Remote models

The Christmas break came and I have finally found some free time to finish some little projects and experiments I started some time ago and I decided to publish preliminary (yet working) results of one of them.

The problem

Qt4 supports the model-view paradigm through the Interview framework, but unfortunately it doesn’t handle models which store data on some remote host. “Hey, of course it can operate on models with remote datasets – just take a look at the SQL models available!” – one might say and basically this is true. But this is just a partial truth – have you tried using those models (for example QSqlTableModel) on a large dataset, let’s say… with thousand records holding some image and textual data? The problem with SQL models is that when the model initialises (when select() is called) it retrieves all the data from the SQL DBMS and blocks the GUI until all of the data is ready. This is not a big problem with local databases – 1k records of 50kB of data each gives 50MB of data that needs to be transfered – over a Fast Ethernet (100Mbps) network it’ll take not more than a few seconds, but try doing the same using a 1Mb network link… After 6 minutes your model will be ready and your application could continue.

Possible solutions

The problem would be easy to overcome if one of two things were true – either QSqlQuery would work asynchronously or QSqlTableModel would begin with an empty model, execute the query in the background and fill the model after the data is ready. The result of both of these solutions would be simmilar – the data would be added to the model dynamically as it is retrieved from the distant server.
Unfortunately QSql*Model classes don’t support such behaviours. In general the same problem applies to all data sources that are blocking.
The proper solution to the problem is to use a thread or set of threads that fetch the data in the background and insert it into the model using signals and slots. Because of queued connections across threads, updating the model is thread safe and it doesn’t block the GUI thread.

As a proof of concept I implemented a subclass of QAbstractTableModel that uses a thread and a simple updater object to transmit data between the application and a distant data storage. The concept uses signals and custom events to do the job and developers only need to implement the updater object and call proper methods in the model.

How does it work?

Here is an example implementation of a remote model:

class MyModel : public RemoteTableModel {
public:
  MyModel() : RemoteTableModel(){
    setUpdaterFactory(new MyUpdaterFactory()); // terrible hack here
    start(); // start filling the model
  }
  int columnCount ( const QModelIndex & parent = QModelIndex() ) const{
    if(parent.isValid()) return 0;
    return 2; // two column flat model
  }
  int rowCount( const QModelIndex &parent = QModelIndex() ) const {
    if(parent.isValid()) return 0;
    return m_rows.count();
  }
  QVariant data ( const QModelIndex & index, int role = Qt::DisplayRole ) const{
    if(!index.isValid() || role!=Qt::DisplayRole) return QVariant();
    int row = index.row();
    if(row>=m_rows.count()) return QVariant();
    if(index.column()==1){
      return m_rows.at(row).title;
    } else {
      return m_rows.at(row).tid;
    }
  }
protected:
  void addRow(const QVariant &data){
    // add a row retrieved from a remote data source
    beginInsertRows(QModelIndex(), m_rows.size(), m_rows.size());
    QVariantList vlist = data.toList();
    m_rows << st(vlist.at(0).toString(), vlist.at(1).toInt());
    endInsertRows();
  }
private:
  /**
   *  Internal data structure
   */
  struct st {
    st(QString t, int i){ title = t; tid = i; }
    QString title; // column 1 data
    int tid;       // column 0 data
  };
  QList m_rows; // data container
};

One also needs to implement an updater object that will do the actual fetching and storing. You can see an example in the tar bundle attached at the end of this post. Basically what it does is to fetch a list of threads in QtCentre’s Qt Programming forum using QtCentre’s archive features over the HTTP protocol. QHttp works asynchronously so I could implement that particular example directly without using threads too, but it’s just an example – I use it successfully to fetch and store data in a SQL database, but I don’t have a public database to use in an example, so the HTTP example has to suffice for now.

Next thing that needs to be done is to split the data transfer functionality from the model interface so that it could be used for hierarchical models or even some other things as well and to clean the implementation a little
(now I’m using an ugly hack to prevent creating a QObject in the context of a wrong thread).

Example code

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One Response to “Remote models”

  1. pavels says:

    Thank you. You made my day.

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