上篇文章介紹了Camera1的使用石洗，本篇介紹Camera2的使用计呈。
Camera2(android.hardware.camera2)是從 Android 5.0 L 版本開始引入的励七，并且廢棄了舊的相機(jī)框架Camera1(android.hardware.Camera)丛楚。
相比于Camera1箩绍，Camera2架構(gòu)上也發(fā)生了變化，API上的使用難度也增加了梳侨。Camera2將相機(jī)設(shè)備模擬成一個(gè)管道，它按順序處理每一幀的請(qǐng)求并返回請(qǐng)求結(jié)果給客戶端日丹。

設(shè)計(jì)框架

來(lái)自官網(wǎng)的模型圖走哺，展示了相關(guān)的工作流程

相機(jī)核心操作模型

優(yōu)點(diǎn)

通過(guò)設(shè)計(jì)框架的改造和優(yōu)化晒旅，Camera2具備了以下優(yōu)點(diǎn):

• 改進(jìn)了新硬件的性能。Supported Hardware Level的概念汪诉，不同廠商對(duì)Camera2的支持程度不同废恋，從低到高有LEGACY谈秫、LIMITED、FULL 和 LEVEL_3四個(gè)級(jí)別
• 以更快的間隔拍攝圖像
• 顯示來(lái)自多個(gè)攝像機(jī)的預(yù)覽
• 直接應(yīng)用效果和濾鏡

開發(fā)流程

框架上的變化鱼鼓，對(duì)整個(gè)使用流程變化也非常大拟烫，首先了解一些主要的開發(fā)類

類

CameraManager

相機(jī)系統(tǒng)服務(wù)，用于管理和連接相機(jī)設(shè)備

CameraDevice

相機(jī)設(shè)備類迄本，和Camera1中的Camera同級(jí)

CameraCharacteristics

主要用于獲取相機(jī)信息硕淑，內(nèi)部攜帶大量的相機(jī)信息，包含攝像頭的正反(LENS_FACING)嘉赎、AE模式置媳、AF模式等，和Camera1中的Camera.Parameters類似

CaptureRequest

相機(jī)捕獲圖像的設(shè)置請(qǐng)求公条，包含傳感器拇囊，鏡頭，閃光燈等

CaptureRequest.Builder

CaptureRequest的構(gòu)造器赃份，使用Builder模式寂拆，設(shè)置更加方便

CameraCaptureSession

請(qǐng)求抓取相機(jī)圖像幀的會(huì)話，會(huì)話的建立主要會(huì)建立起一個(gè)通道抓韩。一個(gè)CameraDevice一次只能開啟一個(gè)CameraCaptureSession纠永。
源端是相機(jī)，另一端是 Target谒拴，Target可以是Preview尝江，也可以是ImageReader。

ImageReader

用于從相機(jī)打開的通道中讀取需要的格式的原始圖像數(shù)據(jù)英上，可以設(shè)置多個(gè)ImageReader炭序。

流程

Camera2開發(fā)流程

獲取CameraManager

CameraManager cameraManager = (CameraManager) getSystemService(Context.CAMERA_SERVICE);

獲取相機(jī)信息

for (String cameraId : cameraManager.getCameraIdList()) {
    CameraCharacteristics characteristics = cameraManager.getCameraCharacteristics(cameraId);

    Integer facing = characteristics.get(CameraCharacteristics.LENS_FACING);
    if (null != facing && facing == CameraCharacteristics.LENS_FACING_FRONT) {
        continue;
    }
    ....
}

這里默認(rèn)選擇前置攝像頭，并獲取相關(guān)相機(jī)信息苍日。

初始化ImageReader

mImageReader = ImageReader.newInstance(largest.getWidth(), largest.getHeight(), ImageFormat.JPEG, 2);
mImageReader.setOnImageAvailableListener(new ImageReader.OnImageAvailableListener() {
    @Override
    public void onImageAvailable(ImageReader reader) {
        Log.d("DEBUG", "##### onImageAvailable: " + mFile.getPath());
        mBackgroundHandler.post(new ImageSaver(reader.acquireNextImage(), mFile));
    }
}, mBackgroundHandler);

ImageReader是獲取圖像數(shù)據(jù)的重要途徑惭聂，通過(guò)它可以獲取到不同格式的圖像數(shù)據(jù)，例如JPEG相恃、YUV辜纲、RAW等。通過(guò)ImageReader.newInstance(int width, int height, int format, int maxImages)創(chuàng)建ImageReader對(duì)象拦耐，有4個(gè)參數(shù)：

