Android Camera2 预览功能实现

1. 概述

最近在做一些关于人脸识别的项目，需要用到 android 相机的预览功能。网上查阅相关资料后，发现 android 5.0 及以后的版本中，原有的 camera api 已经被 camera2 api 所取代。

全新的 camera2 在 camera 的基础上进行了改造，大幅提升了 android 系统的拍照功能。它通过以下几个类与方法来实现相机预览时的工作过程：

• cameramanager ：摄像头管理器，主要用于检测系统摄像头、打开系统摄像头等；
• cameradevice : 用于描述系统摄像头，可用于关闭相机、创建相机会话、发送拍照请求等；
• cameracharacteristics ：用于描述摄像头所支持的各种特性；
• cameracapturesession ：当程序需要预览、拍照时，都需要先通过 cameracapturesession 来实现。该会话通过调用方法 setrepeatingrequest() 实现预览；
• camerarequest ：代表一次捕获请求，用于描述捕获图片的各种参数设置；
• camerarequest.builder ：负责生成 camerarequest 对象。

2. 相机预览

下面通过源码来讲解如何使用 camera2 来实现相机的预览功能。

2.1 相机权限设置

<uses-permission android:name="android.permission.camera" />

2.2 app 布局

• activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<framelayout  xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#000"
    tools:context=".mainactivity">
</framelayout>

• fragment_camera.xml

<?xml version="1.0" encoding="utf-8"?>
<relativelayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".camerafragment">

    <com.lightweh.camera2preview.autofittextureview
        android:id="@+id/textureview"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centervertical="true"
        android:layout_centerhorizontal="true" />

</relativelayout>

2.3 相机自定义view

public class autofittextureview extends textureview {

    private int mratiowidth = 0;
    private int mratioheight = 0;

    public autofittextureview(context context) {
        this(context, null);
    }

    public autofittextureview(context context, attributeset attrs) {
        this(context, attrs, 0);
    }

    public autofittextureview(context context, attributeset attrs, int defstyle) {
        super(context, attrs, defstyle);
    }

    public void setaspectratio(int width, int height) {
        if (width < 0 || height < 0) {
            throw new illegalargumentexception("size cannot be negative.");
        }
        mratiowidth = width;
        mratioheight = height;
        requestlayout();
    }

    @override
    protected void onmeasure(int widthmeasurespec, int heightmeasurespec) {
        super.onmeasure(widthmeasurespec, heightmeasurespec);
        int width = measurespec.getsize(widthmeasurespec);
        int height = measurespec.getsize(heightmeasurespec);
        if (0 == mratiowidth || 0 == mratioheight) {
            setmeasureddimension(width, height);
        } else {
            if (width < height * mratiowidth / mratioheight) {
                setmeasureddimension(width, width * mratioheight / mratiowidth);
            } else {
                setmeasureddimension(height * mratiowidth / mratioheight, height);
            }
        }
    }
}

2.4 动态申请相机权限

public class mainactivity extends appcompatactivity {

    private static final int request_permission = 1;

    @override
    protected void oncreate(bundle savedinstancestate) {
        super.oncreate(savedinstancestate);
        setcontentview(r.layout.activity_main);

        if (haspermission()) {
            if (null == savedinstancestate) {
                setfragment();
            }
        } else {
            requestpermission();
        }
    }

    @override
    public void onrequestpermissionsresult(int requestcode, string permissions[], int[] grantresults) {

        if (requestcode == request_permission) {
            if (grantresults.length == 1 && grantresults[0] == packagemanager.permission_granted) {
                setfragment();
            } else {
                requestpermission();
            }
        } else {
            super.onrequestpermissionsresult(requestcode, permissions, grantresults);
        }
    }
    // 权限判断，当系统版本大于23时，才有必要判断是否获取权限
    private boolean haspermission() {
        if (build.version.sdk_int >= build.version_codes.m) {
            return checkselfpermission(manifest.permission.camera) == packagemanager.permission_granted;
        } else {
            return true;
        }
    }
    // 请求相机权限
    private void requestpermission() {
        if (build.version.sdk_int >= build.version_codes.m) {
            if (shouldshowrequestpermissionrationale(manifest.permission.camera)) {
                toast.maketext(mainactivity.this, "camera permission are required for this demo", toast.length_long).show();
            }
            requestpermissions(new string[]{manifest.permission.camera}, request_permission);
        }
    }
    // 启动相机fragment
    private void setfragment() {
        getsupportfragmentmanager()
                .begintransaction()
                .replace(r.id.container, camerafragment.newinstance())
                .commitnowallowingstateloss();
    }
}

2.5 开启相机预览

首先，在onresume()中，我们需要开启一个 handlerthread，然后利用该线程的 looper 对象构建一个 handler 用于相机回调。

@override
public void onresume() {
    super.onresume();
    startbackgroundthread();

