上篇文章追溯了Android源码中Activity的启动流程，那么Activity启动之后，是如何加载布局的呢？这篇文章我们继续来追溯这一块的Android源码。
Activity->setContentView
public void setContentView(@LayoutRes int layoutResID) { getWindow().setContentView(layoutResID); initWindowDecorActionBar();}这里调用了Window的setContentView方法，而Window是一个抽象类，PhoneWindow是其唯一派生子类，所以这里调用的是PhoneWindow中的setContentView方法。
PhoneWindow->setContentView
@Overridepublic 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) {////////////////////////////////////////////////// TAG1 //创建DecorView，并添加到mContentParent上 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 { //将布局加载到mContentParent上 mLayoutInflater.inflate(layoutResID, mContentParent);/////////////////////// TAG2 } mContentParent.requestApplyInsets(); final Callback cb = getCallback(); if (cb != null && !isDestroyed()) { //回调通知界面加载完毕 cb.onContentChanged(); } mContentParentExplicitlySet = true;}首先看标识TAG1处，这里判断mContentParent是否为空，后面我们可以知道这个mContentParent表示的是DecorView中展示内容的布局（除去标题栏）。到这一步，我们还没有创建过DecorView，也就更不可能生成mContentParent了，所以这里的判断是为true的。也就是会执行installDecor()方法。
Activity->installDecor
private void installDecor() { mForceDecorInstall = false; ///////////////////////////////////////////////////////////////////////// TAG3 if (mDecor == null) { //如果mDecor为空，创建一个DecorView赋给mDecor mDecor = generateDecor(-1); mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS); mDecor.setIsRootNamespace(true); if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) { mDecor.postOnAnimation(mInvalidatePanelMenuRunnable); } } else { mDecor.setWindow(this); } ///////////////////////////////////////////////////////////////////////// TAG4 if (mContentParent == null) { //根据主题以及设置的FEATURE为mDecor添加默认布局 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); //添加其他资源 ...... //设置转场动画 if (hasFeature(FEATURE_ACTIVITY_TRANSITIONS)) { ...... } }}TAG3处没什么好说的，就是创建一个DecorView赋给mDecor。
这里我们重点看TAG4处新建mContentParent的过程，调用的是generateLayout(mDecor)方法。
PhoneWindow->generateLayout(DecorView decor)
protected ViewGroup generateLayout(DecorView decor) { // Apply data from current theme. //获取当前设置的窗口主题 //这也就是为什么我们要在setContentView之前调用requesetFeature的原因 TypedArray a = getWindowStyle(); ......//根据设置加载默认主题 // Inflate the window decor. int layoutResource; int features = getLocalFeatures(); // System.out.println("Features: 0x" + Integer.toHexString(features)); // 根据用户设置的Feature来设置DecorView的布局资源 if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) { layoutResource = R.layout.screen_swipe_dismiss; setCloseOnSwipeEnabled(true); } else if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) { if (mIsFloating) { TypedValue res = new TypedValue(); getContext().getTheme().resolveAttribute( R.attr.dialogTitleIconsDecorLayout, res, true); layoutResource = res.resourceId; } else { layoutResource = R.layout.screen_title_icons; } // XXX Remove this once action bar supports these features. removeFeature(FEATURE_ACTION_BAR); // System.out.println("Title Icons!"); } else if ......一系列的判断 mDecor.startChanging(); //将符合配置的布局创建并添加到DecorView中 mDecor.onResourcesLoaded(mLayoutInflater, layoutResource); ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT); if (contentParent == null) { throw new RuntimeException("Window couldn't find content container view"); } if ((features & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) { ProgressBar progress = getCircularProgressBar(false); if (progress != null) { progress.setIndeterminate(true); } } //右划退出 if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) { registerSwipeCallbacks(contentParent); } // Remaining setup -- of background and title -- that only applies // to top-level windows. // 设置背景和标题 if (getContainer() == null) { ...... } mDecor.finishChanging(); return contentParent;}方法里先是获取了我们设置的窗口主题，并加载相应的主题。然后又根据我们设置的Feature来选择相应的布局资源，最后调用mDecor.onResourcesLoaded(mLayoutInflater, layoutResource)方法将布局实例化并添加到DecorView上。
DecorView->onResourcesLoaded
void onResourcesLoaded(LayoutInflater inflater, int layoutResource) { if (mBackdropFrameRenderer != null) { loadBackgroundDrawablesIfNeeded(); mBackdropFrameRenderer.onResourcesLoaded( this, mResizingBackgroundDrawable, mCaptionBackgroundDrawable, mUserCaptionBackgroundDrawable, getCurrentColor(mStatusColorViewState), getCurrentColor(mNavigationColorViewState)); } mDecorCaptionView = createDecorCaptionView(inflater); final View root = inflater.inflate(layoutResource, null); if (mDecorCaptionView != null) { if (mDecorCaptionView.getParent() == null) { addView(mDecorCaptionView, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT)); } mDecorCaptionView.addView(root, new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT)); } else { // Put it below the color views. addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT)); } mContentRoot = (ViewGroup) root; initializeElevation();}这里通过inflater.inflate方法将布局创建出来，然后通过addView将布局添加到DecorView上，关于inflate的源码后面会进行分析。
到这里，TAG1处的installDecor方法流程就走完了。它创建了Activity的DecorView，设置了DecorView的主题以及布局。
现在，回到PhoneWindow的setContentView方法，看TAG2处的代码。这里调用了mLayoutInflater.inflate(layoutResID, mContentParent)方法，将我们设置的布局加载到mContentParent上。
LayoutInflater->inflate
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) { return inflate(resource, root, root != null);}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(); }}这里通过res.getLayout(resource)方法对布局资源进行了XML解析，将布局资源解析成XmlResourceParser对象，然后调用同名的inflate方法进行下一步操作。
