viewGroup.addView(view, params)将创建的view对象添加到父容器中。通过递归的方式，步骤十分清晰。

我们又见到了createViewFromTag()方法，来看下这个方法的源码：

View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,            boolean ignoreThemeAttr) {    if (name.equals("view")) {        name = attrs.getAttributeValue(null, "class");    }    // Apply a theme wrapper, if allowed and one is specified.    if (!ignoreThemeAttr) {        final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);        final int themeResId = ta.getResourceId(0, 0);        if (themeResId != 0) {            context = new ContextThemeWrapper(context, themeResId);        }        ta.recycle();    }    if (name.equals(TAG_1995)) {        // Let's party like it's 1995!        return new BlinkLayout(context, attrs);    }    try {        View view;        if (mFactory2 != null) {            view = mFactory2.onCreateView(parent, name, context, attrs);        } else if (mFactory != null) {            view = mFactory.onCreateView(name, context, attrs);        } else {            view = null;        }        if (view == null && mPrivateFactory != null) {            view = mPrivateFactory.onCreateView(parent, name, context, attrs);        }        if (view == null) {            final Object lastContext = mConstructorArgs[0];            mConstructorArgs[0] = context;            try {                if (-1 == name.indexOf('.')) {                    view = onCreateView(parent, name, attrs);                } else {                    view = createView(name, null, attrs);                }            } finally {                mConstructorArgs[0] = lastContext;            }        }        return view;    } catch (Exception e) {        //省略异常处理代码    }}复制代码

首先判断节点是否是view(注意这个view是是小写的，和View不同)节点，如果是view节点，则解析“class”属性为类名，这种方式一般用于自定义的View类是内部类的情况。

接下来有个彩蛋，如果标签名为blink，就会返回一个BlinkLayout，这个布局是LayoutInflater的内部类，作用是每500ms会隐藏显示布局内容，达到闪烁的效果。

下面会判断mFactory2和mFactory成员变量是否为空，如果为LayoutInflater设置了Facotry2或Factory，就会调用工厂的方法来创建View，否则会通过标签名中是否包含“.”（也就是标签名是否是全类名）来调用onCreateView()或createView()方法。在onCreateView()方法如下:

protected View onCreateView(String name, AttributeSet attrs)            throws ClassNotFoundException {    return createView(name, "android.view.", attrs);}复制代码

该方法设置前缀为"android.view."后，调用了createView()方法。在LayoutInflater的子类PhoneLayoutInflater中又重写了该方法：

protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {    for (String prefix : sClassPrefixList) {        try {            View view = createView(name, prefix, attrs);            if (view != null) {                return view;            }        } catch (ClassNotFoundException e) {            // In this case we want to let the base class take a crack            // at it.        }    }    return super.onCreateView(name, attrs);}复制代码

sClassPrefixList为：

private static final String[] sClassPrefixList = {     "android.widget.",     "android.webkit.",     "android.app."};复制代码

会尝试为标签添加这些前缀来创建View对象。也就是说如果在XML布局文件中标签没有写全类名，使用的是简写形式，例如<TextView.../>,LayoutInflater会自动补充前缀组成完整类名形式，再去解析View对象。

解析完onCreateView()方法后，再看createView()方法：

public final View createView(String name, String prefix, AttributeSet attrs)            throws ClassNotFoundException, InflateException {        Constructor
     constructor = sConstructorMap.get(name);        if (constructor != null && !verifyClassLoader(constructor)) {            constructor = null;            sConstructorMap.remove(name);        }        Class
     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(                        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);                constructor.setAccessible(true);                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 lastContext = mConstructorArgs[0];            if (mConstructorArgs[0] == null) {                // Fill in the context if not already within inflation.                mConstructorArgs[0] = mContext;            }            Object[] args = mConstructorArgs;            args[1] = attrs;            final View view = constructor.newInstance(args);            if (view instanceof ViewStub) {                // Use the same context when inflating ViewStub later.                final ViewStub viewStub = (ViewStub) view;                viewStub.setLayoutInflater(cloneInContext((Context) args[0]));            }            mConstructorArgs[0] = lastContext;            return view;        } catch (Exception e) {            //省略异常处理代码        }    }复制代码

该方法首先会中sConstructorMap缓存中通过名称获取Constructor对象，如果没有获取到，则会使用ClassLoader通过类名加载Class对象，并获取到Constructor对象加入缓存中。在加载类的时候回通过Filter来校验是否允许加载该类，如果不允许，会直接抛出错误。这个Filter也是系统提供的一个可以添加约束校验的hook点。

获取到Constructor对象后，通过反射constructor.newInstance(args)创建出View对象，并作为结果返回。如果该View是ViewStub会有些其他处理。

至此，我们已经分析出LayoutInflater最终是根据布局节点解析出视图的类名，再通过反射创建出对应的View对象的。