LayoutInflater源码解析
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又来一篇源码分析文章。讲源码分析文章有的时候很虚，因为我只能讲个我看懂的大概流程，所以细节部分可以没有深入研究，看完之后也只能了解个大概。但个人觉得看源码更重要的是思路而不是细节。今天来分析下LayoutInflater的源码。
之所以分析它是因为我们来常经常使用到它，但往往只知道它是加载view的而不知它具体的实现方法。不多说直接分析。
源码分析
平常我们使用LayoutInflater最常见的方式如：
LayoutInflater inflate = LayoutInflater.from(Context context); LayoutInflater inflate = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
这两种方式实质上是一样的。看来LayoutInflater.from的源码：
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;
好吧，它就是一个披着context .getSystemService皮的狼。其实就是简单的封装了下。平常我们在获取系统的一些service如获取传感器之类的都会用到getSystemService，那么在这里context .getSystemService又是具体怎么实现的。
getSystemService
了解这个问题之前我们要清楚，Context是什么，平时我们经常说Context是上下文环境。其实Application，Activity，Service都会存在一个Context。它的具体实现类是ContextImpl。那么直接看ContextImpl：
@Override
public Object getSystemService(String name)
ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
return fetcher == null ? null : fetcher.getService(this);
SYSTEM_SERVICE_MAP是什么。我们继续看ContextImpl的代码：
class ContextImpl extends Context
private final static String TAG = "ContextImpl";
private final static boolean DEBUG = false;
/**
* Map from package name, to preference name, to cached preferences.
*/
private static ArrayMap<String, ArrayMap<String, SharedPreferencesImpl>> sSharedPrefs;
/**
* 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);
if (service != null)
return service;
service = createService(ctx);
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");
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();
private static final HashMap<String, ServiceFetcher> SYSTEM_SERVICE_MAP =
new HashMap<String, ServiceFetcher>();
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);
static
…………
registerService(LAYOUT_INFLATER_SERVICE, new ServiceFetcher()
public Object createService(ContextImpl ctx)
return PolicyManager.makeNewLayoutInflater(ctx.getOuterContext());
);
…………
在虚拟机第一次加载该类时就会注册各种ServiceFatcher,包括LayoutInflater，这个是在一系列的registerService中实现的。然后将它们存储在SYSTEM_SERVICE_MAP这个HashMap中，以后要用只需从中获取，注册是在静态代码块中进行的，也就是说它只会执行一次，保证实例的唯一性。这可以说是用容器来实现的单例模式。最后通过getSystemService获取相应的Service。
我们重点关注到PolicyManager.makeNewLayoutInflate这句代码，它最终调用的是Policy中的makeNewLayoutInflate方法。
public class Policy implements IPolicy
private static final String TAG = "PhonePolicy";
static
// For performance reasons, preload some policy specific classes when
// the policy gets loaded.
for (String s : preload_classes)
try
Class.forName(s);
catch (ClassNotFoundException ex)
Log.e(TAG, "Could not preload class for phone policy: " + s);
public Window makeNewWindow(Context context)
return new PhoneWindow(context);
public LayoutInflater makeNewLayoutInflater(Context context)
return new PhoneLayoutInflater(context);
在这里看到Window具体实现类是PhoneWindow，LayoutInflater的具体实现是PhoneLayoutInflater。
PhoneLayoutInflater
看下PhoneLayoutInflater的代码。
public class PhoneLayoutInflater extends LayoutInflater
private static final String[] sClassPrefixList =
"android.widget.",
"android.webkit.",
"android.app."
;
/**
* Instead of instantiating directly, you should retrieve an instance
* through @link Context#getSystemService
*
* @param context The Context in which in which to find resources and other
* application-specific things.
*
* @see Context#getSystemService
*/
public PhoneLayoutInflater(Context context)
super(context);
protected PhoneLayoutInflater(LayoutInflater original, Context newContext)
super(original, newContext);
/** Override onCreateView to instantiate names that correspond to the
widgets known to the Widget factory. If we don't find a match,
call through to our super class.
