EngineViewPresenter: Only subscribe to SessionManager if not tracking…

… a fixed session.

This fixes the following reference browser issue:
mozilla-mobile/reference-browser#534

Related to mozilla-mobile#1943

components/feature/session/src/main/java/mozilla/components/feature/session/EngineViewPresenter.kt

@@ -24,19 +24,20 @@ class EngineViewPresenter(
         val session = if (sessionId != null) {
             sessionManager.findSessionById(sessionId)
         } else {
+            sessionManager.register(this)
             sessionManager.selectedSession
         }
 
         session?.let { renderSession(it) }
-
-        sessionManager.register(this)
     }
 
     /**
      * Stop presenter from updating view.
      */
     fun stop() {
-        sessionManager.unregister(this)
+        if (sessionId == null) {
+            sessionManager.unregister(this)
+        }
     }
 
     /**

components/feature/session/src/test/java/mozilla/components/feature/session/EngineViewPresenterTest.kt

@@ -10,6 +10,7 @@ import mozilla.components.concept.engine.EngineView
 import org.junit.Test
 import org.mockito.Mockito.`when`
 import org.mockito.Mockito.mock
+import org.mockito.Mockito.never
 import org.mockito.Mockito.spy
 import org.mockito.Mockito.verify
 
@@ -46,4 +47,17 @@ class EngineViewPresenterTest {
         engineViewPresenter.stop()
         verify(sessionManager).unregister(engineViewPresenter)
     }
+
+    @Test
+    fun `Presenter does not register or unregister if presenting fixed session`() {
+        val sessionId = "just-a-test"
+
+        val engineViewPresenter = EngineViewPresenter(sessionManager, engineView, sessionId)
+
+        engineViewPresenter.start()
+        verify(sessionManager, never()).register(engineViewPresenter)
+
+        engineViewPresenter.stop()
+        verify(sessionManager, never()).unregister(engineViewPresenter)
+    }
 }

docs/changelog.md

@@ -13,6 +13,9 @@ permalink: /changelog/
 * [Gecko](https://github.com/mozilla-mobile/android-components/blob/master/buildSrc/src/main/java/Gecko.kt)
 * [Configuration](https://github.com/mozilla-mobile/android-components/blob/master/buildSrc/src/main/java/Config.kt)
 
+* **feature-session**
+  * Fixed an issue causing `EngineViewPresenter` to render a selected `Session` even though it was configured to show a fixed `Session`. This issue caused a crash (`IllegalStateException: Display already acquired`) in the [Reference Browser](https://github.com/mozilla-mobile/reference-browser) when a "Custom Tab" and the "Browser" tried to render the same `Session`.
+
 # 0.41.0
 
 * [Commits](https://github.com/mozilla-mobile/android-components/compare/v0.40.0...v0.41.0)
@@ -30,6 +33,7 @@ permalink: /changelog/
 
 * **feature-browser**
   * Added `BrowserToolbar` attributes to color the menu.
+
   ```xml
   <mozilla.components.browser.toolbar.BrowserToolbar
       android:id="@+id/toolbar"
@@ -50,6 +54,7 @@ permalink: /changelog/
 * **feature-downloads**
   * ⚠️ **This is a breaking API change!**
   * The required permissions are now passed to the `onNeedToRequestPermissions` callback.
+
   ```kotlin
   downloadsFeature = DownloadsFeature(
       requireContext(),
@@ -60,20 +65,24 @@ permalink: /changelog/
       }
   )
   ```
+
   * Removed the `onPermissionsGranted` method in favour of `onPermissionsResult` which handles both granted and denied permissions. This method should be invoked from `onRequestPermissionsResult`:
+
   ```kotlin
    override fun onRequestPermissionsResult(requestCode: Int, permissions: Array<String>, grantResults: IntArray) {
       when (requestCode) {
           REQUEST_CODE_DOWNLOAD_PERMISSIONS -> downloadsFeature.onPermissionsResult(permissions, grantResults)
       }
     }
   ```
+
   * Fixed Downloads feature to work with Custom Tabs by passing in the session ID when applicable.
 
  * **feature-prompts**
    * ⚠️ **This is a breaking API change!**
    * These change are similar to the ones for feature-downloads above and aim to provide a consistent way of handling permission requests.
    * The required permissions are now passed to the `onNeedToRequestPermissions` callback.
+
    ```kotlin
    promptFeature = PromptFeature(
       fragment = this,
@@ -84,7 +93,9 @@ permalink: /changelog/
       }
    )
    ```
+
    * Renamed `onRequestsPermissionsResult` to `onPermissionResult` and allow applications to specify the permission request code. This method should be invoked from `onRequestPermissionsResult`:
+
   ```kotlin
    override fun onRequestPermissionsResult(requestCode: Int, permissions: Array<String>, grantResults: IntArray) {
       when (requestCode) {