• width：圖像數(shù)據(jù)的寬度
• height：圖像數(shù)據(jù)的高度
• format：圖像數(shù)據(jù)的格式耕腾，例如ImageFormat.JPEG，ImageFormat.YUV_420_888等
• maxImages：最大Image個(gè)數(shù)杀糯，Image對(duì)象池的大小扫俺，指定了能從ImageReader獲取Image對(duì)象的最大值，過(guò)多獲取緩沖區(qū)可能導(dǎo)致OOM固翰，所以最好按照最少的需要去設(shè)置這個(gè)值

ImageReader其他相關(guān)的方法和回調(diào)：

• ImageReader.OnImageAvailableListener：有新圖像數(shù)據(jù)的回調(diào)
• acquireLatestImage()：從ImageReader的隊(duì)列里面狼纬，獲取最新的Image羹呵，刪除舊的，如果沒(méi)有可用的Image畸颅，返回null
• acquireNextImage()：獲取下一個(gè)最新的可用Image担巩，沒(méi)有則返回null
• close()：釋放與此ImageReader關(guān)聯(lián)的所有資源
• getSurface()：獲取為當(dāng)前ImageReader生成Image的Surface

打開相機(jī)設(shè)備

try {
    if (!mCameraOpenCloseLock.tryAcquire(2500, TimeUnit.MILLISECONDS)) {
        throw new RuntimeException("Time out waiting to lock camera opening.");
    }

    cameraManager.openCamera(mCameraId, mStateCallback, mBackgroundHandler);
} catch (Exception e) {
    e.printStackTrace();
}

cameraManager.openCamera(@NonNull String cameraId,@NonNull final CameraDevice.StateCallback callback, @Nullable Handler handler)的三個(gè)參數(shù):

• cameraId：攝像頭的唯一標(biāo)識(shí)
• callback：設(shè)備連接狀態(tài)變化的回調(diào)
• handler：回調(diào)執(zhí)行的Handler對(duì)象，傳入null則使用當(dāng)前的主線程Handler

其中callback回調(diào)：

private final CameraDevice.StateCallback mStateCallback = new CameraDevice.StateCallback() {
    @Override
    public void onOpened(@NonNull CameraDevice camera) {
        mCameraOpenCloseLock.release();
        mCameraDevice = camera;
        createCameraPreviewSession();
    }

    @Override
    public void onDisconnected(@NonNull CameraDevice camera) {
        mCameraOpenCloseLock.release();
        camera.close();
        mCameraDevice = null;
    }

    @Override
    public void onError(@NonNull CameraDevice camera, int error) {
        mCameraOpenCloseLock.release();
        camera.close();
        mCameraDevice = null;
    }

    @Override
    public void onClosed(@NonNull CameraDevice camera) {
        super.onClosed(camera);
    }
};

• onOpened：表示相機(jī)打開成功没炒，可以真正開始使用相機(jī)涛癌，創(chuàng)建Capture會(huì)話
• onDisconnected：當(dāng)相機(jī)斷開連接時(shí)回調(diào)該方法，需要進(jìn)行釋放相機(jī)的操作
• onError：當(dāng)相機(jī)打開失敗時(shí)送火，需要進(jìn)行釋放相機(jī)的操作
• onClosed：調(diào)用Camera.close()后的回調(diào)方法

創(chuàng)建Capture會(huì)話

在CameraDevice.StateCallback的onOpened回調(diào)中執(zhí)行：

private void createCameraPreviewSession() {
    SurfaceTexture texture = mTextureView.getSurfaceTexture();
    assert texture != null;
    texture.setDefaultBufferSize(mPreviewSize.getWidth(), mPreviewSize.getHeight());
    Surface surface = new Surface(texture);

    try {
        mPreviewRequestBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
        mPreviewRequestBuilder.addTarget(surface);

        // Here, we create a CameraCaptureSession for camera preview.
        mCameraDevice.createCaptureSession(Arrays.asList(surface, mImageReader.getSurface()),
                new CameraCaptureSession.StateCallback() {

                    @Override
                    public void onConfigured(@NonNull CameraCaptureSession cameraCaptureSession) {
                        // The camera is already closed
                        if (null == mCameraDevice) {
                            return;
                        }

                        // When the session is ready, we start displaying the preview.
                        mCaptureSession = cameraCaptureSession;
                        try {
                            // Auto focus should be continuous for camera preview.
                            mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE,
                                    CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE);
                            // Flash is automatically enabled when necessary.
                            setAutoFlash(mPreviewRequestBuilder);

