通常情況下我們使用LayoutInflater較為常見的地方是ListView的getView中：
View view = LayoutInflater.from(context).inflate(R.layout.xxx, null);
複製程式碼通常情況下我們使用LayoutInflater.from(context)來獲取LayoutInflater服務，我們來看看LayoutInflater服務是如何實現的。
public static LayoutInflater from(Context context) {
LayoutInflater LayoutInflater =
(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
if (LayoutInflater == null) {
throw new AssertionError("LayoutInflater not found.");
}
return LayoutInflater;
}
複製程式碼可以看到from(context)是通過context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)方法，繼續跟蹤之後，得知Context類是個抽象類。
而getView中使用的context的實現類是什麼呢？ 在每個Activity、Service、Application中都存在一個Context物件，所以Context物件的總個數為Activity+Service+1。而ListView通常情況下出現在Activity中，所以我們以Activity的Context物件來分析。
一個Activity的入口是ActivityThread的main函式，在main函式中建立ActivityThread物件，並且啟動Handler，建立新的Activity、新的Context物件，並將該物件傳遞給Activity
public static void main(String[] args) {
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
}
複製程式碼在main函式中，建立並呼叫了attach方法傳遞false(非系統應用)
private void attach(boolean system) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {
ViewRootImpl.addFirstDrawHandler(new Runnable() {
@Override
public void run() {
ensureJitEnabled();
}
});
android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
UserHandle.myUserId());
RuntimeInit.setApplicationObject(mAppThread.asBinder());
final IActivityManager mgr = ActivityManagerNative.getDefault();
try {
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
}
複製程式碼在attach方法中通過Binder機制和ActivityManagerService通訊，最終呼叫handleLaunchActivity方法
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
Activity a = performLaunchActivity(r, customIntent);
}
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
Activity activity = null;
try {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
// 建立activity
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to instantiate activity " + component
+ ": " + e.toString(), e);
}
}
try {
// 建立Application
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
if (activity != null) {
// 建立Context
Context appContext = createBaseContextForActivity(r, activity);
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
// 將相關物件attach到activity中
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor, window);
// 呼叫onCreate方法
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
}
r.paused = true;
mActivities.put(r.token, r);
} catch (SuperNotCalledException e) {
throw e;
}
return activity;
}
private Context createBaseContextForActivity(ActivityClientRecord r, final Activity activity) {
// 建立Context物件
ContextImpl appContext = ContextImpl.createActivityContext(
this, r.packageInfo, r.token, displayId, r.overrideConfig);
return baseContext;
}
複製程式碼最終我們可以發現建立的Context物件的實現類是ContextImpl。我們繼續觀察ContextImpl：
// The system service cache for the system services that are cached per-ContextImpl.
// 獲取系統服務
final Object[] mServiceCache = SystemServiceRegistry.createServiceCache();
複製程式碼在SystemServiceRegistry中：
// service容器
private static final HashMap<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS = new HashMap<String, ServiceFetcher<?>>();
// 註冊伺服器
private static <T> void registerService(String serviceName, Class<T> serviceClass,
ServiceFetcher<T> serviceFetcher) {
SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);
// 將服務放到service容器中
SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);
}
// 靜態程式碼塊，當第一次載入該類的時候就將執行將服務建立出來
static {
registerService(Context.ACCESSIBILITY_SERVICE, AccessibilityManager.class,
new CachedServiceFetcher<AccessibilityManager>() {
@Override
public AccessibilityManager createService(ContextImpl ctx) {
return AccessibilityManager.getInstance(ctx);
}});
...
}
// 獲取系統服務
public static Object getSystemService(ContextImpl ctx, String name) {
ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
return fetcher != null ? fetcher.getService(ctx) : null;
}
複製程式碼在ContextImpl中，當虛擬機器第一次載入的時候就會註冊各種服務，其中就包含LayoutInflater Service，將這些服務以鍵值對的形式儲存在map中，當使用的時候只要通過key就可以獲取對應的服務物件。
在靜態程式碼塊中，我們終於找到了LayoutInflater註冊的程式碼：
registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,
new CachedServiceFetcher<LayoutInflater>() {
@Override
public LayoutInflater createService(ContextImpl ctx) {
return new PhoneLayoutInflater(ctx.getOuterContext());
}});
複製程式碼原來LayoutInflater是通過PhoneLayoutInflater建立出來的，我們再找到PhoneLayoutInflater類：
public class PhoneLayoutInflater extends LayoutInflater {
private static final String[] sClassPrefixList = {
"android.widget.",
"android.webkit.",
"android.app."
};
public PhoneLayoutInflater(Context context) {
super(context);
}
protected PhoneLayoutInflater(LayoutInflater original, Context newContext) {
super(original, newContext);
}
/** 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);
}
public LayoutInflater cloneInContext(Context newContext) {
return new PhoneLayoutInflater(this, newContext);
}
}
複製程式碼核心方法就是onCreateView方法，該方法通過將傳遞過來的View前面加上"android.widget.","android.webkit.","android.app."用來得到該內建View物件的完整路徑，最後根據路徑來建立出對應的View。