Restore Listeners

Create Listener with `loadobj`

Suppose that you create a property listener and want to be able to save and restore the event source and the listener. One approach is to create a listener from the loadobj method.

Use Transient Property to Load Listener

The BankAccount class stores the account balance and an account status. A PostSet listener attached to the AccountBalance property controls the account status.

When the AccountBalance property value changes, the listener callback determines the account status. Important points include:

The BankAccount class defines the AccountManagerListener property to contain the listener handle. This property enables the loadobj method to create a listener and return a reference to it in the object that is loaded into the workspace.
The AccountManagerListener property is Transient because there is no need to store the listener handle with a BankAccount object. Create a listener that is attached to the new BankAccount object created during the load process.
The AccountBalance listener sets the AccountStatus.
Only the AccountManager class can access AccountStatus property.

classdef BankAccount < handle
   properties (SetObservable, AbortSet)
      AccountBalance
   end
   properties (Transient)
      AccountManagerListener
   end
   properties (Access = ?AccountManager)
      AccountStatus
   end
   methods
      function obj = BankAccount(initialBalance)
         obj.AccountBalance = initialBalance;
         obj.AccountStatus = 'New Account';
         obj.AccountManagerListener = AccountManager.addAccount(obj);
      end
   end
   methods (Static)
      function obj = loadobj(obj)
         if isstruct(obj) % Handle error
            initialBalance = obj.AccountBalance;
            obj = BankAccount(initialBalance);
         else
            obj.AccountManagerListener = AccountManager.addAccount(obj);
         end
      end
   end
end

Assume the AccountManager class provides services for various types of accounts. For the BankAccount class, the AccountManager class defines two Static methods:

assignStatus — Callback for the AccountBalance property PostSet listener. This method determines the value of the BankAccount AccountStatus property.
addAccount — Creates the AccountBalance property PostSet listener. The BankAccount constructor and loadobj methods call this method.

classdef AccountManager
   methods (Static)
      function assignStatus(BA,~)
         if BA.AccountBalance < 0 && BA.AccountBalance >= -100
            BA.AccountStatus = 'overdrawn';
         elseif BA.AccountBalance < -100
            BA.AccountStatus = 'frozen';
         else
            BA.AccountStatus = 'open';
         end
      end
      function lh = addAccount(BA)
         lh = addlistener(BA,'AccountBalance','PostSet', ...
            @(src,evt)AccountManager.assignStatus(BA));
      end
   end
end

Using the BankAccount and AccountManager Classes

Create an instance of the BankAccount class.

ba = BankAccount(100)

ba = 

  BankAccount with properties:

            AccountBalance: 100
    AccountManagerListener: [1x1 event.proplistener]
             AccountStatus: 'New Account'

Now set an account value to confirm that the AccountManager sets AccountStatus appropriately:

ba.AccountBalance = -10;
ba.AccountStatus

ans =

overdrawn

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