如何从 C++ 代码请求 Android NDK 相机权限？

Question

我正在用纯 C++ 编写应用程序，并且我有打开相机的代码。并将 AndroidManifest.xml 设置为：

<uses-permission android:name="android.permission.CAMERA"/>
<uses-feature android:name="android.hardware.camera2" android:required="true" />

当运行第一次应用时，它不提供开启权限的提示。安装后我必须手动完成，设置->应用程序->"MyApp"->权限。

如何在不引入 java 代码的情况下在 C++ 中提供提示？非常感谢所有帮助。

Answer 1

原来我也看到了这个

要手动完成，您也可以通过命令行使用 adb shell pm grant com.package.name android.permission.CAMERA
检查是否有权限adb shell dumpsys package com.package.name
This is the series of steps to ask for permissions with Java
要弄清楚如何在没有 Java 的情况下做到这一点，您需要深入研究 AOSP 以了解它是如何 working under the hood... 这并非易事找不到示例。

Answer 2

这可以使用 JNI 调用来完成。请参阅下面的 READ_EXTERNAL_STORAGE 和 WRITE_EXTERNAL_STORAGE 示例（可以轻松适应相机烫发）。

在应用程序启动时调用函数：

 check_android_permissions(struct android_app* app)

（来源如下）。

几个陷阱：

android.Manifest.permission 是嵌套在 android.Manifest 中的 class，因此它的 JNI 名称是 "android/Manifest$permission".
我没有成功访问 ContextCompat (android.support.v4.content.ContextCompat) 和 ActivityCompat (android.support.v4.app.ActivityCompat ) 来自 JNI，所以我使用了 Context (android.content.Context) 和 Activity (android.app.Activity) 直接。因此，需要 Android API 级别 23（Marshmallow，2015 年 5 月）。
需要在classes中检索常量，特别是READ_EXTERNAL_STORAGE和WRITE_EXTERNAL_STORAGE在android.Manifest.permission中是静态字符串，而PERMISSION_GRANTEDa android.content.pm.PackageManager.
我没有实现当用户点击 'do not ask me again' 按钮时如何处理，这需要实现 onRequestPermissionsResult() 回调。



/**
 * \brief Gets the internal name for an android permission.
 * \param[in] lJNIEnv a pointer to the JNI environment
 * \param[in] perm_name the name of the permission, e.g.,
 *   "READ_EXTERNAL_STORAGE", "WRITE_EXTERNAL_STORAGE".
 * \return a jstring with the internal name of the permission,
 *   to be used with android Java functions 
 *   Context.checkSelfPermission() or Activity.requestPermissions()
 */
jstring android_permission_name(JNIEnv* lJNIEnv, const char* perm_name) {
    // nested class permission in class android.Manifest,
    // hence android 'slash' Manifest 'dollar' permission
    jclass ClassManifestpermission = lJNIEnv->FindClass(
       "android/Manifest$permission"
    );
    jfieldID lid_PERM = lJNIEnv->GetStaticFieldID(
       ClassManifestpermission, perm_name, "Ljava/lang/String;"
    );
    jstring ls_PERM = (jstring)(lJNIEnv->GetStaticObjectField(
        ClassManifestpermission, lid_PERM
    )); 
    return ls_PERM;
}

/**
 * \brief Tests whether a permission is granted.
 * \param[in] app a pointer to the android app.
 * \param[in] perm_name the name of the permission, e.g.,
 *   "READ_EXTERNAL_STORAGE", "WRITE_EXTERNAL_STORAGE".
 * \retval true if the permission is granted.
 * \retval false otherwise.
 * \note Requires Android API level 23 (Marshmallow, May 2015)
 */
bool android_has_permission(struct android_app* app, const char* perm_name) {
    JavaVM* lJavaVM = app->activity->vm;
    JNIEnv* lJNIEnv = nullptr; 
    bool lThreadAttached = false;

