阅读setContentView()源码的疑问
首先读源码要带着自己的疑问来读源码，setContentView()到底做了什么，为什么调用后就可以显示出我们想要的布局页面？getWindow().requestFeature(Window.FEATURE_NO_TITLE);为什么要在setContentView()之前调用，下面我就就带着这些疑问来一探究竟。
/**
* Set the activity content from a layout resource. The resource will be
* inflated, adding all top-level views to the activity.
*
* @param layoutResID Resource ID to be inflated.
*
* @see #setContentView(android.view.View)
* @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
*/
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
进入到setContentView()，我们发现调用的是getWindow()的setContentView()方法，getWindow()其实就是一个Window，Window是一个抽象类，从注释我们可以知道Window的唯一实现类是PhoneWindow。
/**
* 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.
*
* <p>The only existing implementation of this abstract class is
* android.view.PhoneWindow, which you should instantiate when needing a
* Window.
*/
public abstract class Window
也就是说Activity的setContentView()调用的是PhoneWindow的setContentView(),我们再看下PhoneWindow的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) {
installDecor(); //1
} 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); //2
}
...
}
在这里会对mContentParent进行判空，mContentParent是一个ViewGroup，从名字和代码②mLayoutInflater.inflate(layoutResID, mContentParent); 我们可以先猜测一下Activity要显示的布局是不是就加载到mContentParent中呢？我们先留个疑问，稍后分析inflate()。
// 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;
注释：这是放置窗口内容的视图。 它是mDecor本身，或mDecor的一个孩子。
当mContentParent为null时，就会执行代码① installDecor();我们再进入到 installDecor()。
private void installDecor() {
mForceDecorInstall = false;
if (mDecor == null) {
mDecor = generateDecor(-1);
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); //*********3**********
...
}
}
}
当我们新启动一个Activity是mDecor 和 mContentParent 都是为空的，分别看下generateDecor()和generateLayout(mDecor)都做了什么。
创建generateDecor
protected DecorView generateDecor(int featureId) {
// System process doesn't have application context and in that case we need to directly use
// the context we have. Otherwise we want the application context, so we don't cling to the
// activity.
Context context;
if (mUseDecorContext) {
Context applicationContext = getContext().getApplicationContext();
if (applicationContext == null) {
context = getContext();
} else {
context = new DecorContext(applicationContext, getContext().getResources());
if (mTheme != -1) {
context.setTheme(mTheme);
}
}
} else {
context = getContext();
}
return new DecorView(context, featureId, this, getAttributes());
}
这里就是创建一个DecorView，DecorView又是什么呢？DecorView继承于FrameLayout，在代码3处将创建的这个DecorView作为参数传到了generateLayout()中。
创建generateLayout
protected ViewGroup generateLayout(DecorView decor) {
// Apply data from current theme.
//获得style属性
TypedArray a = getWindowStyle();
...
...
if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) {
requestFeature(FEATURE_NO_TITLE);
} else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) {
// Don't allow an action bar if there is no title.
requestFeature(FEATURE_ACTION_BAR);
}
if (a.getBoolean(R.styleable.Window_windowActionBarOverlay, false)) {
requestFeature(FEATURE_ACTION_BAR_OVERLAY);
}
if (a.getBoolean(R.styleable.Window_windowActionModeOverlay, false)) {
requestFeature(FEATURE_ACTION_MODE_OVERLAY);
}
if (a.getBoolean(R.styleable.Window_windowSwipeToDismiss, false)) {
requestFeature(FEATURE_SWIPE_TO_DISMISS);
}
...
// Inflate the window decor.
int layoutResource;
int features = getLocalFeatures();
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) {
layoutResource = R.layout.screen_title_icons;
} else {
// Embedded, so no decoration is needed.
layoutResource = R.layout.screen_simple;
// System.out.println("Simple!");
}
...
mDecor.startChanging();
mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT)； //*****4*****
if ((features & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) {
ProgressBar progress = getCircularProgressBar(false);
if (progress != null) {
progress.setIndeterminate(true);
}
}
if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
registerSwipeCallbacks();
}
return contentParent;
}
我们可以看到首先获得style属性，判断是否有WindowNoTitle等等一些属性，然后调用一大堆requestFeature()和setFlags(),就是对Window一些状态属性的设置，mContentParentExplicitlySet是一个标记，在调用setContentView()中就会设置为true,这就解开了文章开头的疑问，为什么要在setContentView()之前设置requestFeature(Window.FEATURE_NO_TITLE);
@Override
public boolean requestFeature(int featureId) {
if (mContentParentExplicitlySet) {
throw new AndroidRuntimeException("requestFeature() must be called before adding content");
}
上面其中还有一段代码 int features = getLocalFeatures()，来获得对Window所设置的features ，系统会根据features 来加载不同的XML文件,这个文件就是将要加载到window decor里面的，再次说明requestFeature()要在setContentView()之前设置，我们来看一下加载的其中一个XML文件R.layout.screen_simple，
R.layout.screen_simple
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
android:orientation="vertical">
<ViewStub android:id="@+id/action_mode_bar_stub"
android:inflatedId="@+id/action_mode_bar"
android:layout="@layout/action_mode_bar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:theme="?attr/actionBarTheme" />
<FrameLayout
android:id="@android:id/content"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:foregroundInsidePadding="false"
android:foregroundGravity="fill_horizontal|top"
android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>
这就是DecorView加载的布局，注意FrameLayout的id，通过代码④找到这个id为ID_ANDROID_CONTENT的ViewGroup，可以发现ID_ANDROID_CONTENT就是FrameLayout。
/**
* The ID that the main layout in the XML layout file should have.
