为开发人员准备的文档,涵盖QGIS的多种功能实现和代码。

CHAPTER5

UsingVectorLayers

Thissectionsummarizesvariousactionsthatcanbedonewithvectorlayers.

5.1Retrievinginformationsaboutattributes

Youcanretrieveinformationsaboutthe eldsassociatedwithavectorlayerbycallingpendingFields()on

aQgsVectorLayerinstance:

5.2Selectingfeatures

InQGISdesktop,featurescanbeselectedindifferentways,theusercanclickonafeature,drawarectangleon

themapcanvasoruseanexpression lter.Selectedfaturesarenormallyhiglightedinadifferentcolor(defaultis

yellow)todrawuser’sattentionontheselection.Sometimescanbeusefultoprogrammaticallyselectfeaturesor

tochangethedefaultcolor.

TochangetheselectioncoloryoucanusesetSelectionColor()methodof

QgsMapCanvasasshownin

thefollowingexample:

Toaddaddfeaturestotheselectedfeatureslistforagivenlayer,youcancallsetSelectedFeatures()

passingtoitthelistoffeaturesIDs:

Tocleartheselection,justpassanemptylist:

5.3IteratingoverVectorLayer

Iteratingoverthefeaturesinavectorlayerisoneofthemostcommontasks.Belowisanexampleofthesimple

basiccodetoperformthistaskandshowingsomeinformationabouteachfeature.thelayervariableisassumed