Android的布局方式有兩種，一種是通過xml布局，一種是通過java代碼布局，兩種布局方式各有各的好處，當然也可以相互混合使用。很多人都習慣用xml布局，那xml布局是如何轉換成view的呢？本文從源碼的角度來簡單分析下整個過程。
首先，創建一個新的項目，默認生成一個activity，其中xml布局很簡單，就一個RelativeLayout套了一個ImageView，代碼及效果如下：
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}
界面1
其中關鍵之處在于調用了父類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實例，那我們直接來看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();
}
}
我們知道每個activity實際都對應一個PhoneWindow，擁有一個頂層的DecorView，DecorView繼承自FrameLayout，作為根View，其中包含了一個標題區域和內容區域，這里的mContentParent就是其內容區域。關于PhoneWindow和DecorView的具體內容，讀者可自行查閱。這段代碼的意思很簡單，如果DecorView的內容區域為null，就先初始化，否則就先把內容區域的子View全部移除，最后再引入layout布局，所以，關鍵在于mLayoutInflater.inflate(layoutResID, mContentParent); 代碼繼續往下看：
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();
}
}
這里首先根據layout布局文件的Id生成xml資源解析器，然后再調用inflate(parser, root, attachToRoot)生成具體的view。XmlResourceParser是繼承自XmlPullParser和AttributeSet的接口，這里的parser其實是XmlBlock的內部類Parser的實例。
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("<merge /> 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，獲取xml根節點名：
final String name = parser.getName();
第39行根據節點名創建臨時View（temp），這個臨時view（temp）也是xml布局的根view：
View temp = createViewFromTag(name, attrs);
第61行，在臨時view（temp）的節點下創建所有子View，顯然這個方法里是通過遍歷xml所有子view節點，調用createViewFromTag方法生成子view并加載到根view中：
rInflate(parser, temp, attrs);
第68到76行，則是判斷，如果inflate方法有父view，則把臨時view（temp）加載到父view中再返回，如果沒有，則直接返回臨時view（temp），我們這里調用inflate方法的時候顯然有父view，即mContentParent，也就是最頂層view DecorView的內容區域。這里最關鍵有兩個方法，一個是createViewFromTag，另一個是rInflate，現在來逐一分析：createViewFromTag實際最終調用的是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 ? "<unknown>" : clazz.getName()));
ie.initCause(e);
throw ie;
}
}
其實這個方法很簡單，就是通過xml節點名，通過反射獲取view的實例再返回，其中先去map中查詢構造函數是否存在，如果存在則直接根據構造函數創建實例，這樣做的好處是不用每次都通過class去獲取構造函數再創建實例，我們看第18行通過類實例獲取構造函數：
constructor = clazz.getConstructor(mConstructorSignature);
其中mConstructorSignature定義如下：
private static final Class[] mConstructorSignature = new Class[] {
Context.class, AttributeSet.class};
很顯然，這里用的是帶有Context和AttributeSet兩個參數的構造函數，這也就是為什么，自定義view一定要重載這個構造函數的原因。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("<include /> cannot be the root element");
}
parseInclude(parser, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> 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();
}
實這個方法也很簡單，就是通過parser解析xml節點再生成對應View的過程。
XML轉換成View的過程就是這樣了，如有錯誤之處，還望指正，回到本文開頭，其實我們還可以這樣寫：
@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
大家發現問題沒，相較于本文開頭的寫法，后面的灰色布局變成全屏了，我們來看看xml代碼：
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="300dip"
android:layout_height="300dip"
android:background="#888888"
tools:context=".MainActivity" >
<ImageView
android:layout_width="200dip"
android:layout_height="200dip"
android:background="#238712"
android:contentDescription="@null" />
</RelativeLayout>
我明明設置了RelativeLayout的寬度和高度分別為300dip，但為什么全屏了？這是因為layout_width和layout_height是相對于父布局而言的，我們這里inflate的時候設置的父布局為null，所以這個屬性設置也就無效了，指定一個父布局就可以了，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);
}
現在，界面顯示效果就和“界面1”相同了。
