Android-从LayoutInflater.from(context)看源码

在Android中，我们经常通过Context获取系统级别的服务，比如WindowsManagerService、ActivityManagerService等，但我们今天使用RecyclerView.Adapter里面onCreateViewHolder来开始查看代码。

  @Override
        public CarSelectorViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            View view = LayoutInflater.from(context).inflate(R.layout.adapter_carselectdialog, null);
            CarSelectorViewHolder holder = new CarSelectorViewHolder(view);
            return holder;
        }

  /**
     * Obtains the LayoutInflater from the given context.
     */
	//点进去发现是 context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    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;
    }

//再次点击就是一个抽象方法
public abstract @Nullable Object getSystemService(@ServiceName @NonNull String name);

下面就用Activity里面来分析

public static void main(String[] args) {
       Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");

       // Install selective syscall interception
       AndroidOs.install();

       // CloseGuard defaults to true and can be quite spammy.  We
       // disable it here, but selectively enable it later (via
       // StrictMode) on debug builds, but using DropBox, not logs.
       CloseGuard.setEnabled(false);

       Environment.initForCurrentUser();

       // Make sure TrustedCertificateStore looks in the right place for CA certificates
       final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
       TrustedCertificateStore.setDefaultUserDirectory(configDir);

       // Call per-process mainline module initialization.
       initializeMainlineModules();

       Process.setArgV0("<pre-initialized>");

       Looper.prepareMainLooper();

       // Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.
       // It will be in the format "seq=114"
       long startSeq = 0;
       if (args != null) {
           for (int i = args.length - 1; i >= 0; --i) {
               if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {
                   startSeq = Long.parseLong(
                           args[i].substring(PROC_START_SEQ_IDENT.length()));
               }
           }
       }
       ActivityThread thread = new ActivityThread();
       thread.attach(false, startSeq);

       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();

       throw new RuntimeException("Main thread loop unexpectedly exited");
   }

1、创建ActivityThread，调用attach。主要看attach方法；

2、looper.prepareMainLooper()，looper.loop()；

   @UnsupportedAppUsage
   private void attach(boolean system, long startSeq) {
       sCurrentActivityThread = this;
       mSystemThread = system;
       if (!system) {
           android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
                                                   UserHandle.myUserId());
           RuntimeInit.setApplicationObject(mAppThread.asBinder());
           final IActivityManager mgr = ActivityManager.getService();
           try {
               mgr.attachApplication(mAppThread, startSeq);
           } catch (RemoteException ex) {
               throw ex.rethrowFromSystemServer();
           }
           // Watch for getting close to heap limit.
           BinderInternal.addGcWatcher(new Runnable() {
               @Override public void run() {
                   if (!mSomeActivitiesChanged) {
                       return;
                   }
                   Runtime runtime = Runtime.getRuntime();
                   long dalvikMax = runtime.maxMemory();
                   long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();
                   if (dalvikUsed > ((3*dalvikMax)/4)) {
                       if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024)
                               + " total=" + (runtime.totalMemory()/1024)
                               + " used=" + (dalvikUsed/1024));
                       mSomeActivitiesChanged = false;
                       try {
                           ActivityTaskManager.getService().releaseSomeActivities(mAppThread);
                       } catch (RemoteException e) {
                           throw e.rethrowFromSystemServer();
                       }
                   }
               }
           });
       } else {
           // Don't set application object here -- if the system crashes,
           // we can't display an alert, we just want to die die die.
           android.ddm.DdmHandleAppName.setAppName("system_process",
                   UserHandle.myUserId());
           try {
               mInstrumentation = new Instrumentation();
               mInstrumentation.basicInit(this);
               ContextImpl context = ContextImpl.createAppContext(
                       this, getSystemContext().mPackageInfo);
               mInitialApplication = context.mPackageInfo.makeApplication(true, null);
               mInitialApplication.onCreate();
           } catch (Exception e) {
               throw new RuntimeException(
                       "Unable to instantiate Application():" + e.toString(), e);
           }
       }

       ViewRootImpl.ConfigChangedCallback configChangedCallback
               = (Configuration globalConfig) -> {
           synchronized (mResourcesManager) {
               // TODO (b/135719017): Temporary log for debugging IME service.
               if (Build.IS_DEBUGGABLE && mHasImeComponent) {
                   Log.d(TAG, "ViewRootImpl.ConfigChangedCallback for IME, "
                           + "config=" + globalConfig);
               }

               // We need to apply this change to the resources immediately, because upon returning
               // the view hierarchy will be informed about it.
               if (mResourcesManager.applyConfigurationToResourcesLocked(globalConfig,
                       null /* compat */)) {
                   updateLocaleListFromAppContext(mInitialApplication.getApplicationContext(),
                           mResourcesManager.getConfiguration().getLocales());