*/
@Override 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);
它重定了onCreateView 方法，其实就是为系统View加上相应的前缀。如TextView读出的完整路径会是android.widget.TextView。再调用createView方法，通过类的完整路径来构造View对象。具体的实现过程我们可以来看看setContentView。
setContentView
首先我们来看一幅android的结构图：
Acitivity的一个界面中最外层是PhoneWindow，它里面是一个DecorView。它是界面中所有view的根view。它包括两部分，ActionBar和ContentView，而ContentView就是我们平常接触最多的，setContentView设置的就是它的内容。而我们在xml中定义的所有view都是显示在它上面的。
先来看下PhoneWindow中的setContentView方法。
public void setContentView(int layoutResID)
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null)
installDecor();
else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS))
mContentParent.removeAllViews();
if (hasFeature(FEATURE_CONTENT_TRANSITIONS))
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
else
mLayoutInflater.inflate(layoutResID, mContentParent);
final Callback cb = getCallback();
if (cb != null && !isDestroyed())
cb.onContentChanged();
首先调用的是installDecor方法。从名字中我们就可以猜到，它应该就是加载DecorView。看下代码：
private void installDecor()
if (mDecor == null)
mDecor = generateDecor();
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0)
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
if (mContentParent == null)
mContentParent = generateLayout(mDecor);
// Set up decor part of UI to ignore fitsSystemWindows if appropriate.
mDecor.makeOptionalFitsSystemWindows();
final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(
R.id.decor_content_parent);
if (decorContentParent != null)
mDecorContentParent = decorContentParent;
mDecorContentParent.setWindowCallback(getCallback());
if (mDecorContentParent.getTitle() == null)
mDecorContentParent.setWindowTitle(mTitle);
final int localFeatures = getLocalFeatures();
for (int i = 0; i < FEATURE_MAX; i++)
if ((localFeatures & (1 << i)) != 0)
mDecorContentParent.initFeature(i);
mDecorContentParent.setUiOptions(mUiOptions);
if ((mResourcesSetFlags & FLAG_RESOURCE_SET_ICON) != 0 ||
(mIconRes != 0 && !mDecorContentParent.hasIcon()))
mDecorContentParent.setIcon(mIconRes);
else if ((mResourcesSetFlags & FLAG_RESOURCE_SET_ICON) == 0 &&
mIconRes == 0 && !mDecorContentParent.hasIcon())
mDecorContentParent.setIcon(
getContext().getPackageManager().getDefaultActivityIcon());
mResourcesSetFlags |= FLAG_RESOURCE_SET_ICON_FALLBACK;
if ((mResourcesSetFlags & FLAG_RESOURCE_SET_LOGO) != 0 ||
(mLogoRes != 0 && !mDecorContentParent.hasLogo()))
mDecorContentParent.setLogo(mLogoRes);
// Invalidate if the panel menu hasn't been created before this.
// Panel menu invalidation is deferred avoiding application onCreateOptionsMenu
// being called in the middle of onCreate or similar.
// A pending invalidation will typically be resolved before the posted message
// would run normally in order to satisfy instance state restoration.
PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);
if (!isDestroyed() && (st == null || st.menu == null))
invalidatePanelMenu(FEATURE_ACTION_BAR);
else
mTitleView = (TextView)findViewById(R.id.title);
if (mTitleView != null)
mTitleView.setLayoutDirection(mDecor.getLayoutDirection());
if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0)
View titleContainer = findViewById(
R.id.title_container);
if (titleContainer != null)
titleContainer.setVisibility(View.GONE);
else
mTitleView.setVisibility(View.GONE);
if (mContentParent instanceof FrameLayout)
((FrameLayout)mContentParent).setForeground(null);
else
mTitleView.setText(mTitle);
if (mDecor.getBackground() == null && mBackgroundFallbackResource != 0)
mDecor.setBackgroundFallback(mBackgroundFallbackResource);
// Only inflate or create a new TransitionManager if the caller hasn't
// already set a custom one.
