ViewTestRule - for testing of isolated views using instrumented tests on android.

ViewTestRule.kt

package ch.olmero.jo.test

import android.content.Context
import android.view.View
import android.view.ViewGroup
import androidx.test.platform.app.InstrumentationRegistry
import androidx.test.rule.ActivityTestRule
import ch.olmero.jo.EmptyTestActivity


/** EmptyTestActivity must be available in the build you are testing. It's just an empty activity. */
class ViewTestRule<T : View>(
    private val createView: (context: Context) -> T
) : ActivityTestRule<EmptyTestActivity>(EmptyTestActivity::class.java) {

    private val instrumentation = InstrumentationRegistry.getInstrumentation()

    /** 
    * When interacting with the view, you may need to use InstrumentationRegistry.getInstrumentation().runOnMainSync(..)
    */
    var view: T? = null
        private set

    override fun afterActivityLaunched() {
        super.afterActivityLaunched()
        instrumentation.runOnMainSync {
            val context = activity ?: throw IllegalStateException("no activity")
            view = createView(context)
            context.setContentView(view,
                ViewGroup.LayoutParams(
                    ViewGroup.LayoutParams.MATCH_PARENT,
                    ViewGroup.LayoutParams.MATCH_PARENT
                )
            )
        }
        instrumentation.waitForIdleSync()
    }
}