                   // This actually changed the resources! Tell everyone about it.
                   if (mPendingConfiguration == null
                           || mPendingConfiguration.isOtherSeqNewer(globalConfig)) {
                       mPendingConfiguration = globalConfig;
                       sendMessage(H.CONFIGURATION_CHANGED, globalConfig);
                   }
               }
           }
       };
       ViewRootImpl.addConfigCallback(configChangedCallback);
   }

@UnsupportedAppUsage
   public static IActivityManager getService() {
       return IActivityManagerSingleton.get();
   }

ActivityManager.getService()获取到是一个IActivityManager单例

attachApplication()调用在ActivityManagerService文件里面

@Override
   public final void attachApplication(IApplicationThread thread, long startSeq) {
       if (thread == null) {
           throw new SecurityException("Invalid application interface");
       }
       synchronized (this) {
           int callingPid = Binder.getCallingPid();
           final int callingUid = Binder.getCallingUid();
           final long origId = Binder.clearCallingIdentity();
           attachApplicationLocked(thread, callingPid, callingUid, startSeq);
           Binder.restoreCallingIdentity(origId);
       }
   }

Binder.restoreCallingIdentity(origId);是binder中的native方法

attachApplicationLocked(thread, callingPid, callingUid, startSeq);

/**
 * Extended implementation of activity launch. Used when server requests a launch or relaunch.
 */
@Override
public Activity handleLaunchActivity(ActivityClientRecord r,
        PendingTransactionActions pendingActions, Intent customIntent) {
    // If we are getting ready to gc after going to the background, well
    // we are back active so skip it.
    unscheduleGcIdler();
    mSomeActivitiesChanged = true;

    if (r.profilerInfo != null) {
        mProfiler.setProfiler(r.profilerInfo);
        mProfiler.startProfiling();
    }

    // Make sure we are running with the most recent config.
    handleConfigurationChanged(null, null);

    if (localLOGV) Slog.v(
        TAG, "Handling launch of " + r);

    // Initialize before creating the activity
    if (!ThreadedRenderer.sRendererDisabled
            && (r.activityInfo.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0) {
        HardwareRenderer.preload();
    }
    WindowManagerGlobal.initialize();

    // Hint the GraphicsEnvironment that an activity is launching on the process.
    GraphicsEnvironment.hintActivityLaunch();

    final Activity a = performLaunchActivity(r, customIntent);

    if (a != null) {
        r.createdConfig = new Configuration(mConfiguration);
        reportSizeConfigurations(r);
        if (!r.activity.mFinished && pendingActions != null) {
            pendingActions.setOldState(r.state);
            pendingActions.setRestoreInstanceState(true);
            pendingActions.setCallOnPostCreate(true);
        }
    } else {
        // If there was an error, for any reason, tell the activity manager to stop us.
        try {
            ActivityTaskManager.getService()
                    .finishActivity(r.token, Activity.RESULT_CANCELED, null,
                            Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
        } catch (RemoteException ex) {
            throw ex.rethrowFromSystemServer();
        }
    }

    return a;
}

handleConfigurationChanged()、WindowManagerGlobal.initialize()和GraphicsEnvironment.hintActivityLaunch()等方法；

performLaunchActivity()；

构建activity核心代码，performLaunchActivity

/**  Core implementation of activity launch. */
  private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
      ActivityInfo aInfo = r.activityInfo;
      if (r.packageInfo == null) {
          r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                  Context.CONTEXT_INCLUDE_CODE);
      }

      ComponentName component = r.intent.getComponent();
      if (component == null) {
          component = r.intent.resolveActivity(
              mInitialApplication.getPackageManager());
          r.intent.setComponent(component);
      }

      if (r.activityInfo.targetActivity != null) {
          component = new ComponentName(r.activityInfo.packageName,
                  r.activityInfo.targetActivity);
      }

      ContextImpl appContext = createBaseContextForActivity(r);
      Activity activity = null;
      try {
          java.lang.ClassLoader cl = appContext.getClassLoader();
          activity = mInstrumentation.newActivity(
                  cl, component.getClassName(), r.intent);
          StrictMode.incrementExpectedActivityCount(activity.getClass());
          r.intent.setExtrasClassLoader(cl);
          r.intent.prepareToEnterProcess();
          if (r.state != null) {
              r.state.setClassLoader(cl);
          }
      } catch (Exception e) {
          if (!mInstrumentation.onException(activity, e)) {
              throw new RuntimeException(
                  "Unable to instantiate activity " + component
                  + ": " + e.toString(), e);
          }
      }

      try {
          Application app = r.packageInfo.makeApplication(false, mInstrumentation);

          if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
          if (localLOGV) Slog.v(
                  TAG, r + ": app=" + app
                  + ", appName=" + app.getPackageName()
                  + ", pkg=" + r.packageInfo.getPackageName()
                  + ", comp=" + r.intent.getComponent().toShortString()
                  + ", dir=" + r.packageInfo.getAppDir());

          if (activity != null) {
              CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
              Configuration config = new Configuration(mCompatConfiguration);
              if (r.overrideConfig != null) {
                  config.updateFrom(r.overrideConfig);
              }
              if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
                      + r.activityInfo.name + " with config " + config);
              Window window = null;
              if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
                  window = r.mPendingRemoveWindow;
                  r.mPendingRemoveWindow = null;
                  r.mPendingRemoveWindowManager = null;
              }

