Android：AssetFileDescriptor 类

一、概述

Android 应用程序是将代码逻辑和界面资源进行分离的，Android应用程序资源可以分为两大类，分别是 assets 和 res。

• assets类资源放在工程根目录的assets子目录下，它里面保存的是一些原始的文件，可以以任何方式来进行组织。这些文件最终会被原装不动地打包在apk文件中。如果我们要在程序中访问这些文件，那么就需要指定文件名来访问。
• res类资源放在工程根目录的res子目录下，它里面保存的文件大多数都会被编译，并且都会被赋予资源ID。这样我们就可以在程序中通过ID来访问res类的资源。

文件描述符 AssetFileDescriptor 是 AssetManager 的入口。提供了您自己打开的文件描述符，可用于读取数据，以及该文件中数据的偏移量和长度。常用方法如下(参考自官方文档)：

AssetFileDescriptor(ParcelFileDescriptor fd, long startOffset, long length)
AssetFileDescriptor(ParcelFileDescriptor fd, long startOffset, long length, Bundle extras)

close()：Convenience for calling getParcelFileDescriptor().close().

FileInputStream createInputStream()：Create and return a new auto-close input stream for this asset.This will either return a full asset AutoCloseInputStream, or an underlying ParcelFileDescriptor.AutoCloseInputStream depending on whether the the object represents a complete file or sub-section of a file. You should only call this once for a particular asset.

FileOutputStream createOutputStream()：Create and return a new auto-close output stream for this asset.This will either return a full asset AutoCloseOutputStream, or an underlying ParcelFileDescriptor.AutoCloseOutputStream depending on whether the the object represents a complete file or sub-section of a file. You should only call this once for a particular asset.

int describeContents()：Describe the kinds of special objects contained in this Parcelable instance's marshaled representation.For example, if the object will include a file descriptor in the output of writeToParcel(android.os.Parcel, int), the return value of this method must include the CONTENTS_FILE_DESCRIPTOR bit.

long getDeclaredLength()：Return the actual number of bytes that were declared when the AssetFileDescriptor was constructed. Will be UNKNOWN_LENGTH if the length was not declared, meaning data should be read to the end of the file.

Bundle getExtras()：Returns any additional details that can be used to interpret the underlying file descriptor.
FileDescriptor getFileDescriptor()：Returns the FileDescriptor that can be used to read the data in the file.
long getLength()：Returns the total number of bytes of this asset entry's data.May be UNKNOWN_LENGTH if the asset extends to the end of the file. If the AssetFileDescriptor was constructed with UNKNOWN_LENGTH, this will use ParcelFileDescriptor.getStatSize() to find the total size of the file, returning that number if found or UNKNOWN_LENGTH if it could not be determined.

ParcelFileDescriptor getParcelFileDescriptor()：The AssetFileDescriptor contains its own ParcelFileDescriptor, which in addition to the normal FileDescriptor object also allows you to close the descriptor when you are done with it.
long getStartOffset()：Returns the byte offset where this asset entry's data starts.
String toString()：Returns a string representation of the object.In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.

The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of: getClass().getName() + '@' + Integer.toHexString(hashCode())

void writeToParcel(Parcel out, int flags)：Flatten this object in to a Parcel.

二、实例

1、使用 AssetFileDescriptor 来读取(android)app的raw文件夹下的数据

try {
    AssetFileDescriptor fd = getResources().openRawResouceFd(R.raw.greet_1);
    mp.setDataSource(fd.getFileDescriptor(), fd.getStartOffset(), fd.getLength());
    mp.start();
    fd.close();
} catch (IllegalArgumentException e) {
    // handle exception
} catch (IllegalStateException e) {
    // handle exception
} catch (IOException e) {
    // handle exception
}

2、解码 bmp 图像

private static Bitmap decodeBitmap(Context context, Uri theUri, int sampleSize) {
    BitmapFactory.Options options = new BitmapFactory.Options();
    options.inSampleSize = sampleSize;

    AssetFileDescriptor fileDescriptor = null;
    try {
        fileDescriptor = context.getContentResolver().openAssetFileDescriptor(theUri, "r");
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    }

    Bitmap actuallyUsableBitmap = BitmapFactory.decodeFileDescriptor(
            fileDescriptor.getFileDescriptor(), null, options);

    Log.d(TAG, options.inSampleSize + " sample method bitmap ... " +
            actuallyUsableBitmap.getWidth() + " " + actuallyUsableBitmap.getHeight());

    return actuallyUsableBitmap;
}