    // when the screen is turned off and turned back on, the surfacetexture is 
    // already available, and "onsurfacetextureavailable" will not be called. in 
    // that case, we can open a camera and start preview from here (otherwise, we 
    // wait until the surface is ready in the surfacetexturelistener).
    if (mtextureview.isavailable()) {
        opencamera(mtextureview.getwidth(), mtextureview.getheight());
    } else {
        mtextureview.setsurfacetexturelistener(msurfacetexturelistener);
    }
}

private void startbackgroundthread() {
    mbackgroundthread = new handlerthread("camerabackground");
    mbackgroundthread.start();
    mbackgroundhandler = new handler(mbackgroundthread.getlooper());
}

同时，在 onpause() 中有对应的 handlerthread 关闭方法。

当屏幕关闭后重新开启，surfacetexture 已经就绪，此时不会触发 onsurfacetextureavailable 回调。因此，我们判断 mtextureview 如果可用，则直接打开相机，否则等待 surfacetexture 回调就绪后再开启相机。

private void opencamera(int width, int height) {
    if (contextcompat.checkselfpermission(getactivity(), manifest.permission.camera)
            != packagemanager.permission_granted) {
        return;
    }
    setupcameraoutputs(width, height);
    configuretransform(width, height);
    activity activity = getactivity();
    cameramanager manager = (cameramanager) activity.getsystemservice(context.camera_service);
    try {
        if (!mcameraopencloselock.tryacquire(2500, timeunit.milliseconds)) {
            throw new runtimeexception("time out waiting to lock camera opening.");
        }
        manager.opencamera(mcameraid, mstatecallback, mbackgroundhandler);
    } catch (cameraaccessexception e) {
        e.printstacktrace();
    } catch (interruptedexception e) {
        throw new runtimeexception("interrupted while trying to lock camera opening.", e);
    }
}

开启相机时，我们首先判断是否具备相机权限，然后调用 setupcameraoutputs 函数对相机参数进行设置（包括指定摄像头、相机预览方向以及预览尺寸的设定等），接下来调用 configuretransform 函数对预览图片的大小和方向进行调整，最后获取 cameramanager 对象开启相机。因为相机有可能会被其他进程同时访问，所以在开启相机时需要加锁。

private final cameradevice.statecallback mstatecallback = new cameradevice.statecallback() {

    @override
    public void onopened(@nonnull cameradevice cameradevice) {
        mcameraopencloselock.release();
        mcameradevice = cameradevice;
        createcamerapreviewsession();
    }

    @override
    public void ondisconnected(@nonnull cameradevice cameradevice) {
        mcameraopencloselock.release();
        cameradevice.close();
        mcameradevice = null;
    }

    @override
    public void onerror(@nonnull cameradevice cameradevice, int error) {
        mcameraopencloselock.release();
        cameradevice.close();
        mcameradevice = null;
        activity activity = getactivity();
        if (null != activity) {
            activity.finish();
        }
    }
};

相机开启时还会指定相机的状态变化回调函数 mstatecallback，如果相机成功开启，则开始创建相机预览会话。

private void createcamerapreviewsession() {
    try {
        // 获取 texture 实例
        surfacetexture texture = mtextureview.getsurfacetexture();
        assert texture != null;

        // 设置 textureview 缓冲区大小
        texture.setdefaultbuffersize(mpreviewsize.getwidth(), mpreviewsize.getheight());

        // 获取 surface 显示预览数据
        surface surface = new surface(texture);

        // 构建适合相机预览的请求
        mpreviewrequestbuilder = mcameradevice.createcapturerequest(cameradevice.template_preview);
        
        // 设置 surface 作为预览数据的显示界面
        mpreviewrequestbuilder.addtarget(surface);

        // 创建相机捕获会话用于预览
        mcameradevice.createcapturesession(arrays.aslist(surface),
                new cameracapturesession.statecallback() {

                    @override
                    public void onconfigured(@nonnull cameracapturesession cameracapturesession) {
                        // 如果相机关闭则返回
                        if (null == mcameradevice) {
                            return;
                        }

                        // 如果会话准备好则开启预览
                        mcapturesession = cameracapturesession;
                        try {
                            // 自动对焦
                          mpreviewrequestbuilder.set(capturerequest.control_af_mode,
                                 capturerequest.control_af_mode_continuous_picture);

                            mpreviewrequest = mpreviewrequestbuilder.build();
                            // 设置反复捕获数据的请求，预览界面一直显示画面
                            mcapturesession.setrepeatingrequest(mpreviewrequest,
                                    null, mbackgroundhandler);
                        } catch (cameraaccessexception e) {
                            e.printstacktrace();
                        }
                    }

                    @override
                    public void onconfigurefailed(
                            @nonnull cameracapturesession cameracapturesession) {
                        showtoast("failed");
                    }
                }, null
        );
    } catch (cameraaccessexception e) {
        e.printstacktrace();
    }
}

以上便是 camera2 api 实现相机预览的主要过程。

3. demo 源码

github：camera2preview

4. 参考

• https://github.com/googlesamples/android-camera2basic