大家都知道MMKV性能好，因為是直接操作內存。
內存其實就是一個數(shù)組結構，根據(jù)地址去尋址查找數(shù)據(jù)的，就跟數(shù)組通過index查找數(shù)據(jù)一樣，
但是數(shù)組修改是很不方便的，比如刪除中間一個數(shù)據(jù)，受影響的都要往前移動，會影響性能。
MMKV每次修改數(shù)據(jù)，不會去修改文件原有數(shù)據(jù)，而是在尾部追加。
但這樣會導致文件無限膨脹，MMKV在每次追加數(shù)據(jù)的時候，會去檢查數(shù)據(jù)和文件大小。
修改數(shù)據(jù)的時候檢查大小，完成檢查整理擴容之后，再追加數(shù)據(jù)：

bool MMKV::appendDataWithKey(const MMBuffer &data, MMKVKey_t key) {
#ifdef MMKV_APPLE
    auto keyData = [key dataUsingEncoding:NSUTF8StringEncoding];
    size_t keyLength = keyData.length;
#else
    size_t keyLength = key.length();
#endif
    // size needed to encode the key
    size_t size = keyLength + pbRawVarint32Size((int32_t) keyLength);
    // size needed to encode the value
    size += data.length() + pbRawVarint32Size((int32_t) data.length());

    SCOPED_LOCK(m_exclusiveProcessLock);

    bool hasEnoughSize = ensureMemorySize(size);
    if (!hasEnoughSize || !isFileValid()) {
        return false;
    }

#ifdef MMKV_IOS
    auto ret = protectFromBackgroundWriting(m_output->curWritePointer(), size, ^{
      m_output->writeData(MMBuffer(keyData, MMBufferNoCopy));
      m_output->writeData(data); // note: write size of data
    });
    if (!ret) {
        return false;
    }
#else
#ifdef MMKV_APPLE
    m_output->writeData(MMBuffer(keyData, MMBufferNoCopy));
#else
    m_output->writeString(key);
#endif
    m_output->writeData(data); // note: write size of data
#endif

    auto ptr = (uint8_t *) m_file->getMemory() + Fixed32Size + m_actualSize;
    if (m_crypter) {
        m_crypter->encrypt(ptr, ptr, size);
    }
    m_actualSize += size;
    updateCRCDigest(ptr, size);

    return true;
}

當前表數(shù)據(jù)超過文件大小，或者表數(shù)據(jù)的1.5倍超過文件大小，則將文件大小翻倍，直到滿足上述條件：

// since we use append mode, when -[setData: forKey:] many times, space may not be enough
// try a full rewrite to make space
bool MMKV::ensureMemorySize(size_t newSize) {
    if (!isFileValid()) {
        MMKVWarning("[%s] file not valid", m_mmapID.c_str());
        return false;
    }

    // make some room for placeholder
    constexpr size_t ItemSizeHolderSize = 4;
    if (m_dic.empty()) {
        newSize += ItemSizeHolderSize;
    }
    if (newSize >= m_output->spaceLeft() || m_dic.empty()) {
        // try a full rewrite to make space
        auto fileSize = m_file->getFileSize();
        MMBuffer data = MiniPBCoder::encodeDataWithObject(m_dic);
        size_t lenNeeded = data.length() + Fixed32Size + newSize;
        size_t avgItemSize = lenNeeded / std::max<size_t>(1, m_dic.size());
        size_t futureUsage = avgItemSize * std::max<size_t>(8, (m_dic.size() + 1) / 2);
        // 1. no space for a full rewrite, double it
        // 2. or space is not large enough for future usage, double it to avoid frequently full rewrite
        if (lenNeeded >= fileSize || (lenNeeded + futureUsage) >= fileSize) {
            size_t oldSize = fileSize;
            do {
                fileSize *= 2;
            } while (lenNeeded + futureUsage >= fileSize);
            MMKVInfo("extending [%s] file size from %zu to %zu, incoming size:%zu, future usage:%zu", m_mmapID.c_str(),
                     oldSize, fileSize, newSize, futureUsage);

            // if we can't extend size, rollback to old state
            if (!m_file->truncate(fileSize)) {
                return false;
            }

            // check if we fail to make more space
            if (!isFileValid()) {
                MMKVWarning("[%s] file not valid", m_mmapID.c_str());
                return false;
            }
        }
        return doFullWriteBack(std::move(data));
    }
    return true;
}

最后將整個表數(shù)據(jù)以pb格式全部寫入文件:

bool MMKV::doFullWriteBack(MMBuffer &&allData) {
#ifdef MMKV_IOS
    unsigned char oldIV[AES_KEY_LEN];
    unsigned char newIV[AES_KEY_LEN];
    if (m_crypter) {
        memcpy(oldIV, m_crypter->m_vector, sizeof(oldIV));
#else
    unsigned char newIV[AES_KEY_LEN];
    if (m_crypter) {
#endif
        AESCrypt::fillRandomIV(newIV);
        m_crypter->resetIV(newIV, sizeof(newIV));
        auto ptr = allData.getPtr();
        m_crypter->encrypt(ptr, ptr, allData.length());
    }

    auto ptr = (uint8_t *) m_file->getMemory();
    delete m_output;
    m_output = new CodedOutputData(ptr + Fixed32Size, m_file->getFileSize() - Fixed32Size);
#ifdef MMKV_IOS
    auto ret = protectFromBackgroundWriting(m_output->curWritePointer(), allData.length(), ^{
      m_output->writeRawData(allData); // note: don't write size of data
    });
    if (!ret) {
        // revert everything
        if (m_crypter) {
            m_crypter->resetIV(oldIV);
        }
        delete m_output;
        m_output = new CodedOutputData(ptr + Fixed32Size, m_file->getFileSize() - Fixed32Size);
        m_output->seek(m_actualSize);
        return false;
    }
#else
    m_output->writeRawData(allData); // note: don't write size of data
#endif

    m_actualSize = allData.length();
    if (m_crypter) {
        recaculateCRCDigestWithIV(newIV);
    } else {
        recaculateCRCDigestWithIV(nullptr);
    }
    m_hasFullWriteback = true;
    // make sure lastConfirmedMetaInfo is saved
    sync(MMKV_SYNC);
    return true;
}