一、引子
在xml文件中，如果想使用TextView、WebView、Button、RelativeLayout等系統View，直接寫類名即可，但如果是自定義的View，則需要寫帶有包名的全類名，否則運行時會報錯。這是爲什麼呢？
二、查找原因
我們知道Android是通過LayoutInflater.inflate()方法解析xml文件創建View的，從這個方法找下答案：
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
調用了LayoutInflater類的inflate(XmlPullParser, ViewGroup, Boolean)方法
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
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;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
final String name = parser.getName();
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); // ①
ViewGroup.LayoutParams params = null;
if (root != null) {
// 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);
}
}
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true); // ②
// 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;
}
}
}
// 省略異常處理代碼
finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
}
return result;
}
}
對於xml文件中最外層標籤名不爲merge的佈局而言，主要做兩件事；
①創建xml最外層標籤名的View；
②解析最外層標籤內部的View。
2.1 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) {
// 省略其他代碼
try {
if (-1 == name.indexOf('.')) {
view = onCreateView(parent, name, attrs);// A
} else {
view = createView(name, null, attrs); // B
}
} finally {
mConstructorArgs[0] = lastContext;
}
return view;
}
可以看到如果標籤名字不帶"."，也就是標籤就是類名（SimpleName），不包含包名，則執行onCreateView(parent, name, attrs)，否則執行createView(name, null, attrs)。
2.1.1 createView(name, null, attrs)
先看下createView(name, null, attrs)。
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Constructor<? extends View> constructor = sConstructorMap.get(name);
if (constructor != null && !verifyClassLoader(constructor)) {
constructor = null;
sConstructorMap.remove(name);
}
Class<? extends View> 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).asSubclass(View.class);
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
sConstructorMap.put(name, constructor);
}
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;
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
// Use the same context when inflating ViewStub later.
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
mConstructorArgs[0] = lastContext;
return view;
}
主要是通過完全限定類名（包含包名）加載Class，然後通過反射調用其構造函數生成View。
2.1.2 onCreateView(parent, name, attrs)
protected View onCreateView(String name, AttributeSet attrs)
throws ClassNotFoundException {
return createView(name, "android.view.", attrs);
}
onCreateView()方法非常簡單，調用的是createView()方法，只是prefix不爲null。因此，對於LayoutInflater類而言，如果不含包名的類名作爲標籤，則加上"android.view."來構造全限定類名。
2.2 問題解決了嗎？
問題似乎是解決了，但TextView、ImageView、ListView、GridView等都是在android.widget包下，如果直接加上"android.view"，加載class時會報ClassNotFoundException的！Android是如何解決的呢？
2.2.1 LayoutInflater.from()
看下LayoutInflater的創建方法。
public static LayoutInflater from(Context context) {
LayoutInflater LayoutInflater =
(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
return LayoutInflater;
}
直接看下ContextImpl.getSystemService()方法
@Override
public Object getSystemService(String name) {
return SystemServiceRegistry.getSystemService(this, name);
}
跟下去
public static Object getSystemService(ContextImpl ctx, String name) {
ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
return fetcher != null ? fetcher.getService(ctx) : null;
}
SYSTEM_SERVICE_FETCHERS是HashMap類型變量，
private static final HashMap<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS =
new HashMap<String, ServiceFetcher<?>>();
數據在SystemServiceRegistry類的靜態語句塊中填充
registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,
new CachedServiceFetcher<LayoutInflater>() {
@Override
public LayoutInflater createService(ContextImpl ctx) {
return new PhoneLayoutInflater(ctx.getOuterContext());
}});
registerService()方法
private static <T> void registerService(String serviceName, Class<T> serviceClass,
ServiceFetcher<T> serviceFetcher) {
SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);
SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);
}
因此
public static LayoutInflater from(Context context) {
ServiceFetcher<?> fetcher = new CachedServiceFetcher<LayoutInflater>() {
@Override
public LayoutInflater createService(ContextImpl ctx) {
return new PhoneLayoutInflater(ctx.getOuterContext());
}};
return fetcher != null ? fetcher.getService(ctx) : null;
}
看下CachedServiceFetcher類
static abstract class CachedServiceFetcher<T> implements ServiceFetcher<T> {
CachedServiceFetcher() {
}
@Override
public final T getService(ContextImpl ctx) {
// 省略同步和異常處理代碼
T service = null;
@ServiceInitializationState int newState = ContextImpl.STATE_NOT_FOUND;
// This thread is the first one to get here. Instantiate the service
// *without* the cache lock held.
service = createService(ctx);
return service;
}
}
}
public abstract T createService(ContextImpl ctx) throws ServiceNotFoundException;
}
因此LayoutInflater.from()方法返回的是PhoneLayoutInflater類。
2.2.2 PhoneLayoutInflater
public class PhoneLayoutInflater extends LayoutInflater {
private static final String[] sClassPrefixList = {
"android.widget.",
"android.webkit.",
"android.app."
};
/** Override onCreateView to instantiate names that correspond to the
widgets known to the Widget factory. If we don't find a match,
call through to our super class.
*/
@Override protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {
for (String prefix : sClassPrefixList) {
try {
View view = createView(name, prefix, attrs);
if (view != null) {
return view;
}
} catch (ClassNotFoundException e) {
// In this case we want to let the base class take a crack
// at it.
}
}
return super.onCreateView(name, attrs);
}
}
PhoneLayoutInflater非常簡單，複寫了onCreateView()方法，依次嘗試"android.widget.", “android.webkit.”, "android.app."三個前綴，如果在這三個包名下有找到指定的類，則直接創建並返回，否則執行父類（LayoutInflater）的onCreateView()方法。
三、總結
結論很簡單，xml佈局中系統View標籤不需要帶包名的原因是，LayoutInflater及其子類PhoneLayoutInflater會在創建View對象時替開發者加上！