Android LayoutInflater源码分析及使用（一）

说到LayoutInflater可能大家首先想到的是加载layout，一般我们会习惯性的调用View中的这个静态方法

public static View inflate(Context context, int resource, ViewGroup root) {
LayoutInflater factory = LayoutInflater.from(context);
return factory.inflate(resource, root);
}

或者直接得到LayoutInflater对象，然后调用它的下面方法

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();
}
}

可是LayoutInflater是个抽象方法，我们是不能直接初始化的，一般都是这样来得到这个对象的

LayoutInflater LayoutInflater =
(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

我们通过查看源码，发现上面的方法在Context中也是也抽象方法，并没有真正的实现

public abstract Object getSystemService(String name);

今天我们就来一步步分析怎么得到LayoutInflater这个对象的，至于他的inflate方法我们也是经常遇到的，这篇我们先跳过，下一篇在作详解，

上面说到我们在Context中发现他是一个抽象类，那么我们就去它的子类中查找，在ContextWrapper中我们终于找到了，

@Override
public Object getSystemService(String name) {
return mBase.getSystemService(name);
}

那么我们在看看mBase是什么，可能很失望，它就是传进来的Context

protected void attachBaseContext(Context base) {
if (mBase != null) {
throw new IllegalStateException("Base context already set");
}
mBase = base;
}

我们继续跟踪，找到ContextWrapper的一个子类ContextThemeWrapper，发现getSystemService方法正好有我们要找的LayoutInflater，可是却没什么用，因为它需要实例化之后才能克隆，

@Override public Object getSystemService(String name) {
if (LAYOUT_INFLATER_SERVICE.equals(name)) {
if (mInflater == null) {
mInflater = LayoutInflater.from(mBase).cloneInContext(this);
}
return mInflater;
}
return mBase.getSystemService(name);
}

我们继续找它的下一个子类Activity，找到了，可是并没有我们要找的LayoutInflater

@Override
public Object getSystemService(String name) {
if (getBaseContext() == null) {
throw new IllegalStateException(
"System services not available to Activities before onCreate()");
}

if (WINDOW_SERVICE.equals(name)) {
return mWindowManager;
} else if (SEARCH_SERVICE.equals(name)) {
ensureSearchManager();
return mSearchManager;
}
return super.getSystemService(name);
}

饶了一大圈白费力气结果还是没找到。不要气馁，回到刚才我们说的ContextWrapper类中的getSystemService方法，我们来看一下mBase是怎么传进来的，最终我们在Activity的这个方法找到了它

final void attach(Context context, ActivityThread aThread,
Instrumentation instr, IBinder token, int ident,
Application application, Intent intent, ActivityInfo info,
CharSequence title, Activity parent, String id,
NonConfigurationInstances lastNonConfigurationInstances,
Configuration config) {
attachBaseContext(context);
…………………………
}

在之前我们讲过Activity的attach方法是在ActivityThreaad的performLaunchActivity方法中传进来的，

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
…………………………
Context appContext = createBaseContextForActivity(r, activity);
…………………………
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config);
…………………………

return activity;
}

也就是说ContextWrapper中的mBase其实就是在这里传进来的，我们继续查看createBaseContextForActivity方法

private Context createBaseContextForActivity(ActivityClientRecord r,
final Activity activity) {
ContextImpl appContext = ContextImpl.createActivityContext(this, r.packageInfo, r.token);
……………………
return baseContext;
}

看到了吧，其实Context的真正实现类是ContextImpl

/**
* Common implementation of Context API, which provides the base
* context object for Activity and other application components.
*/
class ContextImpl extends Context {

找到它就好办了，我们来查找一下getSystemService方法

@Override
public Object getSystemService(String name) {
ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
return fetcher == null ? null : fetcher.getService(this);
}

