Add ArgumentCaptor.singleValue (#529)

Co-authored-by: Björn Michael <bjoern.michael@optimax-energy.de>

mockito-kotlin/src/main/kotlin/org/mockito/kotlin/ArgumentCaptor.kt

@@ -190,6 +190,13 @@ class KArgumentCaptor<out T : Any?>(
     val lastValue: T
         get() = captor.lastValue
 
+    /**
+     * The *only* captured value of the argument,
+     * or throws an exception if no value or more than one value was captured.
+     */
+    val singleValue: T
+        get() = captor.singleValue
+
     val allValues: List<T>
         get() = captor.allValues
 
@@ -223,3 +230,6 @@ val <T> ArgumentCaptor<T>.thirdValue: T
 
 val <T> ArgumentCaptor<T>.lastValue: T
     get() = allValues.last()
+
+val <T> ArgumentCaptor<T>.singleValue: T
+    get() = allValues.single()

tests/src/test/kotlin/test/ArgumentCaptorTest.kt

@@ -3,6 +3,9 @@ package test
 import com.nhaarman.expect.expect
 import com.nhaarman.expect.expectErrorWithMessage
 import org.junit.Test
+import org.mockito.ArgumentCaptor
+import org.mockito.ArgumentMatchers
+import org.mockito.ArgumentMatchers.anyLong
 import org.mockito.kotlin.*
 import java.util.*
 
@@ -126,6 +129,35 @@ class ArgumentCaptorTest : TestBase() {
         expect(captor.lastValue).toBeNull()
     }
 
+    @Test
+    fun argumentCaptor_singleValue() {
+        /* Given */
+        val date: Date = mock()
+
+        /* When */
+        date.time = 5L
+
+        /* Then */
+        val captor = argumentCaptor<Long>()
+        verify(date).time = captor.capture()
+        expect(captor.singleValue).toBe(5L)
+    }
+
+    @Test(expected = IllegalArgumentException::class)
+    fun argumentCaptor_singleValue_properlyFails() {
+        /* Given */
+        val date: Date = mock()
+        val captor = argumentCaptor<Long>()
+        doNothing().whenever(date).time = captor.capture()
+
+        /* When */
+        date.time = 5L
+        date.time = 7L
+
+        /* Then */
+        expect(captor.singleValue).toBe(5)
+    }
+
     @Test
     fun argumentCaptor_multipleValues() {
         /* Given */