*/
public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;
最后又返回contentParent，也就是说mContentParent 就是这个id为content的FrameLayout。我们看下面的视图
Activity加载UI-类图关系和视图结构.png
我们总结一下图片，每一个Activity对应一个Window,PhoneWindow是Window的子类，DecorView是PhoneWindow的内部类，加载Activity显示的视图，现在对窗口的视图有了清晰的了解。
LayoutInflater 把xml添加到Decorview分析
1.include 为什么不能xml资源布局的根节点？
2.merge 为什么作为xml资源布局的根节点？
我们来看下setContentView()中mLayoutInflater.inflate(layoutResID, mContentParent)的部分，LayoutInflater的inflater(layoutResID，mContentParent)方法
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();
}
}
/**
* Inflate a new view hierarchy from the specified XML node. Throws
* {@link InflateException} if there is an error.
* <p>
* <em><strong>Important</strong></em> For performance
* reasons, view inflation relies heavily on pre-processing of XML files
* that is done at build time. Therefore, it is not currently possible to
* use LayoutInflater with an XmlPullParser over a plain XML file at runtime.
*
* @param parser XML dom node containing the description of the view
* hierarchy.
* @param root Optional view to be the parent of the generated hierarchy (if
* <em>attachToRoot</em> is true), or else simply an object that
* provides a set of LayoutParams values for root of the returned
* hierarchy (if <em>attachToRoot</em> is false.)
* @param attachToRoot Whether the inflated hierarchy should be attached to
* the root parameter? If false, root is only used to create the
* correct subclass of LayoutParams for the root view in the XML.
* @return The root View of the inflated hierarchy. If root was supplied and
* attachToRoot is true, this is root; otherwise it is the root of
* the inflated XML file.
*/
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); //获得的是Activity显示布局的属性
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
View result = root; //将DecorView布局中的FrameLayout赋值给result
...
// Look for the root node. //找到根节点
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
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
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
// Create layout params that match root, if supplied //创建与root匹配的布局参数（如果提供）
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);
}
}
// Inflate all children under temp against its context. //填充temp下面的子View
rInflateChildren(parser, temp, attrs, true);
...
}
...
return result;
}
}
上面三段代码依次调用，如果是meger标签时调用 rInflate(parser, root, inflaterContext, attrs, false);如果不是meger标签时会调用View temp = createViewFromTag(root, name, inflaterContext, attrs); 上面有一句注释// Temp is the root view that was found in the xml，意思是：Temp是在xml中找到的根视图，也就是我们Activity显示布局的根视图，然后调用root.generateLayoutParams(attrs);来解析根布局的参数并进行设置， 下面我们看下rInflateChildren(parser, temp, attrs, true)
/**
* Recursive method used to inflate internal (non-root) children. This
* method calls through to {@link #rInflate} using the parent context as
* the inflation context.
* <strong>Note:</strong> Default visibility so the BridgeInflater can
* call it.
*/
final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}
我们发现最后都是调用rInflate()只是最后一个参数finishInflate不同，meger传false，反之。
/**
* Recursive method used to descend down the xml hierarchy and instantiate
* views, instantiate their children, and then call onFinishInflate().
* <p>
* <strong>Note:</strong> Default visibility so the BridgeInflater can
* override it.
*/
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
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)) {
parseRequestFocus(parser, parent);
} 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 {
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflateChildren(parser, view, attrs, true);
viewGroup.addView(view, params);
}
}
if (finishInflate) {
parent.onFinishInflate();
}
}
我们发现是一个while循环，也就是通过遍历去解析的，看下解析过程会判断节点的名称，其中判断到TAG_INCLUDE和TAG_MERGE标签时我们会看到抛出两个异常，这就解释了开始提出的两个问题。最后else还是重复了rInflateChildren(),然后添加到viewGroup里面，这个过程用下面图片说明一下解析XML 的大致过程。
LayoutInflater.png
现在我们用AS开发默认都是继承AppCompatActivity，那么AppCompatActivity的setContentView()也是这样呢！！答案肯定不是的，我们下篇分析！
（第一篇源码分析文章，希望读者给出建议！）