if (hasFeature(FEATURE_ACTIVITY_TRANSITIONS))
if (mTransitionManager == null)
final int transitionRes = getWindowStyle().getResourceId(
R.styleable.Window_windowContentTransitionManager,
0);
if (transitionRes != 0)
final TransitionInflater inflater = TransitionInflater.from(getContext());
mTransitionManager = inflater.inflateTransitionManager(transitionRes,
mContentParent);
else
mTransitionManager = new TransitionManager();
mEnterTransition = getTransition(mEnterTransition, null,
R.styleable.Window_windowEnterTransition);
mReturnTransition = getTransition(mReturnTransition, USE_DEFAULT_TRANSITION,
R.styleable.Window_windowReturnTransition);
mExitTransition = getTransition(mExitTransition, null,
R.styleable.Window_windowExitTransition);
mReenterTransition = getTransition(mReenterTransition, USE_DEFAULT_TRANSITION,
R.styleable.Window_windowReenterTransition);
mSharedElementEnterTransition = getTransition(mSharedElementEnterTransition, null,
R.styleable.Window_windowSharedElementEnterTransition);
mSharedElementReturnTransition = getTransition(mSharedElementReturnTransition,
USE_DEFAULT_TRANSITION,
R.styleable.Window_windowSharedElementReturnTransition);
mSharedElementExitTransition = getTransition(mSharedElementExitTransition, null,
R.styleable.Window_windowSharedElementExitTransition);
mSharedElementReenterTransition = getTransition(mSharedElementReenterTransition,
USE_DEFAULT_TRANSITION,
R.styleable.Window_windowSharedElementReenterTransition);
if (mAllowEnterTransitionOverlap == null)
mAllowEnterTransitionOverlap = getWindowStyle().getBoolean(
R.styleable.Window_windowAllowEnterTransitionOverlap, true);
if (mAllowReturnTransitionOverlap == null)
mAllowReturnTransitionOverlap = getWindowStyle().getBoolean(
R.styleable.Window_windowAllowReturnTransitionOverlap, true);
if (mBackgroundFadeDurationMillis < 0)
mBackgroundFadeDurationMillis = getWindowStyle().getInteger(
R.styleable.Window_windowTransitionBackgroundFadeDuration,
DEFAULT_BACKGROUND_FADE_DURATION_MS);
if (mSharedElementsUseOverlay == null)
mSharedElementsUseOverlay = getWindowStyle().getBoolean(
R.styleable.Window_windowSharedElementsUseOverlay, true);
可以看到这里不仅初始化mContentParent，而且在之前先调用generateDecor();初始化了一个mDecor，mDecor是DecorView对象，为FrameLayout的子类。并通过findViewById进行获取控件。
generateLayout(mDecor)猜测应该是用来获取到了我们的ContentView的；具体我们来看下源码。
generateLayout
protected ViewGroup generateLayout(DecorView decor)
// Apply data from current theme.
TypedArray a = getWindowStyle();
//...... //依据主题style设置一堆值进行设置
int layoutResource;
int features = getLocalFeatures();
View in = mLayoutInflater.inflate(layoutResource, null);
decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
mContentRoot = (ViewGroup) in;
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
if (contentParent == null)
throw new RuntimeException("Window couldn't find content container view");
//...... //继续一堆属性设置，完事返回contentParent
return contentParent;
之前我们设置界面主题（如Notitle）主要有两种方式，一种是在xml中设置：
android:theme="@android:style/Theme.Black.NoTitleBar" 一种是调用requestFeature。
requestWindowFeature(Window.FEATURE_NO_TITLE);第一种方法对应的就是getWindowStyle方法。而第二种就是在getLocalFeatures完成的。这就是为什么requestWindowFeature要写在setContentView方法之前的原因。
接着通过mDecor.findViewById传入R.id.content，返回mDecor（布局）中的id为content的View，就是我们前面说的ContentView。可以看到我们的mDecor是一个FrameLayout，然后会根据theme去选择系统中的布局文件，将布局文件通过inflate转化为view，加入到mDecor中；这些布局文件中都包含一个id为content的FrameLayout，将其引用返回给mContentParent。
等我们的mContentParent有值了以后，再次回到setContentView中，它会调用以下方法。
mLayoutInflater.inflate(layoutResID, mContentParent);接下来重点看inflate是如何加载布局的。
inflate
public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)
synchronized (mConstructorArgs)
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context)mConstructorArgs[0];
mConstructorArgs[0] = mContext;
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("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
rInflate(parser, root, attrs, false, false);
else
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, attrs, false);
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
rInflate(parser, temp, attrs, true, 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)
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;
代码很长，主要分为以下几步：
1.解析XML的根标签。
2.如果是merge，调用rInflate进行解析。它会把merge所有子view添加到根标签中。
3.如果是普通标签，调用createViewFromTag进行解析。
4.调用rInflate解析temp根元素下的子view。并添加到temp中。
最后返回root。
看下createViewFromTag是如何加载标签的。
View createViewFromTag(View parent, String name, AttributeSet attrs, boolean inheritContext)
if (name.equals("view"))
name = attrs.getAttributeValue(null, "class");
Context viewContext;
if (parent != null && inheritContext)
viewContext = parent.getContext();
else
viewContext = mContext;
// Apply a theme wrapper, if requested.
