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　　安卓N版本ASessionDescription::setTo函數原文：

bool ASessionDescription::setTo(const void *data, size_t size) {
    //調用parse函數來解析緩衝區的內容
    //布爾變量mIsValid用來表示該會話描述對象是否是有效的
    mIsValid = parse(data, size);

    if (!mIsValid) {
        //如果變量mIsValid的值爲fasle
        //說明parse函數解析遇到了錯誤
        //清楚mTracks和mFormats容器裏的內容
        //後面給出ASessionDescription.h的原文看下該兩個容器的定義
        mTracks.clear();
        mFormats.clear();
    }

    return mIsValid;
}

　　ASessionDescription.h的原文：
　　

#ifndef A_SESSION_DESCRIPTION_H_

#define A_SESSION_DESCRIPTION_H_

#include <sys/types.h>

#include <media/stagefright/foundation/ABase.h>
#include <utils/KeyedVector.h>
#include <utils/RefBase.h>
#include <utils/Vector.h>

namespace android {

struct AString;

struct ASessionDescription : public RefBase {
    ASessionDescription();

    bool setTo(const void *data, size_t size);
    bool isValid() const;

    // Actually, 1 + number of tracks, as index 0 is reserved for the
    // session description root-level attributes.
    size_t countTracks() const;
    void getFormat(size_t index, AString *value) const;

    void getFormatType(
            size_t index, unsigned long *PT,
            AString *desc, AString *params) const;

    bool getDimensions(
            size_t index, unsigned long PT,
            int32_t *width, int32_t *height) const;

    bool getDurationUs(int64_t *durationUs) const;

    static void ParseFormatDesc(
            const char *desc, int32_t *timescale, int32_t *numChannels);

    bool findAttribute(size_t index, const char *key, AString *value) const;

    // parses strings of the form
    //   npt      := npt-time "-" npt-time? | "-" npt-time
    //   npt-time := "now" | [0-9]+("." [0-9]*)?
    //
    // Returns true iff both "npt1" and "npt2" times were available,
    // i.e. we have a fixed duration, otherwise this is live streaming.
    static bool parseNTPRange(const char *s, float *npt1, float *npt2);

protected:
    virtual ~ASessionDescription();

private:
    //Attribs是一個類型爲KeyedVector<AString,AString>的容器
    //即key和value都是AString類型的容器
    typedef KeyedVector<AString,AString> Attribs;

    bool mIsValid;
    //mTracks是KeyedVector<AString,AString>類型容器的容器
    //即mTracks的每個元素是KeyedVector<AString,AString>類型的容器
    Vector<Attribs> mTracks;
    Vector<AString> mFormats;

    bool parse(const void *data, size_t size);

    DISALLOW_EVIL_CONSTRUCTORS(ASessionDescription);
};

}  // namespace android

#endif  // A_SESSION_DESCRIPTION_H_

　　小結：ASessionDescription對象的主要任務是通過定義的parse函數來解析對應緩衝區裏的內容，該內容是服務端的迴應消息裏攜帶的。