                            // Finally, we start displaying the camera preview.
                            mPreviewRequest = mPreviewRequestBuilder.build();
                            mCaptureSession.setRepeatingRequest(mPreviewRequest,
                                    mCaptureCallback, mBackgroundHandler);
                        } catch (CameraAccessException e) {
                            e.printStackTrace();
                        }
                    }

                    @Override
                    public void onConfigureFailed(
                            @NonNull CameraCaptureSession cameraCaptureSession) {
                        Toast.makeText(Camera2Activity.this, "configureFailed", Toast.LENGTH_SHORT).show();
                    }
                }, null
        );
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

這段的代碼核心方法是mCameraDevice.createCaptureSession()創(chuàng)建Capture會(huì)話拳话，它接受了三個(gè)參數(shù)：

• outputs：用于接受圖像數(shù)據(jù)的surface集合，這里傳入的是一個(gè)preview的surface
• callback：用于監(jiān)聽 Session 狀態(tài)的CameraCaptureSession.StateCallback對(duì)象
• handler：用于執(zhí)行CameraCaptureSession.StateCallback的Handler對(duì)象种吸，傳入null則使用當(dāng)前的主線程Handler

創(chuàng)建CaptureRequest

CaptureRequest是向CameraCaptureSession提交Capture請(qǐng)求時(shí)的信息載體弃衍，其內(nèi)部包括了本次Capture的參數(shù)配置和接收?qǐng)D像數(shù)據(jù)的Surface。

mPreviewRequestBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
mPreviewRequestBuilder.addTarget(surface);

通過(guò)CameraDevice.createCaptureRequest()創(chuàng)建CaptureRequest.Builder對(duì)象坚俗，傳入一個(gè)templateType參數(shù)镜盯，templateType用于指定使用何種模板創(chuàng)建CaptureRequest.Builder對(duì)象，templateType的取值：

• TEMPLATE_PREVIEW：預(yù)覽模式
• TEMPLATE_STILL_CAPTURE：拍照模式
• TEMPLATE_RECORD：視頻錄制模式
• TEMPLATE_VIDEO_SNAPSHOT：視頻截圖模式
• TEMPLATE_MANUAL：手動(dòng)配置參數(shù)模式

除了模式的配置猖败，CaptureRequest還可以配置很多其他信息速缆，例如圖像格式、圖像分辨率恩闻、傳感器控制艺糜、閃光燈控制、3A(自動(dòng)對(duì)焦-AF幢尚、自動(dòng)曝光-AE和自動(dòng)白平衡-AWB)控制等破停。在createCaptureSession的回調(diào)中可以進(jìn)行設(shè)置

// Auto focus should be continuous for camera preview.
mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE,
        CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE);
// Flash is automatically enabled when necessary.
setAutoFlash(mPreviewRequestBuilder);

// Finally, we start displaying the camera preview.
mPreviewRequest = mPreviewRequestBuilder.build();

代碼中設(shè)置了AF為設(shè)置未圖片模式下的連續(xù)對(duì)焦，并設(shè)置自動(dòng)閃光燈尉剩。最后通過(guò)build()方法生成CaptureRequest對(duì)象真慢。

預(yù)覽

Camera2中，通過(guò)連續(xù)重復(fù)的Capture實(shí)現(xiàn)預(yù)覽功能理茎，每次Capture會(huì)把預(yù)覽畫面顯示到對(duì)應(yīng)的Surface上晤碘。連續(xù)重復(fù)的Capture操作通過(guò)mCaptureSession.setRepeatingRequest(mPreviewRequest,mCaptureCallback, mBackgroundHandler)實(shí)現(xiàn)，該方法有三個(gè)參數(shù)：

• request：CaptureRequest對(duì)象
• listener：監(jiān)聽Capture 狀態(tài)的回調(diào)
• handler：用于執(zhí)行CameraCaptureSession.CaptureCallback的Handler對(duì)象功蜓，傳入null則使用當(dāng)前的主線程Handler

停止預(yù)覽使用mCaptureSession.stopRepeating()方法。

拍照

設(shè)置上面的request宠蚂，session后式撼，就可以真正的開始拍照操作

mCaptureSession.capture(mPreviewRequestBuilder.build(), mCaptureCallback, mBackgroundHandler);

該方法也有三個(gè)參數(shù)，和mCaptureSession.setRepeatingRequest一樣：

• request：CaptureRequest對(duì)象
• listener：監(jiān)聽Capture 狀態(tài)的回調(diào)
• handler：用于執(zhí)行CameraCaptureSession.CaptureCallback的Handler對(duì)象求厕，傳入null則使用當(dāng)前的主線程Handler