LayoutInflater->inflate
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; // 循环查找顶节点 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, inflaterContext, attrs, false); } else { // Temp is the root view that was found in the xml // 先生成顶节点布局 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. // 实例化所有子View，以temp顶节点为父布局，递归生成View树 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) { // 将生成的View树添加到root父布局上， // setContentView时这里的root是mContentParent 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) { // 如果root根视图为空，则返回在XML中找到的顶视图 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; }}先说下这个方法的整体流程，首先，循环上面解析好的布局资源XmlResourceParser对象，取出布局的根节点，调用createViewFromTag方法先生成根节点View。然后再调用rInflateChildren方法递归的生成整个布局View树。最后将生成的View树添加到DecorView的root根布局上。
先来看下createViewFromTag方法是如何生成View对象的。
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 (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 { // 生成View对象 if (-1 == name.indexOf('.')) { // Android系统自带View view = onCreateView(parent, name, attrs); } else { // 自定义View view = createView(name, null, attrs); } } finally { mConstructorArgs[0] = lastContext; } } return view; } catch (InflateException e) { throw e; } catch (ClassNotFoundException e) { final InflateException ie = new InflateException(attrs.getPositionDescription() + ": Error inflating class " + name, e); ie.setStackTrace(EMPTY_STACK_TRACE); throw ie; } catch (Exception e) { final InflateException ie = new InflateException(attrs.getPositionDescription() + ": Error inflating class " + name, e); ie.setStackTrace(EMPTY_STACK_TRACE); throw ie; }}这里正常情况下，最后会调用onCreateView或createView方法来生成View对象，这里的onCreateView方法中会为View的节点名添加"android.view."前缀，然后再调用createView方法生成View对象。做这一步操作是因为createView方法是通过反射生成的对象，所以需要完整的包名+类名，而Android系统自带的View都是放在android.view包下。
LayoutInflater->createView
protected View onCreateView(View parent, String name, AttributeSet attrs) throws ClassNotFoundException { return onCreateView(name, attrs);}protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException { return createView(name, "android.view.", attrs);}public final View createView(String name, String prefix, AttributeSet attrs) throws ClassNotFoundException, InflateException { // 获取静态全局变量中是否有缓存相应的构造方法 Constructor<? extends View> constructor = sConstructorMap.get(name); if (constructor != null && !verifyClassLoader(constructor)) { constructor = null; sConstructorMap.remove(name); } Class<? extends View> 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 // 通过反射机制获取View的Class对象 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); } } // 获取构造方法，用于View的实例化 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 // 通过反射机制获取View的Class对象 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; // 实例化View对象 final View view = constructor.newInstance(args); if (view instanceof ViewStub) { // Use the same context when inflating ViewStub later. final ViewStub viewStub = (ViewStub) view; // 如果是ViewStub 就为其设置LayoutInflater 以便后续inflate viewStub.setLayoutInflater(cloneInContext((Context) args[0])); } mConstructorArgs[0] = lastContext; return view; } catch (NoSuchMethodException e) { final InflateException ie = new InflateException(attrs.getPositionDescription() + ": Error inflating class " + (prefix != null ? (prefix + name) : name), e); ie.setStackTrace(EMPTY_STACK_TRACE); throw ie; } catch (ClassCastException e) { // If loaded class is not a View subclass final InflateException ie = new InflateException(attrs.getPositionDescription() + ": Class is not a View " + (prefix != null ? (prefix + name) : name), e); ie.setStackTrace(EMPTY_STACK_TRACE); throw ie; } catch (ClassNotFoundException e) { // If loadClass fails, we should propagate the exception. throw e; } catch (Exception e) { final InflateException ie = new InflateException( attrs.getPositionDescription() + ": Error inflating class " + (clazz == null ? "<unknown>" : clazz.getName()), e); ie.setStackTrace(EMPTY_STACK_TRACE); throw ie; } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); }}createView中先是通过ClassLoader的反射机制，获取到对应的Class对象，然后获取到它的构造方法。最后通过构造方法实例化这个View对象并返回。
这就是createViewFromTag生成View对象的流程。最后我们来看LayoutInflater->inflate中的rInflateChildren方法是如何生成View树的。
LayoutInflater->rInflateChildren
final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException { rInflate(parser, parent, parent.getContext(), attrs, finishInflate);}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("<include /> cannot be the root element"); } parseInclude(parser, context, parent, attrs); } else if (TAG_MERGE.equals(name)) { throw new InflateException("<merge /> must be the root element"); } else { // 生成View对象 final View view = createViewFromTag(parent, name, context, attrs); // 父布局 final ViewGroup viewGroup = (ViewGroup) parent; final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs); // 递归调用rInflateChildren，递归生成View树 rInflateChildren(parser, view, attrs, true); viewGroup.addView(view, params); } } if (pendingRequestFocus) { parent.restoreDefaultFocus(); } if (finishInflate) { parent.onFinishInflate(); }}可以看到，方法里循环取出子节点。特殊节点这里不做分析，我们看最后的else里。
首先先调用上面已经分析过的createViewFromTag方法生成当前的节点View对象，然后这里会把当前这个View作为父节点递归调用rInflateChildren方法，如果当前View有子节点，就会递归的生成子节点并调用addView方法层层的插入对应的父节点。最终就会生成一个完整的View树，整体的页面布局也就显现出来了。