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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(" 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 ? "" : 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(" 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 {

// 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。