Showing QGIS map in Qt/C++ custom widget with QgsMapCanvas?

Question

I'm trying to add a QGis map in Qt/C++ widget using QgsMapCanvas, but I have some problems...

I'm working with the following environment: - Ubuntu 12.04 - QGis 2.8.1 - Qt Creator 2.4.1 (based on Qt 4.8.0 - 32 bit)

I found online a sample code that should do what I'm looking for, but something doesn't work..

The code is the following:

int main(int argc, char **argv)
{
    QgsApplication app(argc, argv, true);

    QString myPluginsDir        = "/home/pesce/qgis";
    QString myLayerPath         = "/home/pesce/qgis_sample_data/shapefiles/alaska.shp";
    QString myLayerBaseName     = "al";
    QString myProviderName      = "ogr";

    // Instantiate Provider Registry
    QgsProviderRegistry * preg = QgsProviderRegistry::instance(myPluginsDir);

    QgsVectorLayer *mypLayer =  new QgsVectorLayer(myLayerPath,myLayerBaseName,myProviderName);

    QgsSingleSymbolRendererV2 *mypRenderer = new QgsSingleSymbolRendererV2(QgsSymbolV2::defaultSymbol(mypLayer->geometryType()));

    QList <QgsMapCanvasLayer> myLayerSet;

    mypLayer->setRendererV2(mypRenderer);

    if(mypLayer->isValid()){
        qDebug("Layer is valid");
    } else {
        qDebug("Layer is NOT valid");
    }

    // Add the Vector Layer to the Layer Registry
    QgsMapLayerRegistry::instance()->addMapLayer(mypLayer, TRUE);
    // Add the Layer to the Layer Set
    myLayerSet.append(QgsMapCanvasLayer(mypLayer, TRUE));

    // Create the Map Canvas
    QgsMapCanvas * mypMapCanvas = new QgsMapCanvas(0, 0);
    mypMapCanvas->setExtent(mypLayer->extent());
    mypMapCanvas->enableAntiAliasing(true);
    mypMapCanvas->setCanvasColor(QColor(255, 255, 255));
    mypMapCanvas->freeze(false);
    // Set the Map Canvas Layer Set
    mypMapCanvas->setLayerSet(myLayerSet);
    mypMapCanvas->setVisible(true);
    mypMapCanvas->refresh();

    return app.exec();
}

The code is correctly compiled and the program correctly starts, but the map is empty..nothing is show... the selected layer (/home/pesce/qgis_sample_data/shapefiles/alaska.shp) seems to be not valid (mypLayer->isValid() returns "false")... but it's a .shp file....it should be valid!

Which kind of files are supported?

Answer 1

Make sure you have setup the environment for QGIS properly before you start using the providers.

In particular you will have to call

QgsApplication::setPrefixPath( prefixPath )
QgsApplication::initQgis()

before using providers.

See also this answer

Answer 2

i think it is your method when you delcare your myplayer.

try this way :

QString myFileName = QFileDialog::getOpenFileName(this, tr("Open File"),
                       QCoreApplication::applicationDirPath () + "/data",
                       tr("GeoTiff (*.shp)"));
  QFileInfo myRasterFileInfo(myFileName);

  qDebug() << "myFileName =" << myFileName;

QgsVectorLayer * mypLayer =  new QgsVectorLayer(myRasterFileInfo.filePath(), myRasterFileInfo.completeBaseName(), myProviderName);

QgsSingleSymbolRendererV2 *mypRenderer = new QgsSingleSymbolRendererV2(QgsSymbolV2::defaultSymbol(mypLayer->geometryType()));

it works in 2.8