Initial implementation of dev mode.

Huge thanks to @atterpac!

v3/pkg/application/application_dev.go

@@ -0,0 +1,38 @@
+//go:build !production
+
+package application
+
+import (
+	"errors"
+	"github.com/wailsapp/wails/v3/internal/assetserver"
+	"net/http"
+	"time"
+)
+
+func (a *App) preRun() error {
+	// Check for frontend server url
+	frontendURL := assetserver.GetDevServerURL()
+	if frontendURL != "" {
+		// We want to check if the frontend server is running by trying to http get the url
+		// and if it is not, we wait 500ms and try again for a maximum of 10 times. If it is
+		// still not available, we return an error.
+		// This is to allow the frontend server to start up before the backend server.
+		client := http.Client{}
+		a.Logger.Info("Waiting for frontend dev server to start...", "url", frontendURL)
+		for i := 0; i < 10; i++ {
+			_, err := client.Get(frontendURL)
+			if err == nil {
+				a.Logger.Info("Connected to frontend dev server!")
+				return nil
+			}
+			// Wait 500ms
+			time.Sleep(500 * time.Millisecond)
+			if i%2 == 0 {
+				a.Logger.Info("Retrying...")
+			}
+		}
+		a.Logger.Info("failed!")
+		return errors.New("unable to connect to frontend server. Please check it is running")
+	}
+	return nil
+}