QGIS-2.8-PyQGISDeveloperCookbook-en(21)

为开发人员准备的文档,涵盖QGIS的多种功能实现和代码。

5.3.3Iteratingoverasubsetoffeatures

Ifyouwanttoiterateoveragivensubsetoffeaturesinalayer,suchasthosewithinagivenarea,youhavetoadd

aQgsFeatureRequestobjecttothegetFeatures()call.Here’sanexample

Ifyouneedanattribute-based lterinstead(orinaddition)ofaspatialonelikeshownintheexampleabove,you

canbuildanQgsExpressionobjectandpassittotheQgsFeatureRequestconstructor.Here’sanexample

Therequestcanbeusedtode nethedataretrievedforeachfeature,sotheiteratorreturnsallfeatures,butreturns

partialdataforeachofthem.

Tip:Ifyouonlyneedasubsetoftheattributesoryoudon’tneedthegeometryinformations,youcansigni cantlyincreasethespeedofthefeaturesrequestbyusingQgsFeatureRequest.NoGeometry agorspecifyinga

subsetofattributes(possiblyempty)likeshownintheexampleabove.

5.4ModifyingVectorLayers

Mostvectordataproviderssupportediting

oflayerdata.Sometimestheysupportjustasubsetofpossibleediting

ethecapabilities()functionto ndoutwhatsetoffunctionalityissupported

Byusinganyofthefollowingmethodsforvectorlayerediting,thechangesaredirectlycommittedtotheunderly-

ingdatastore(a le,databaseetc).Incaseyouwouldliketodoonlytemporarychanges,skiptothenextsection

thatexplainshowtodomodi cationswitheditingbuffer.

Note:IfyouareworkinginsideQGIS(eitherfromtheconsoleorfromaplugin),itmightbenecessarytoforcearedrawofthemapcanvasinordertoseethechangesyou’vedonetothegeometry,tothestyleortotheattributes:

5.4.1AddFeatures

CreatesomeQgsFeatureinstancesandpassalistofthemtoprovider’saddFeatures()method.Itwill

returntwovalues:result(true/false)andlistofaddedfeatures(theirIDissetbythedatastore)