Android 源码 Camera2 获取 CameraManager 对象

CameraManager 类是开放给 App 使用相机的入口类，它包括了获取相机 ID 列表、打开和关闭相机等操作，用于检测、表征和连接到 CameraDevice。

下面是 APP 获取 CameraManager 类的代码。CAMERA_SERVICE 定义在 Context 中，CAMERA_SERVICE = “camera”。

val manager = activity.getSystemService(Context.CAMERA_SERVICE) as CameraManager

由于传入的 name 既不等于 WINDOW_SERVICE，也不等于 SEARCH_SERVICE，因此会调用其父类 ContextThemeWrapper 的 getSystemService 查找。

frameworks/base/core/java/android/app/Activity.java

public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory2,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks2,
        Window.OnWindowDismissedCallback {
    ......
    @Override
    public Object getSystemService(@ServiceName @NonNull 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);
    }
    ......
}

结合以前的源码分析，不难知道最终会调用 ContextImpl 内的方法 getSystemService(…)。

frameworks/base/core/java/android/app/ContextImpl.java

class ContextImpl extends Context {
    ......
    @Override
    public Object getSystemService(String name) {
        return SystemServiceRegistry.getSystemService(this, name);
    }
    ......
}

SystemServiceRegistry 类管理所有可以由 Context#getSystemService 返回的系统服务。SYSTEM_SERVICE_FETCHERS 是一个 HashMap，其 key 类型为 String，value 是 ServiceFetcher<?>。那么这个 HashMap 在哪里填充呢？

frameworks/base/core/java/android/app/SystemServiceRegistry.java

final class SystemServiceRegistry {
    ......
    private static final HashMap<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS =
            new HashMap<String, ServiceFetcher<?>>();    
    ......
    public static Object getSystemService(ContextImpl ctx, String name) {
        ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
        return fetcher != null ? fetcher.getService(ctx) : null;
    }    
    ......
}

原来是在 SystemServiceRegistry 类静态块中添加的。其中就包括了 CAMERA_SERVICE。CachedServiceFetcher 继承自 ServiceFetcher。CachedServiceFetcher 是一个抽象类，其 createService(…) 方法待实现。其 getService(…) 方法首先从 ContextImpl 成员变量 mServiceCache 获取 cache 数组，接着从对应的 mCacheIndex 拿到 Object，第一次获取由于这个“坑位”在 CachedServiceFetcher 构造函数中刚增加（mCacheIndex = sServiceCacheSize++），所以 service 必然为 null，接着就会调用 createService(…) 创建相应的对象，并给对应的缓存“坑位”赋值。

frameworks/base/core/java/android/app/SystemServiceRegistry.java

final class SystemServiceRegistry {
    ......
    private static int sServiceCacheSize;
    ......
    static {
        ......
        registerService(Context.CAMERA_SERVICE, CameraManager.class,
                new CachedServiceFetcher<CameraManager>() {
            @Override
            public CameraManager createService(ContextImpl ctx) {
                return new CameraManager(ctx);
            }});        
        ......
    }
    ......
    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);
    }    
    ......
    static abstract class CachedServiceFetcher<T> implements ServiceFetcher<T> {
        private final int mCacheIndex;

        public CachedServiceFetcher() {
            mCacheIndex = sServiceCacheSize++;
        }

        @Override
        @SuppressWarnings("unchecked")
        public final T getService(ContextImpl ctx) {
            final Object[] cache = ctx.mServiceCache 中，实际上 ;
            synchronized (cache) {
                // Fetch or create the service.
                Object service = cache[mCacheIndex];
                if (service == null) {
                    service = createService(ctx);
                    cache[mCacheIndex] = service;
                }
                return (T)service;
            }
        }

        public abstract T createService(ContextImpl ctx);
    }
    ......
}

其中 cache 数组指向 ContextImpl 类的成员变量 mServiceCache，实际上 mServiceCache 赋值再次调用了 SystemServiceRegistry 类的 createServiceCache() 方法。

frameworks/base/core/java/android/app/ContextImpl.java

class ContextImpl extends Context {
    ......
    // The system service cache for the system services that are cached per-ContextImpl.
    final Object[] mServiceCache = SystemServiceRegistry.createServiceCache();    
    ......
}

创建用于缓存每个上下文服务实例的数组。

frameworks/base/core/java/android/app/SystemServiceRegistry.java

final class SystemServiceRegistry {
    ......
    public static Object[] createServiceCache() {
        return new Object[sServiceCacheSize];
    }    
    ......
}

现在得到了一个 CameraManager 对象。

frameworks/base/core/java/android/hardware/camera2/CameraManager.java

public final class CameraManager {
    ......
    private final Context mContext;
    private final Object mLock = new Object();

    /**
     * @hide
     */
    public CameraManager(Context context) {
        synchronized(mLock) {
            mContext = context;
        }
    }    
    ......
}