Difference between app:srcCompat and android:src in Android's layout XML

app:srcCompat

is the most foolproof method of integrating vector drawables into your app.Vector drawables allow you to replace multiple png assets with a single vector graphic, defined in XML. While previously limited to Lollipop and higher devices

Note

As of Android Support Library 23.3.0, support vector drawables can only be loaded via app:srcCompat .

you need to add vectorDrawables.useSupportLibrary = true to your build.gradle file

    // Gradle Plugin 2.0+  
 android {  
   defaultConfig {  
     vectorDrawables.useSupportLibrary = true  
    }  
 }  

android:src

Sets a drawable as the content of this ImageView.It will display in its original size. No automatic scaling .

If you are using android:src="@drawable/some_vector" without vectorDrawables.useSupportLibrary = true in build.gradle file and your app have vector images (vector drawable), then while building the apk file Android gradle plugin generates a lot of *.png files for different screens (hdpi, xhdpi...) from each of your vector drawable (only for API =< 19). The result - bigger size of apk.

When using app:srcCompat="@drawable/some_vector" with vectorDrawables.useSupportLibrary = true android uses vector drawable files without generating *.png files.

You can check this with Android Studio apk analyzer tool. Just build apk with and without vectorDrawables.useSupportLibrary = true.

I think this is the main difference.