为开发人员准备的文档,涵盖QGIS的多种功能实现和代码。

CHAPTER3

LoadingLayers

Let’sopensomelayerswithdata.QGISrecognizesvectorandrasterlayers.Additionally,customlayertypesare

available,butwearenotgoingtodiscussthemhere.

3.1VectorLayers

Toloadavectorlayer,specifylayer’sdatasourceidenti er,nameforthelayerandprovider’sname:

Thedatasourceidenti erisastringanditisspeci yer’snameisusedinthe

layerlistwidget.Itisimportanttocheckwhetherthelayerhasbeenloadedsuccessfully.Ifitwasnot,aninvalid

layerinstanceisreturned.

ThequickestwaytoopenanddisplayavectorlayerinQGISistheaddVectorLayerfunctionofthe

QgisInterface:

,"ogr")Thiscreatesanewlayerandaddsittothemaplayerregistry(makingitappearinthelayerlist)inonestep.The

functionreturnsthelayerinstanceorNoneifthelayercouldn’tbeloaded.

Thefollowinglistshowshowtoaccessvariousdatasourcesusingvectordataproviders:

OGRlibrary(shape lesandmanyother leformats)—data

sourceisthepathtothe le

)

PostGISdatabase—datasourceisastringwithallinformationneededtocreateaconnectiontoPostgreSQL

database.QgsDataSourceURIclasscangeneratethisstringforyou.NotethatQGIShastobecompiled

withPostgressupport,otherwisethisproviderisn’tavailable.

CSVorotherdelimitedtext les—toopena lewithasemicolonasadelimiter,with eld“x”forx-

coordinateand eld“y”withy-coordinateyouwouldusesomethinglikethis