這里設(shè)置了mCaptureCallback：

private CameraCaptureSession.CaptureCallback mCaptureCallback = new CameraCaptureSession.CaptureCallback() {
    @Override
    public void onCaptureProgressed(@NonNull CameraCaptureSession session, @NonNull CaptureRequest request, @NonNull CaptureResult partialResult) {
        process(partialResult);
    }

    @Override
    public void onCaptureCompleted(@NonNull CameraCaptureSession session, @NonNull CaptureRequest request, @NonNull TotalCaptureResult result) {
        process(result);
    }

    private void process(CaptureResult result) {
        switch (mState) {
            case STATE_PREVIEW: {
                // We have nothing to do when the camera preview is working normally.
                break;
            }
            case STATE_WAITING_LOCK: {
                Integer afState = result.get(CaptureResult.CONTROL_AF_STATE);
                Log.d("DEBUG", "##### process STATE_WAITING_LOCK: " + afState);
                if (afState == null) {
                    captureStillPicture();
                } else if (CaptureResult.CONTROL_AF_STATE_FOCUSED_LOCKED == afState ||
                        CaptureResult.CONTROL_AF_STATE_NOT_FOCUSED_LOCKED == afState) {
                    // CONTROL_AE_STATE can be null on some devices
                    Integer aeState = result.get(CaptureResult.CONTROL_AE_STATE);
                    if (aeState == null ||
                            aeState == CaptureResult.CONTROL_AE_STATE_CONVERGED) {
                        mState = STATE_PICTURE_TAKEN;
                        captureStillPicture();
                    } else {
                        runPrecaptureSequence();
                    }
                }
                break;
            }
            case STATE_WAITING_PRECAPTURE: {
                // CONTROL_AE_STATE can be null on some devices
                Integer aeState = result.get(CaptureResult.CONTROL_AE_STATE);
                if (aeState == null ||
                        aeState == CaptureResult.CONTROL_AE_STATE_PRECAPTURE ||
                        aeState == CaptureRequest.CONTROL_AE_STATE_FLASH_REQUIRED) {
                    mState = STATE_WAITING_NON_PRECAPTURE;
                }
                break;
            }
            case STATE_WAITING_NON_PRECAPTURE: {
                // CONTROL_AE_STATE can be null on some devices
                Integer aeState = result.get(CaptureResult.CONTROL_AE_STATE);
                if (aeState == null || aeState != CaptureResult.CONTROL_AE_STATE_PRECAPTURE) {
                    mState = STATE_PICTURE_TAKEN;
                    captureStillPicture();
                }
                break;
            }
        }
    }
};

通過(guò)設(shè)置mState來(lái)區(qū)分當(dāng)前狀態(tài)著隆，是在預(yù)覽還是拍照

關(guān)閉相機(jī)

退到后臺(tái)或者當(dāng)前頁(yè)面被關(guān)閉的時(shí)候扰楼，已經(jīng)不需要使用相機(jī)了，需要進(jìn)行相機(jī)關(guān)閉操作美浦，釋放資源弦赖，

private void closeCamera() {
    try {
        mCameraOpenCloseLock.acquire();
        if (null != mCaptureSession) {
            mCaptureSession.close();
            mCaptureSession = null;
        }
        if (null != mCameraDevice) {
            mCameraDevice.close();
            mCameraDevice = null;
        }
        if (null != mImageReader) {
            mImageReader.close();
            mImageReader = null;
        }
    } catch (InterruptedException e) {
        throw new RuntimeException("Interrupted while trying to lock camera closing.", e);
    } finally {
        mCameraOpenCloseLock.release();
    }
}

先后對(duì)CaptureSession，CameraDevice浦辨，ImageReader進(jìn)行close操作蹬竖，釋放資源。
這里僅僅對(duì)Camera2基本使用流程做了介紹流酬，一些更高級(jí)的用法需要大家自行去實(shí)踐币厕。在Camera1中需要對(duì)畫面進(jìn)行方向矯正，而Camera2是否需要呢芽腾，關(guān)于相機(jī)Orientation相關(guān)的知識(shí)旦装，通過(guò)后面的章節(jié)再進(jìn)行介紹。

文章中涉及到的代碼

參考：

• Camera
• Detecting camera features with Camera2
• Android Camera 編程從入門到精通
• Android Camera2 教程 · 第一章 · 概覽