perf: reduce webView startup time

AnkiDroid/src/main/java/com/ichi2/anki/AnkiDroidApp.kt

@@ -67,6 +67,7 @@ import com.ichi2.utils.ExceptionUtil
 import com.ichi2.utils.LanguageUtil
 import com.ichi2.utils.measureTime
 import com.ichi2.utils.setWebContentsDebuggingEnabled
+import com.ichi2.utils.startUpWebView
 import com.ichi2.widget.DayRolloverAlarm
 import com.ichi2.widget.WidgetNotificationScheduler
 import com.ichi2.widget.cardanalysis.CardAnalysisWidget
@@ -172,8 +173,6 @@ open class AnkiDroidApp :
             showThemedToast(this.applicationContext, getString(R.string.user_is_a_robot), false)
         }
 
-        setWebContentsDebuggingEnabled(Prefs.isWebDebugEnabled)
-
         setupContextMenus()
 
         setup("makeBackendUsable") { makeBackendUsable(this) }
@@ -185,6 +184,8 @@ open class AnkiDroidApp :
         if (!checkWebViewAvailable()) {
             return
         }
+        // after the probe: startUpWebView throws on its executor if the WebView provider is missing (5794)
+        setupWebView()
 
         // Forget the last deck that was used in the CardBrowser
         CardBrowser.clearLastDeckId()
@@ -447,6 +448,21 @@ open class AnkiDroidApp :
         }
     }
 
+    /**
+     * Starts asynchronously loading the WebView on a background thread.
+     *
+     * TODO: This only handles a subset of WebView init, and will not produce a performance
+     *  improvement until this pattern is used for all WebView init.
+     */
+    private fun setupWebView() =
+        setup("setupWebView") {
+            startUpWebView(
+                context = this,
+                onSuccess = { setWebContentsDebuggingEnabled(Prefs.isWebDebugEnabled) },
+                onFailure = { e -> Timber.w(e, "startUpWebView") },
+            )
+        }
+
     /**
      * @return the app version, OS version and device model, provided when syncing.
      */

AnkiDroid/src/main/java/com/ichi2/utils/WebViewUtils.kt

@@ -27,12 +27,17 @@ import androidx.annotation.VisibleForTesting
 import androidx.appcompat.app.AlertDialog
 import androidx.core.content.pm.PackageInfoCompat
 import androidx.webkit.WebViewCompat
+import androidx.webkit.WebViewOutcomeReceiver
+import androidx.webkit.WebViewStartUpConfig
+import androidx.webkit.WebViewStartUpResult
+import androidx.webkit.WebViewStartupException
 import com.ichi2.anki.R
 import com.ichi2.anki.common.crashreporting.CrashReportService
 import com.ichi2.anki.utils.openUrl
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.withContext
 import timber.log.Timber
+import java.util.concurrent.Executors
 
 @JvmInline
 value class WebViewVersion(
@@ -231,3 +236,34 @@ fun setWebContentsDebuggingEnabled(enabled: Boolean) =
         // android.util.AndroidRuntimeException: android.webkit.WebViewFactory$MissingWebViewPackageException: Failed to load WebView provider: No WebView installed
         Timber.w(e, "setWebContentsDebuggingEnabled")
     }
+
+/**
+ * Starts the WebView on a background thread, so later WebView usage is faster.
+ *
+ * Exactly one of [onSuccess] or [onFailure] is called, on the main thread, once startup completes.
+ *
+ * Background work runs on a dedicated thread, released once startup completes.
+ *
+ * @see WebViewCompat.startUpWebView
+ */
+fun startUpWebView(
+    context: Context,
+    onSuccess: () -> Unit,
+    onFailure: (Throwable) -> Unit,
+) {
+    val executor = Executors.newSingleThreadExecutor()
+    val config = WebViewStartUpConfig.Builder(executor).build()
+    val callback =
+        object : WebViewOutcomeReceiver<WebViewStartUpResult, WebViewStartupException> {
+            override fun onResult(result: WebViewStartUpResult?) {
+                executor.shutdown()
+                onSuccess()
+            }
+
+            override fun onError(e: WebViewStartupException) {
+                executor.shutdown()
+                onFailure(e)
+            }
+        }
+    WebViewCompat.startUpWebView(context, config, callback)
+}