android - My R in library is red but I can build and run app normally -

i use compile project(':pickphotos') include library named pickphotos.

the r in pickphotos red though have me.crosswall.photo.pick.r.i can build , run app normally.

when use alt+enter , shows solution:

when use move 'r' in pickphotos, shows r:

but if pick me.crosswall.photo.pick.r,i repeat r error in next build.

below build.gradle of pickphotos:

apply plugin: 'com.android.library'  android {     compilesdkversion 23     buildtoolsversion '23.0.2'      compileoptions {         sourcecompatibility javaversion.version_1_7         targetcompatibility javaversion.version_1_7     }      defaultconfig {         minsdkversion 14         targetsdkversion 23         versioncode 1         versionname "1.0"     }     buildtypes {         release {             minifyenabled false             proguardfiles getdefaultproguardfile('proguard-android.txt'), 'proguard-rules.pro'         }     }      sourcesets {         main {             manifest.srcfile 'androidmanifest.xml'             java.srcdirs = ['src']             res.srcdirs = ['res']         }     } }  dependencies {  } 

i can find app/build/generated/source/r:

there no mark item in menu in android studio:

i resloved it.

just make project.

r created when make project.

i think when run project or build project,android studio doesn't make project reason.