Qt 4.8: Fetch More Example

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- Qt 4.8
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# Fetch More Example

Files:

- itemviews/fetchmore/filelistmodel.cpp
- itemviews/fetchmore/filelistmodel.h
- itemviews/fetchmore/window.cpp
- itemviews/fetchmore/window.h
- itemviews/fetchmore/main.cpp
- itemviews/fetchmore/fetchmore.pro

The Fetch More example shows how two add items to an item view model on demand.

The user of the example can enter a directory in the Directory line edit. The contents of the directory will be listed in the list view below.

When you have large - or perhaps even infinite - data sets, you will need to add items to the model in batches, and preferably only when the items are needed by the view (i.e., when they are visible in the view).

In this example, we implement `FileListModel` - an item view model containing the entries of a directory. We also have `Window`, which sets up the GUI and feeds the model with directories.

Let's take a tour of `FileListModel`'s code.

## FileListModel Class Definition

The `FileListModel` inherits QAbstractListModel and contains the contents of a directory. It will add items to itself only when requested to do so by the view.

     class FileListModel : public QAbstractListModel
     {
         Q_OBJECT

     public:
         FileListModel(QObject *parent = 0);

         int rowCount(const QModelIndex &parent = QModelIndex()) const;
         QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const;

     signals:
         void numberPopulated(int number);

     public slots:
         void setDirPath(const QString &path);

     protected:
         bool canFetchMore(const QModelIndex &parent) const;
         void fetchMore(const QModelIndex &parent);

     private:
         QStringList fileList;
         int fileCount;
     };

The secret lies in the reimplementation of fetchMore() and canFetchMore() from QAbstractItemModel. These functions are called by the item view when it needs more items.

The `setDirPath()` function sets the directory the model will work on. We emit `numberPopulated()` each time we add a batch of items to the model.

We keep all directory entries in `fileList`. `fileCount` is the number of items that have been added to the model.

## FileListModel Class Implementation

We start by checking out the `setDirPath()`.

     void FileListModel::setDirPath(const QString &path)
     {
         QDir dir(path);

         fileList = dir.entryList();
         fileCount = 0;
         reset();
     }

We use a QDir to get the contents of the directory. We need to inform QAbstractItemModel that we want to remove all items - if any - from the model.

     bool FileListModel::canFetchMore(const QModelIndex & /* index */) const
     {
         if (fileCount < fileList.size())
             return true;
         else
             return false;
     }

The `canFetchMore()` function is called by the view when it needs more items. We return true if there still are entries that we have not added to the model; otherwise, we return false.

And now, the `fetchMore()` function itself:

     void FileListModel::fetchMore(const QModelIndex & /* index */)
     {
         int remainder = fileList.size() - fileCount;
         int itemsToFetch = qMin(100, remainder);

         beginInsertRows(QModelIndex(), fileCount, fileCount+itemsToFetch-1);

         fileCount += itemsToFetch;

         endInsertRows();

         emit numberPopulated(itemsToFetch);
     }

We first calculate the number of items to fetch. beginInsertRows() and endInsertRows() are mandatory for QAbstractItemModel to keep up with the row insertions. Finally, we emit `numberPopulated()`, which is picked up by `Window`.

To complete the tour, we also look at `rowCount()` and `data()`.

     int FileListModel::rowCount(const QModelIndex & /* parent */) const
     {
         return fileCount;
     }

     QVariant FileListModel::data(const QModelIndex &index, int role) const
     {
         if (!index.isValid())
             return QVariant();

         if (index.row() >= fileList.size() || index.row() < 0)
             return QVariant();

         if (role == Qt::DisplayRole)
             return fileList.at(index.row());
         else if (role == Qt::BackgroundRole) {
             int batch = (index.row() / 100) % 2;
             if (batch == 0)
                 return qApp->palette().base();
             else
                 return qApp->palette().alternateBase();
         }
         return QVariant();
     }

Notice that the row count is only the items we have added so far, i.e., not the number of entries in the directory.

In `data()`, we return the appropriate entry from the `fileList`. We also separate the batches with a different background color.