Android的布局方式有两种，一种是通过xml布局，一种是通过java代码布局，两种布局方式各有各的好处，当然也可以相互混合使用。很多人都习惯用xml布局，那xml布局是如何转换成view的呢？本文从源码的角度来简单分析下整个过程。
首先，创建一个新的项目，默认生成一个activity，其中xml布局很简单，就一个RelativeLayout套了一个ImageView，代码及效果如下：
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}
界面1
其中关键之处在于调用了父类Activity的setContentView方法：
/**
* 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.
*/
public void setContentView(int layoutResID) {
getWindow().setContentView(layoutResID);
}
getWindow返回的是PhoneWindow实例，那我们直接来看PhoneWindow中的setContentView方法：
@Override
public void setContentView(int layoutResID) {
if (mContentParent == null) {
installDecor();
} else {
mContentParent.removeAllViews();
}
mLayoutInflater.inflate(layoutResID, mContentParent);
final Callback cb = getCallback();
if (cb != null) {
cb.onContentChanged();
}
}
我们知道每个activity实际都对应一个PhoneWindow，拥有一个顶层的DecorView，DecorView继承自FrameLayout，作为根View，其中包含了一个标题区域和内容区域，这里的mContentParent就是其内容区域。关于PhoneWindow和DecorView的具体内容，读者可自行查阅。这段代码的意思很简单，如果DecorView的内容区域为null，就先初始化，否则就先把内容区域的子View全部移除，最后再引入layout布局，所以，关键在于mLayoutInflater.inflate(layoutResID, mContentParent); 代码继续往下看：
public View inflate(int resource, ViewGroup root) {
return inflate(resource, root, root != null);
}
public View inflate(int resource, ViewGroup root, boolean attachToRoot) {
if (DEBUG) System.out.println("INFLATING from resource: " + resource);
XmlResourceParser parser = getContext().getResources().getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
这里首先根据layout布局文件的Id生成xml资源解析器，然后再调用inflate(parser, root, attachToRoot)生成具体的view。XmlResourceParser是继承自XmlPullParser和AttributeSet的接口，这里的parser其实是XmlBlock的内部类Parser的实例。
public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context)mConstructorArgs[0];
mConstructorArgs[0] = mContext;
View result = root;
try {
// Look for the root node.
int type;
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, attrs);
} else {
// Temp is the root view that was found in the xml
View temp = createViewFromTag(name, 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
rInflate(parser, temp, attrs);
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) {
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) {
result = temp;
}
}
} catch (XmlPullParserException e) {
InflateException ex = new InflateException(e.getMessage());
ex.initCause(e);
throw ex;
} catch (IOException e) {
InflateException ex = new InflateException(
parser.getPositionDescription()
+ ": " + e.getMessage());
ex.initCause(e);
throw ex;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
}
return result;
}
}
第21行，获取xml根节点名：
final String name = parser.getName();
第39行根据节点名创建临时View(temp)，这个临时view(temp)也是xml布局的根view：
View temp = createViewFromTag(name, attrs);
第61行，在临时view(temp)的节点下创建所有子View，显然这个方法里是通过遍历xml所有子view节点，调用createViewFromTag方法生成子view并加载到根view中：
rInflate(parser, temp, attrs);
第68到76行，则是判断，如果inflate方法有父view，则把临时view(temp)加载到父view中再返回，如果没有，则直接返回临时view(temp)，我们这里调用inflate方法的时候显然有父view，即mContentParent，也就是最顶层view DecorView的内容区域。这里最关键有两个方法，一个是createViewFromTag，另一个是rInflate，现在来逐一分析：createViewFromTag实际最终调用的是createView方法：
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Constructor constructor = sConstructorMap.get(name);
Class clazz = null;
try {
if (constructor == null) {
// Class not found in the cache, see if it's real, and try to add it
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name);
if (mFilter != null && clazz != null) {
boolean allowed = mFilter.onLoadClass(clazz);
if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
}
constructor = clazz.getConstructor(mConstructorSignature);
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
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name);
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[] args = mConstructorArgs;
args[1] = attrs;
return (View) constructor.newInstance(args);
} catch (NoSuchMethodException e) {
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class "
+ (prefix != null ? (prefix + name) : name));
ie.initCause(e);
throw ie;
} catch (ClassNotFoundException e) {
// If loadClass fails, we should propagate the exception.
throw e;
} catch (Exception e) {
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class "
+ (clazz == null ? "" : clazz.getName()));
ie.initCause(e);
throw ie;
}
}
其实这个方法很简单，就是通过xml节点名，通过反射获取view的实例再返回，其中先去map中查询构造函数是否存在，如果存在则直接根据构造函数创建实例，这样做的好处是不用每次都通过class去获取构造函数再创建实例，我们看第18行通过类实例获取构造函数：
constructor = clazz.getConstructor(mConstructorSignature);
其中mConstructorSignature定义如下：
private static final Class[] mConstructorSignature = new Class[] {
Context.class, AttributeSet.class};
很显然，这里用的是带有Context和AttributeSet两个参数的构造函数，这也就是为什么，自定义view一定要重载这个构造函数的原因。最后就是rInflate方法：
private void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs)
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_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
throw new InflateException(" cannot be the root element");
}
parseInclude(parser, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException(" must be the root element");
} else {
final View view = createViewFromTag(name, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflate(parser, view, attrs);
viewGroup.addView(view, params);
}
}
parent.onFinishInflate();
}
实这个方法也很简单，就是通过parser解析xml节点再生成对应View的过程。
XML转换成View的过程就是这样了，如有错误之处，还望指正，回到本文开头，其实我们还可以这样写：
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
View content = LayoutInflater.from(this).inflate(R.layout.activity_main, null);
setContentView(content);
}
界面2
大家发现问题没，相较于本文开头的写法，后面的灰色布局变成全屏了，我们来看看xml代码：
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="300dip"
android:layout_height="300dip"
android:background="#888888"
tools:context=".MainActivity" >
android:layout_width="200dip"
android:layout_height="200dip"
android:background="#238712"
android:contentDescription="@null" />
我明明设置了RelativeLayout的宽度和高度分别为300dip，但为什么全屏了？这是因为layout_width和layout_height是相对于父布局而言的，我们这里inflate的时候设置的父布局为null，所以这个属性设置也就无效了，指定一个父布局就可以了，例如：
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
RelativeLayout rootView = new RelativeLayout(this);
View content = LayoutInflater.from(this).inflate(R.layout.activity_main, rootView);
setContentView(content);
}
现在，界面显示效果就和“界面1”相同了。
