setContentView(layoutId)执行操作？
我们自定义的Activity通常继承自系统的的Activity。自然setContentView()方法去系统的Activity查找：
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
} 我们可以看到系统Activity的setContentView调用了getWindow()的setContentView方法，我们先看getWindow()，实际上getWindow()返回了一个Window：
/**
* Abstract base class for a top-level window look and behavior policy. An
* instance of this class should be used as the top-level view added to the
* window manager. It provides standard UI policies such as a background, title
* area, default key processing, etc.
* 抽象基类，用于顶级窗口外观和行为策略。一个
* 这个类的实例应该被用作添加到的顶层视图。
* 窗口管理器。它提供了标准的UI策略，例如背景、标题。
* 区域、默认密钥处理等。
* The only existing implementation of this abstract class is
* android.view.PhoneWindow, which you should instantiate when needing a
* Window.
* 这个抽象类的惟一现有实现是。
* android.view。PhoneWindow，在需要时应该实例化。
* 窗口
*/
public abstract class Window {}
上面的注释告诉我们Window是一个抽象类，唯一的实现类是PhoneWindow，那我们就去看PhoneWindow的setContentView()方法：
@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) {//mContentParent为空时
installDecor();//该方法实例化DecorView和mContnetParent
} 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);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
当mContentParent为空时就会调用installDecor()方法去实例化。有人问mContentParent是什么？
// This is the view in which the window contents are placed. It is either
// mDecor itself, or a child of mDecor where the contents go.
ViewGroup mContentParent;mContentParent是一个ViewGroup容器，根据注释这是放置窗口视图内容的容器。
我们再来看installDecor()方法：
private void installDecor() {
mForceDecorInstall = false;
if (mDecor == null) {
mDecor = generateDecor(-1);//获取DecorView实例
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
}
} else {
mDecor.setWindow(this);
}
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);//获取mContentParent实例
// Set up decor part of UI to ignore fitsSystemWindows if appropriate.
mDecor.makeOptionalFitsSystemWindows();
//以下省略n行代码......
}
}以上方法创建了DecorView、ContentParent
我们看看generateDecor()方法：
protected DecorView generateDecor(int featureId) {
//省略......无关代码
return new DecorView(context, featureId, this, getAttributes());
}可以看到该方法创建了一个新的DecorView实例 DecorView就是一个FrameLayout
接下来看看generateLayout()方法：
protected ViewGroup generateLayout(DecorView decor) {
// Apply data from current theme.
//根据不同样式设置设置不同属性 省略......
//这里就是为什么代码中我们设置去标题栏要在setContentView之前
// Inflate the window decor.
int layoutResource;
int features = getLocalFeatures();
// System.out.println("Features: 0x" + Integer.toHexString(features));
//根据不同特征加载不同的布局
if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
layoutResource = R.layout.screen_swipe_dismiss;
} 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;//加载布局
} 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!");
//特征判断
} else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0
&& (features & (1 << FEATURE_ACTION_BAR)) == 0) {
// Special case for a window with only a progress bar (and title).
// XXX Need to have a no-title version of embedded windows.
layoutResource = R.layout.screen_progress;
// System.out.println("Progress!");
} else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {
// Special case for a window with a custom title.
// If the window is floating, we need a dialog layout
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogCustomTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = R.layout.screen_custom_title;
}
// XXX Remove this once action bar supports these features.
removeFeature(FEATURE_ACTION_BAR);
} else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {
// If no other features and not embedded, only need a title.
// If the window is floating, we need a dialog layout
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {
layoutResource = a.getResourceId(
R.styleable.Window_windowActionBarFullscreenDecorLayout,
R.layout.screen_action_bar);
} else {
layoutResource = R.layout.screen_title;
}
// System.out.println("Title!");
} else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
layoutResource = R.layout.screen_simple_overlay_action_mode;
} else {
// Embedded, so no decoration is needed.
layoutResource = R.layout.screen_simple;
// System.out.println("Simple!");
}
//加载资源布局到DecorView中
mDecor.startChanging();
mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
//这里就是我们创建的contentParent
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
if (contentParent == null) {
throw new RuntimeException("Window couldn't find content container view");
}
//省略......
mDecor.finishChanging();
return contentParent;
}我们可以拿其中一个布局文件看看：screen_simple
FrameLayout就是需要我们把布局文件解析以后通过addView添加进去的。
接下来就是最上面我们下一步操作：
mLayoutInflater.inflate(layoutResID, mContentParent);来看一下inflate()方法:可以看到我们把传进去的layoutResID和ViewGroup容器放入这个方法
public View inflate(@LayoutRes int resource) {
return inflate(resource, root, root != null);
}又调到：
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) + ")");
}
//通过资源文件拿到XmlResourceParser对象即xml文件
final XmlResourceParser parser = res.getLayout(resource);
try {
//最终调用方法解析xml文件
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}最终调用方法：
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;
//循环遍历xml文件，直到开始标签 即view标签
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
//判断是否是起始标签
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("**************************");
}
//merge标签另作处理
if (TAG_MERGE.equals(name)) {
//root是id为content的frameLayout的布局
if (root == null || !attachToRoot) {
throw new InflateException("最重要方法：
final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}
//最终解析XM文件的方法
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
//循环遍历xml文件，直到开始标签 即view标签
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();
//根据不同类型去调用不同方法去解析
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)) {//include去调用parseInclude解析
if (parser.getDepth() == 0) {
throw new InflateException("