    // Get JNIEnv from lJavaVM using GetEnv to test whether
    // thread is attached or not to the VM. If not, attach it
    // (and note that it will need to be detached at the end
    //  of the function).
    switch (lJavaVM->GetEnv((void**)&lJNIEnv, JNI_VERSION_1_6)) {
      case JNI_OK:
        break;
      case JNI_EDETACHED: {
        jint lResult = lJavaVM->AttachCurrentThread(&lJNIEnv, nullptr);
        if(lResult == JNI_ERR) {
          throw std::runtime_error("Could not attach current thread");
        }
        lThreadAttached = true;
      } break;
      case JNI_EVERSION:
        throw std::runtime_error("Invalid java version");
    }

    bool result = false;

    jstring ls_PERM = android_permission_name(lJNIEnv, perm_name);

    jint PERMISSION_GRANTED = jint(-1);
    {
       jclass ClassPackageManager = lJNIEnv->FindClass(
          "android/content/pm/PackageManager"
       );
       jfieldID lid_PERMISSION_GRANTED = lJNIEnv->GetStaticFieldID(
          ClassPackageManager, "PERMISSION_GRANTED", "I"
       );
       PERMISSION_GRANTED = lJNIEnv->GetStaticIntField(
          ClassPackageManager, lid_PERMISSION_GRANTED
       );
    }
    {
       jobject activity = app->activity->clazz;
       jclass ClassContext = lJNIEnv->FindClass(
          "android/content/Context"
       );
       jmethodID MethodcheckSelfPermission = lJNIEnv->GetMethodID(
          ClassContext, "checkSelfPermission", "(Ljava/lang/String;)I"
       );
       jint int_result = lJNIEnv->CallIntMethod(
           activity, MethodcheckSelfPermission, ls_PERM
       );
       result = (int_result == PERMISSION_GRANTED);
    }

    if(lThreadAttached) {
      lJavaVM->DetachCurrentThread();
    }

    return result;
}

/**
 * \brief Query file permissions.
 * \details This opens the system dialog that lets the user
 *  grant (or deny) the permission.
 * \param[in] app a pointer to the android app.
 * \note Requires Android API level 23 (Marshmallow, May 2015)
 */
void android_request_file_permissions(struct android_app* app) {
    JavaVM* lJavaVM = app->activity->vm;
    JNIEnv* lJNIEnv = nullptr; 
    bool lThreadAttached = false;

    // Get JNIEnv from lJavaVM using GetEnv to test whether
    // thread is attached or not to the VM. If not, attach it
    // (and note that it will need to be detached at the end
    //  of the function).
    switch (lJavaVM->GetEnv((void**)&lJNIEnv, JNI_VERSION_1_6)) {
      case JNI_OK:
        break;
      case JNI_EDETACHED: {
        jint lResult = lJavaVM->AttachCurrentThread(&lJNIEnv, nullptr);
        if(lResult == JNI_ERR) {
          throw std::runtime_error("Could not attach current thread");
        }
        lThreadAttached = true;
      } break;
      case JNI_EVERSION:
        throw std::runtime_error("Invalid java version");
      }

    jobjectArray perm_array = lJNIEnv->NewObjectArray(
      2,
      lJNIEnv->FindClass("java/lang/String"),
      lJNIEnv->NewStringUTF("")
    );

    lJNIEnv->SetObjectArrayElement(
      perm_array, 0,
      android_permission_name(lJNIEnv, "READ_EXTERNAL_STORAGE")
    );

    lJNIEnv->SetObjectArrayElement(
      perm_array, 1,
      android_permission_name(lJNIEnv, "WRITE_EXTERNAL_STORAGE")        
    );

    jobject activity = app->activity->clazz;

    jclass ClassActivity = lJNIEnv->FindClass(
       "android/app/Activity"
    );

    jmethodID MethodrequestPermissions = lJNIEnv->GetMethodID(
       ClassActivity, "requestPermissions", "([Ljava/lang/String;I)V"
    );

    // Last arg (0) is just for the callback (that I do not use)
    lJNIEnv->CallVoidMethod(
       activity, MethodrequestPermissions, perm_array, 0
    );

    if(lThreadAttached) {
       lJavaVM->DetachCurrentThread();
    }
}

void check_android_permissions(struct android_app* app) {
    bool OK = android_has_permission(
       app, "READ_EXTERNAL_STORAGE"
    ) && android_has_permission(
       app, "WRITE_EXTERNAL_STORAGE"
    );
    if(!OK) {
       android_request_file_permissions(app);
    }
}

如何从 C++ 代码请求 Android NDK 相机权限？

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