final TypedArray ta = viewContext.obtainStyledAttributes(attrs, ATTRS_THEME);
final int themeResId = ta.getResourceId(0, 0);
if (themeResId != 0)
viewContext = new ContextThemeWrapper(viewContext, themeResId);
ta.recycle();
if (name.equals(TAG_1995))
// Let's party like it's 1995!
return new BlinkLayout(viewContext, attrs);
if (DEBUG) System.out.println("******** Creating view: " + name);
try
View view;
if (mFactory2 != null)
view = mFactory2.onCreateView(parent, name, viewContext, attrs);
else if (mFactory != null)
view = mFactory.onCreateView(name, viewContext, attrs);
else
view = null;
if (view == null && mPrivateFactory != null)
view = mPrivateFactory.onCreateView(parent, name, viewContext, attrs);
if (view == null)
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = viewContext;
try
if (-1 == name.indexOf('.'))
view = onCreateView(parent, name, attrs);
else
view = createView(name, null, attrs);
finally
mConstructorArgs[0] = lastContext;
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;
上面这个方法中，如果传过来的名字没有“.”，会认为是一个内置view。会调用onCreateView来解析这个View。PhoneLayoutInflater的onCreateView就是为内置的View加上前缀，如android.widget等。然后再调用createView()来进行view的构造。
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
Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);
//constructor是从缓存中取出的构造函数
if (constructor == null)
// Class not found in the cache, see if it's real, and try to add it
//如果prefix不为空就构造view路径并加载
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);
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
//通过反射构造view
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;
args[1] = attrs;
constructor.setAccessible(true);
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]));
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);
又是一大堆代码，但它其实就是使用view的完整路径将类加载到虚拟机中，通过构造函数来创建view对象，这个过程是通过反射。最后返回view。这个就解析了单个view。那如果是一棵树，则交给rInflate来处理。看下代码：
void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,
boolean finishInflate, boolean inheritContext) 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_TAG.equals(name))
parseViewTag(parser, parent, attrs);
else if (TAG_INCLUDE.equals(name))
if (parser.getDepth() == 0)
throw new InflateException("<include /> cannot be the root element");
parseInclude(parser, parent, attrs, inheritContext);
else if (TAG_MERGE.equals(name))
throw new InflateException("<merge /> must be the root element");
else
final View view = createViewFromTag(parent, name, attrs, inheritContext);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflate(parser, view, attrs, true, true);
viewGroup.addView(view, params);
if (finishInflate) parent.onFinishInflate();
/**
* Parses a <code><request-focus></code> element and requests focus on
* the containing View.
*/
private void parseRequestFocus(XmlPullParser parser, View view)
throws XmlPullParserException, IOException
int type;
view.requestFocus();
final int currentDepth = parser.getDepth();
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > currentDepth) && type != XmlPullParser.END_DOCUMENT)
// Empty
普希金通过深度优先遍历来构造视图树。每解析一个view就会递归调用rInflate。view的结构一层包一层，其实就是标准的组合设计模式实现的。整个视图构建完后就会在onResume之后，内容就会出现在界面中。
到此，LayoutInflater加载View的过程就分析完了。还是比较简单易懂的。
布局加载优化
通过上面的分析，我们可以提出几点常用的布局优化手段：
1.尽量使用相对布局，减少不必要层级结构。
2.使用merge属性。使用它可以有效的将某些符合条件的多余的层级优化掉，使用merge标签还是有一些限制的，具体是：merge只能用在布局XML文件的根元素；使用merge来inflate一个布局时，必须指定一个ViewGroup作为其父元素，并且要设置inflate的attachToRoot参数为true。；不能在ViewStub中使用merge标签。
3.使用ViewStub。一个轻量级的页面，我们通常使用它来做预加载处理，来改善页面加载速度和提高流畅性，ViewStub本身不会占用层级，它最终会被它指定的层级取代。ViewStub也是有一些缺点，譬如：ViewStub只能Inflate一次，之后ViewStub对象会被置为空。VIewStub中不能嵌套merge标签。
4.使用include。这个标签是为了布局重用。
当然还有很多优化手段和工具可以使用，在此不一一罗列。关于布局还是有很大的学问要去深入研究的。
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