              // Activity resources must be initialized with the same loaders as the
              // application context.
              appContext.getResources().addLoaders(
                      app.getResources().getLoaders().toArray(new ResourcesLoader[0]));

              appContext.setOuterContext(activity);
              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, r.configCallback,
                      r.assistToken);

              if (customIntent != null) {
                  activity.mIntent = customIntent;
              }
              r.lastNonConfigurationInstances = null;
              checkAndBlockForNetworkAccess();
              activity.mStartedActivity = false;
              int theme = r.activityInfo.getThemeResource();
              if (theme != 0) {
                  activity.setTheme(theme);
              }

              activity.mCalled = false;
              if (r.isPersistable()) {
                  mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
              } else {
                  mInstrumentation.callActivityOnCreate(activity, r.state);
              }
              if (!activity.mCalled) {
                  throw new SuperNotCalledException(
                      "Activity " + r.intent.getComponent().toShortString() +
                      " did not call through to super.onCreate()");
              }
              r.activity = activity;
              mLastReportedWindowingMode.put(activity.getActivityToken(),
                      config.windowConfiguration.getWindowingMode());
          }
          r.setState(ON_CREATE);

          // updatePendingActivityConfiguration() reads from mActivities to update
          // ActivityClientRecord which runs in a different thread. Protect modifications to
          // mActivities to avoid race.
          synchronized (mResourcesManager) {
              mActivities.put(r.token, r);
          }

      } catch (SuperNotCalledException e) {
          throw e;

      } catch (Exception e) {
          if (!mInstrumentation.onException(activity, e)) {
              throw new RuntimeException(
                  "Unable to start activity " + component
                  + ": " + e.toString(), e);
          }
      }

      return activity;
  }

1、activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent); 创建activity

2、Application app = r.packageInfo.makeApplication(false, mInstrumentation); 创建application

3、window = r.mPendingRemoveWindow;创建window;

4、activity.attach()把window application window context 都加入

5、mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);

private ContextImpl createBaseContextForActivity(ActivityClientRecord r) {
    final int displayId;
    try {
        displayId = ActivityTaskManager.getService().getDisplayId(r.token);
    } catch (RemoteException e) {
        throw e.rethrowFromSystemServer();
    }

    ContextImpl appContext = ContextImpl.createActivityContext(
            this, r.packageInfo, r.activityInfo, r.token, displayId, r.overrideConfig);

    // The rotation adjustments must be applied before creating the activity, so the activity
    // can get the adjusted display info during creation.
    if (r.mPendingFixedRotationAdjustments != null) {
        handleFixedRotationAdjustments(r.token, r.mPendingFixedRotationAdjustments,
                r.overrideConfig);
        r.mPendingFixedRotationAdjustments = null;
    }

    final DisplayManagerGlobal dm = DisplayManagerGlobal.getInstance();
    // For debugging purposes, if the activity's package name contains the value of
    // the "debug.use-second-display" system property as a substring, then show
    // its content on a secondary display if there is one.
    String pkgName = SystemProperties.get("debug.second-display.pkg");
    if (pkgName != null && !pkgName.isEmpty()
            && r.packageInfo.mPackageName.contains(pkgName)) {
        for (int id : dm.getDisplayIds()) {
            if (id != Display.DEFAULT_DISPLAY) {
                Display display =
                        dm.getCompatibleDisplay(id, appContext.getResources());
                appContext = (ContextImpl) appContext.createDisplayContext(display);
                break;
            }
        }
    }
    return appContext;
}

1、 ContextImpl appContext = ContextImpl.createActivityContext(
this, r.packageInfo, r.activityInfo, r.token, displayId, r.overrideConfig);

2、appContext = (ContextImpl) appContext.createDisplayContext(display);

3、无论如何，都会使用到contextimpl这个类。

从contextImpl看出，虚拟机第一次加载类事会注册各种ServiceFatcher,这些服务以键值队形式存在HashMap中，用户先根据key找到ServiceFetcher,然后通过ServiceFetcher createService获取具体的服务对象。第一次获取时，会将该对象缓存到一个列表中。