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("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 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
// 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("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。