Yggdrasil account offline logging in

HMCLCore/src/main/java/org/jackhuang/hmcl/auth/Account.java

@@ -57,6 +57,14 @@ public abstract class Account {
      */
     public abstract AuthInfo logIn(MultiCharacterSelector selector, Proxy proxy) throws AuthenticationException;
 
+    public abstract boolean canPlayOffline();
+
+    /**
+     * Play offline.
+     * @return the specific offline player's info.
+     */
+    public abstract AuthInfo playOffline();
+
     public abstract void logOut();
 
     protected abstract Map<Object, Object> toStorageImpl();

HMCLCore/src/main/java/org/jackhuang/hmcl/auth/OfflineAccount.java

@@ -69,6 +69,16 @@ public class OfflineAccount extends Account {
         // Offline account need not log out.
     }
 
+    @Override
+    public boolean canPlayOffline() {
+        return false;
+    }
+
+    @Override
+    public AuthInfo playOffline() {
+        throw new IllegalStateException();
+    }
+
     @Override
     public Map<Object, Object> toStorageImpl() {
         return Lang.mapOf(

HMCLCore/src/main/java/org/jackhuang/hmcl/auth/yggdrasil/YggdrasilAccount.java

@@ -171,6 +171,19 @@ public final class YggdrasilAccount extends Account {
             userProperties.putAll(user.getProperties());
     }
 
+    @Override
+    public boolean canPlayOffline() {
+        return isLoggedIn() && getSelectedProfile() != null && !canPlayOnline();
+    }
+
+    @Override
+    public AuthInfo playOffline() {
+        if (!canPlayOffline())
+            throw new IllegalStateException("Current account " + this + " cannot play offline.");
+
+        return new AuthInfo(selectedProfile, accessToken, userType, GSON.toJson(userProperties));
+    }
+
     @Override
     public void logOut() {
         password = null;