最近發現從QDataStream向QByteArray中寫入數據常常是寫不進去的，通過查看QT的源碼:

 

QDataStream &operator>>(QDataStream &in, QByteArray &ba)
{
    ba.clear();
    quint32 len;
    in >> len;
    if (len == 0xffffffff)
        return in;

    const quint32 Step = 1024 * 1024;
    quint32 allocated = 0;

    do {
        int blockSize = qMin(Step, len - allocated);
        ba.resize(allocated + blockSize);
        if (in.readRawData(ba.data() + allocated, blockSize) != blockSize) {
            ba.clear();
            in.setStatus(QDataStream::ReadPastEnd);
            return in;
        }
        allocated += blockSize;
    } while (allocated < len);

    return in;
}

 

發現原來其中有一句：

 

1	if  (in.readRawData(ba.data() + allocated, blockSize) != blockSize) {

它檢查讀入的數據是不是整數個blockSize，如果不是，那么下面它會將QByteArray清空：

 

1	ba.clear();

 

所以如果想從QDataStream讀入數據島QByteArray中，可以采用下面的方法：

 

QDataStream &operator>>(QDataStream &in, QByteArray &ba)
{
    ba.clear();
    quint32 len;
    in >> len;
    if (len == 0xffffffff)
        return in;

    const quint32 Step = 1024 * 1024;
    quint32 allocated = 0;

    do {
        int blockSize = qMin(Step, len - allocated);
        ba.resize(allocated + blockSize);
        if (in.readRawData(ba.data() + allocated, blockSize) < blockSize) {
            return in;
        }
        allocated += blockSize;
    } while (allocated < len);

    return in;
}

 

 

最后回過頭來看一下將QByteArray寫入QDataStream的方法

 

QDataStream &operator<<(QDataStream &out, const QByteArray &ba)
{
    if (ba.isNull() && out.version() >= 6) {
        out << (quint32)0xffffffff;
        return out;
    }
    return out.writeBytes(ba, ba.size());
}

 

其實是將QByteArray中所有的數據都寫入QDatStream中，並沒有擴展QByteArray的大小到blockSize，

所以針對這種情況，我們在使用QDataStream向QByteArray中讀寫數據的時候，需要手動將QByteArray

擴展到blockSize(1024*1024)大小。