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

@Override
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) {////////////////////////////////////////////////// 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。