看到没，其实SYSTEM_SERVICE_MAP就是个HashMap，那它究竟是怎么保存进去的，我们继续查看，发现有一个方法叫registerService，在这里面保存的

private static void registerService(String serviceName, ServiceFetcher fetcher) {
if (!(fetcher instanceof StaticServiceFetcher)) {
fetcher.mContextCacheIndex = sNextPerContextServiceCacheIndex++;
}
SYSTEM_SERVICE_MAP.put(serviceName, fetcher);
}

我们还发现了有一个静态代码块，存进去的比较多，我们随便找几个我们平时比较常见的看看

static {
…………………………
registerService(ACTIVITY_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
return new ActivityManager(ctx.getOuterContext(), ctx.mMainThread.getHandler());
}});

registerService(ALARM_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
IBinder b = ServiceManager.getService(ALARM_SERVICE);
IAlarmManager service = IAlarmManager.Stub.asInterface(b);
return new AlarmManager(service, ctx);
}});

registerService(AUDIO_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
return new AudioManager(ctx);
}});

…………………………

registerService(INPUT_SERVICE, new StaticServiceFetcher() {
public Object createStaticService() {
return InputManager.getInstance();
}});

registerService(DISPLAY_SERVICE, new ServiceFetcher() {
@Override
public Object createService(ContextImpl ctx) {
return new DisplayManager(ctx.getOuterContext());
}});

registerService(INPUT_METHOD_SERVICE, new StaticServiceFetcher() {
public Object createStaticService() {
return InputMethodManager.getInstance();
}});

………………………………
………………………………

registerService(LAYOUT_INFLATER_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
return PolicyManager.makeNewLayoutInflater(ctx.getOuterContext());
}});

………………………………
………………………………

// Note: this was previously cached in a static variable, but
// constructed using mMainThread.getHandler(), so converting
// it to be a regular Context-cached service...
registerService(POWER_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
IBinder b = ServiceManager.getService(POWER_SERVICE);
IPowerManager service = IPowerManager.Stub.asInterface(b);
return new PowerManager(ctx.getOuterContext(),
service, ctx.mMainThread.getHandler());
}});

registerService(SEARCH_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
return new SearchManager(ctx.getOuterContext(),
ctx.mMainThread.getHandler());
}});

…………………………

registerService(TELEPHONY_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
return new TelephonyManager(ctx.getOuterContext());
}});

…………………………

registerService(VIBRATOR_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
return new SystemVibrator(ctx);
}});

registerService(WALLPAPER_SERVICE, WALLPAPER_FETCHER);

registerService(WIFI_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
IBinder b = ServiceManager.getService(WIFI_SERVICE);
IWifiManager service = IWifiManager.Stub.asInterface(b);
return new WifiManager(ctx.getOuterContext(), service);
}});

registerService(WIFI_P2P_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
IBinder b = ServiceManager.getService(WIFI_P2P_SERVICE);
IWifiP2pManager service = IWifiP2pManager.Stub.asInterface(b);
return new WifiP2pManager(service);
}});
…………………………

registerService(CAMERA_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
return new CameraManager(ctx);
}
});
…………………………
}

然后我们在找我们需要的LayoutInflater，就是两条杠之间的，我们找到PolicyManager类，然后找到相应的方法

public static LayoutInflater makeNewLayoutInflater(Context context) {
return sPolicy.makeNewLayoutInflater(context);
}

sPolicy其实就是Policy，我们来查看一下它的源码，找到相应的方法

public LayoutInflater makeNewLayoutInflater(Context context) {
return new PhoneLayoutInflater(context);
}

看这里我们终于明白了，其实我们通过getSystemService得到的LayoutInflater 就是PhoneLayoutInflater，其实他继承的是LayoutInflater，我们可以看一下

public class PhoneLayoutInflater extends LayoutInflater {

好了，到此为止LayoutInflater LayoutInflater =(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);这个方法我们已经分析完了，其实他得到的就是PhoneLayoutInflater这个类，有时间我们在来分析一下它的inflate方法，也是我们最常用到的一个方法。