Closes #2174: ReaderViewFeature crashes with SystemEngine

Author: csadilek

components/concept/engine/src/main/java/mozilla/components/concept/engine/Engine.kt

@@ -64,13 +64,14 @@ interface Engine {
      * @param ext the [WebExtension] to install.
      * @param onSuccess (optional) callback invoked if the extension was installed successfully.
      * @param onError (optional) callback invoked if there was an error installing the extension.
-     * @throws UnsupportedOperationException if this engine doesn't support web extensions.
+     * This callback is invoked with an [UnsupportedOperationException] in case the engine doesn't
+     * have web extension support.
      */
     fun installWebExtension(
         ext: WebExtension,
         onSuccess: ((WebExtension) -> Unit) = { },
         onError: ((WebExtension, Throwable) -> Unit) = { _, _ -> }
-    ): Unit = throw UnsupportedOperationException("Web extension support is not available in this engine")
+    ): Unit = onError(ext, UnsupportedOperationException("Web extension support is not available in this engine"))
 
     /**
      * Provides access to the settings of this engine.

components/concept/engine/src/test/java/mozilla/components/concept/engine/EngineTest.kt

@@ -8,14 +8,15 @@ import android.content.Context
 import android.util.AttributeSet
 import mozilla.components.concept.engine.webextension.WebExtension
 import org.json.JSONObject
-import org.junit.Assert.fail
+import org.junit.Assert.assertNotNull
+import org.junit.Assert.assertTrue
 import org.junit.Test
 import java.lang.UnsupportedOperationException
 
 class EngineTest {
 
     @Test
-    fun `throws exception if webextensions not supported`() {
+    fun `invokes error callback if webextensions not supported`() {
         val engine = object : Engine {
             override fun createView(context: Context, attrs: AttributeSet?): EngineView {
                 throw NotImplementedError("Not needed for test")
@@ -41,32 +42,9 @@ class EngineTest {
                 get() = throw NotImplementedError("Not needed for test")
         }
 
-        try {
-            engine.installWebExtension(WebExtension("my-ext", "resource://path"))
-            fail("Expected UnsupportedOperationException")
-        } catch (_: UnsupportedOperationException) {
-            // expected
-        }
-
-        try {
-            engine.installWebExtension(WebExtension("my-ext", "resource://path")) { _, _ -> }
-            fail("Expected UnsupportedOperationException")
-        } catch (_: UnsupportedOperationException) {
-            // expected
-        }
-
-        try {
-            engine.installWebExtension(WebExtension("my-ext", "resource://path"), onSuccess = { })
-            fail("Expected UnsupportedOperationException")
-        } catch (_: UnsupportedOperationException) {
-            // expected
-        }
-
-        try {
-            engine.installWebExtension(WebExtension("my-ext", "resource://path"), onSuccess = { }) { _, _ -> }
-            fail("Expected UnsupportedOperationException")
-        } catch (_: UnsupportedOperationException) {
-            // expected
-        }
+        var exception: Throwable? = null
+        engine.installWebExtension(WebExtension("my-ext", "resource://path"), onError = { _, e -> exception = e })
+        assertNotNull(exception)
+        assertTrue(exception is UnsupportedOperationException)
     }
 }