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2. 创建OpenSLES音频播放测试项目并完成引擎初始化~1

发布时间：2021-06-04 15:07:27编辑：雪饮阅读（）

 上篇中實現了安卓视频播放窗口去掉标题栏、全屏、横屏等的處理並在雷電模擬器4中進行了測試。但是目前來説視頻還有最致命的問題就是沒有聲音，雖然聲音也解碼了。

但是這裏音頻的播放還要依賴于OpenSL ES。
OpenSL ES (Open Sound Library for Embedded Systems)是无授权费、跨平台、针对嵌入式系统精心优化的硬件音频加速API。它为嵌入式移动多媒体设备上的本地应用程序开发者提供标准化, 高性能,低响应时间的音频功能实现方法，并实现软/硬件音频性能的直接跨平台部署，降低执行难度，促进高级音频市场的发展。简单来说OpenSL ES是一个嵌入式跨平台免费的音频处理库。
Android的OpenSL ES库是在NDK的platforms文件夹对应android平台先相应cpu类型里面。
這裏主要以一個獨立的項目實現下OpenSL ES 下的SLEngineItf引擎的創建，為上篇項目中的視頻中的音頻的同步播放打下堅實的基礎。
同樣新建一個支持c++的安卓項目于android studio中。
主要關注cpp/native-lib.cpp:
#include <jni.h>
#include <string>
#include <SLES/OpenSLES.h>
#include <SLES/OpenSLES_Android.h>
#include <android/log.h>
#define LOGD(FORMAT,...) __android_log_print(ANDROID_LOG_ERROR,"ywl5320",FORMAT,##__VA_ARGS__);
/*
 * SLObjectItf：OpenSL ES中最重要的接口類
 * */
static SLObjectItf engineSL = NULL;
/*
 * SLEngineItf:OpenSL ES中具体的引擎（接口類）
 * */
SLEngineItf CreateSL()
{
    /*SLresult：返回結果
     * 下面三個方法：
     * slCreateEngine、Realize、GetInterface
     * 返回結果都是SLresult類型，所以這裏統一定義一個返回結果類型
     * */
    SLresult re;
    SLEngineItf en;
    /*
     * slCreateEngine方法用於创建一个引擎接口对象，這裏一般只關注第一個參數，其餘5個參數一般設置為0即可
     * */
    re = slCreateEngine(&engineSL,0,0,0,0,0);
    if(re != SL_RESULT_SUCCESS) return NULL;
    //创建好引擎接口对象后，需要用SLObjectItf的Realize方法来实现engineObject,Realize方法的第二個參數async表示是否異步，這裏使用同步
    re = (*engineSL)->Realize(engineSL,SL_BOOLEAN_FALSE);
    if(re != SL_RESULT_SUCCESS) return NULL;
    //GetInterface方法来初始化SLEngnineItf对象实例，GetInterface的第二個參數是一個SL接口id，這裏一般用常量SL_IID_ENGINE
    re = (*engineSL)->GetInterface(engineSL,SL_IID_ENGINE,&en);
    if(re != SL_RESULT_SUCCESS) return NULL;
    return en;
}
extern "C" JNIEXPORT jstring JNICALL
Java_com_example_audio_1opensles_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */) {
    std::string hello = "Hello from C++";

   //1 创建引擎
    SLEngineItf eng = CreateSL();
    if(eng)
    {
        LOGD("CreateSL success！ ");
    }
    else
    {
        LOGD("CreateSL failed！ ");
    }
    return env->NewStringUTF(hello.c_str());
}
和cpp/ CMakeLists.txt:

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html

# Sets the minimum version of CMake required to build the native library.

cmake_minimum_required(VERSION 3.10.2)

# Declares and names the project.

project("audio_opensles")

# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds them for you.
# Gradle automatically packages shared libraries with your APK.

add_library( # Sets the name of the library.
             native-lib

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             native-lib.cpp )

# Searches for a specified prebuilt library and stores the path as a
# variable. Because CMake includes system libraries in the search path by
# default, you only need to specify the name of the public NDK library
# you want to add. CMake verifies that the library exists before
# completing its build.

find_library( # Sets the name of the path variable.
              log-lib

              # Specifies the name of the NDK library that
              # you want CMake to locate.
              log )

# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in this
# build script, prebuilt third-party libraries, or system libraries.

target_link_libraries( # Specifies the target library.
                       native-lib
                        OpenSLES
                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )

 


編譯並運行到雷電4上出現這一幕就算是引擎創建成功了。