unity使用文件流读取streamingassets下的资源

目的：读取streamingassets下的文件中指定的一段字节

已知：文件中的起始位置，和需要读取的长度

1.android下读取

1.1 不能直接使用C#的FileStream，读取失败

var buffer = new byte[size];
FileStream stream = File.OpenRead(path);
stream.Read(buffer , pos, size);

报错：IsolatedStorageException: Could not find a part of the path "/jar:file:/data/app/com.xxx.xxxx-1/base.apk!/assets/xxx.pack".

 

1.2 可以使用Unity原生接口与Android交互

主要过程：

Java

public class XXXPlugin extends UnityPlayerNativeActivity {
    protected AssetManager assetManager;
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		assetManager = getAssets();
    }

	//返回字节数组
	public byte[] LoadBytes(String path,int offset,int len)
	{
         //可以缓存起来，不需每次都open
         InputStream inputStream = assetManager.open(path);
         try {
                 byte buf[] = new byte[len];
                 inputStream.reset();
                 
                //注意skip、read的可靠性
                 inputStream.skip(offset);
                 inputStream.read(buf,0,len)

                 inputStream.close();
                 return buf;
    
			} 
		catch (IOException e) {
				Log.v ("unity", e.getMessage());
		}
		return null;
	}

}

注意skip、read的可靠性，每次调用不一定能返回正确长度，可能需多次调用。参考how-does-the-skip-method-in-inputstream-work

C#:

		
            public static byte[] read_streamingpath_bytes(string path,IntPtr ptr, int pos, int size) 
            {
                using (AndroidJavaClass cls = new AndroidJavaClass("com.XXX.XXXPlugin";) ) 
                {
                   AndroidJavaObject  m_AndroidJavaObject = cls.GetStatic("mainActivity");

                    byte[] s = m_AndroidJavaObject.Call("LoadBytes", path, pos, size);

                    return s;
                }
                return null;
            }

这种方法是在Java分配内存。

1.3 更灵活的方法，使用JNI，可以从C#传递指针到C++

在Android Studio中生成库libNativeLib.so文件，参考NativeReadBytes

C++

#include "com_XXX_NativeHelper.h"
#include 
#include 
#include 
#include 
#include 

#ifdef __cplusplus
extern "C" {
#endif

static AAssetManager *assetManager = nullptr;

JNIEXPORT void JNICALL Java_com_XXX_NativeHelper_SetAssetManager
        (JNIEnv *env, jobject jobj, jobject jassetManager) {
    assetManager = AAssetManager_fromJava(env, jassetManager);
}
 
JNIEXPORT int32_t JNICALL ReadAssetsBytesWithOffset(uint32_t pathKey, char* fileName, unsigned char** result, int32_t offset, int32_t length){
    if(assetManager == nullptr){
        return -1;
    }
    AAsset* asset = asset = AAssetManager_open(assetManager, fileName, AASSET_MODE_UNKNOWN);
    if(asset == nullptr){
        return -1;
    }
    off_t size = AAsset_getLength(asset);
    if(size > 0){
        try {
            AAsset_seek(asset, offset, SEEK_SET);
            AAsset_read(asset, *result, length);
        }catch (std::bad_alloc){
            *result = nullptr;
            return -1;
        }
    }
    AAsset_close(asset);
    return (int32_t)length;
}
#ifdef __cplusplus
}
#endif

Java

//XXXPlugin.java
public class XXXPlugin extends UnityPlayerNativeActivity {
    static{
        System.loadLibrary("NativeLib");
    }
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		mainActivity = this;
		
		
        //设置AssetManager， C++中要使用
		NativeHelper.SetAssetManager(getAssets());
   }

}

//NativeHelper.java
public class NativeHelper {
    public static native void SetAssetManager(AssetManager assetManager);
}

C#

    public class ReadNativeByte
    {

#if UNITY_ANDROID
        [DllImport("NativeLib")]
        public static extern int ReadAssetsBytesWithOffset(uint pathKey,string name, ref IntPtr ptr, int offset, int length);
#endif
    }

2.IOS下读取

可以直接在C#下读取，StreamAsseting只有读的权限，用OpenRead

byte[] bytes = new byte[len];
FileStream stream = File.OpenRead(path);
stream.Seek(offset, SeekOrigin.Begin);
stream.Read(bytes, 0, len);