{"id":615,"date":"2025-07-08T13:50:21","date_gmt":"2025-07-08T13:50:21","guid":{"rendered":"https:\/\/xojo.itbib4you.be\/?page_id=615"},"modified":"2025-07-11T09:19:34","modified_gmt":"2025-07-11T09:19:34","slug":"subclasses","status":"publish","type":"page","link":"https:\/\/xojo.itbib4you.be\/index.php\/subclasses\/","title":{"rendered":"Subclasses"},"content":{"rendered":"\n
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Overview<\/a><\/div>\n<\/div>\n\n\n\n

Introduction<\/h2>\n\n\n\n

In the previous lessons, we discussed in detail how to define classes and how to use objects created from these classes. In this lesson, we will learn how to create and use subclasses<\/mark>. A subclass<\/mark> inherits properties and methods from a parent class, called the superclass<\/mark>. Using some small examples, we will illustrate the usefulness of subclasses in object-oriented programming (OOP). <\/p>\n\n\n\n

How to create a subclass?<\/h2>\n\n\n\n

For the class DesktopTextField<\/mark> , many properties, methods and events are already available by default. However, it can sometimes be very useful to add new methods to this class. Since it is not possible to add new methods to existing Xojo classes, we have to create a new subclass. <\/p>\n\n\n\n

A subclass can be created in a simple way:<\/p>\n\n\n

\n
\"Define<\/figure>\n<\/div>\n\n\n
    \n
  1. Insert a new Class to your project (MainMenuBar<\/em> – Insert<\/em> – Class<\/em> or via \"insert\" button in toolbar);<\/li>\n\n\n\n
  2. Name the new class (e.g. MyTextField<\/mark>) in the Inspector;<\/li>\n\n\n\n
  3. Select the superclass (e.g. DesktopTextField<\/mark>)in the Inspector.<\/li>\n<\/ol>\n\n\n\n

    MyTextField<\/mark> is now a subclass from the superclass DesktopTextField<\/mark>. This means that an instance of MyTextField<\/mark> can be added to a window. MyTextField<\/mark> inherits all properties (e.g. Text<\/mark>) , methods (e.g. AddText<\/mark>) and events (e.g. TextChanged<\/mark>) from the superclass DesktopTextField<\/mark>.<\/p>\n\n\n

    \n
    \"MyTextField1\"<\/figure>\n<\/div>\n\n\n

    Now you can drag and drop MyTextField<\/mark> from the navigator onto a Window. The default name of the object (new instance of MyTextField) will be MyTextField1<\/mark>, but can be changed. To keep the example simple, I kept the default name.<\/p>\n\n\n\n

    <\/div>\n\n\n\n

    How to add methods to a subclass?<\/h3>\n\n\n
    \n
    \"New<\/figure>\n<\/div>\n\n\n

    Now you can add new methods to the subclass named MyTextField<\/mark>. Four new methods were added in the example (ToCurrency<\/mark>, ToLowerCase<\/mark>, ToTitleCase<\/mark>, ToUpperCase<\/mark>). <\/p>\n\n\n\n

    Our discussion will be limited to the methods ToTitleCase<\/mark> and ToCurrency<\/mark>. <\/p>\n\n\n\n

    <\/div>\n\n\n\n

    ToTitleCase<\/h4>\n\n\n\n
    \/\/ToTitleCase As String<\/mark>
    Var<\/mark> t As String<\/mark> = Self<\/mark>.Text.Titlecase
    Return<\/mark> t<\/pre>\n\n\n\n
    This method is very simple. The Text<\/mark> property of MyTextField<\/mark> (denoted by the keyword Self<\/mark>) is converted by the String<\/mark> method TitleCase<\/mark> and assigned to the localString<\/mark> variable t<\/mark>. The converted string is then returned to the calling method.<\/p>\n\n\n\n
    In the MyTextField1<\/mark> object, the new method can now be called via the TextChanged<\/mark> event:<\/p>\n\n\n\n
    \/\/TextChanged event<\/mark>
    Me<\/mark>.Text = Me<\/mark>.ToTitleCase<\/pre>\n\n\n\n
    Now if you type the name “john de long” into MyTextField1<\/mark> the name is automatically converted to “John De Long”. <\/p>\n\n\n\n
    ToCurrency<\/h4>\n\n\n\n
    A monetary amount entered as text in MyTextField1<\/mark> can easily be converted to a Currency<\/mark> data type via the method ToCurrency<\/mark> . <\/p>\n\n\n\n
    The ToCurrency<\/mark> method looks as follows:<\/p>\n\n\n\n
    \/\/ToCurrency As Currency<\/mark>
    Var<\/mark> v As Currency<\/mark>
    Try<\/mark>
      v = Currency<\/mark>.FromString(Self<\/mark>.Text, Locale.Current)
    Catch<\/mark> error As<\/mark> RuntimeException
      MessageBox(\"Error \"+error.Message)
    End Try<\/mark>
    Return<\/mark> v<\/pre>\n\n\n\n
    The FromString<\/mark> method is used to convert the value of the Text<\/mark> property of MyTextField<\/mark> into a Currency<\/mark> value. The parameter Locale.Current<\/mark> takes into account the local representation of money amounts (meaning of comma and period).<\/p>\n\n\n\n
    If the conversion fails a RunTimeException<\/mark> is raised. This error condition  is caught by the Try..Catch..End Try construction. The converted value v<\/mark> is returned to the calling method.<\/p>\n\n\n\n
    An empty Currency<\/mark> value doesn't exist. The above code will return 0<\/mark> if the conversion fails! <\/pre>\n\n\n\n
    In the MyTextField1<\/mark> object, the new method can be called via the KeyDown<\/mark> event:<\/p>\n\n\n\n
    \/\/KeyDown(key As String) As Boolean<\/mark>
    If<\/mark> key = Chr(3) Or<\/mark> key = Chr(13) Then
      Var<\/mark> amount As Currency<\/mark> = Me<\/mark>.ToCurrency
      MessageBox(Str(amount))
    End If<\/mark>
    Return False<\/mark><\/pre>\n\n\n\n
    The KeyDown<\/mark> event waits until End-Of-Text (ETX) (Chr(3)<\/mark>) is reached or Carriage Return (Chr(13)<\/mark>) is pressed. The ToCurrency<\/mark> method converts the Text<\/mark> of TextField1<\/mark> into the Currency<\/mark> value named amount<\/mark>. In Europe the Text<\/mark> value \u20ac1.234,55 entered in MyTextField1<\/mark> is converted to 1234.56. This value can now be used to perform calculations.<\/p>\n\n\n\n
    The Return<\/mark> statement returns False<\/mark> . Returning False<\/mark> means the key reaches the control. Returning True<\/mark> means the key is intercepted.<\/p>\n\n\n\n
    How to add properties to a subclass?<\/h3>\n\n\n\n
    Properties<\/mark> can be added to a subclass in the usual way. For example, you can add the CurrencyValue<\/mark> property (type Currency<\/mark> – scope Private<\/mark>) to store the currency amount calculated from the Text<\/mark> property.<\/p>\n\n\n\n
    In this case, modify the ToCurrency<\/mark> method as follows:<\/p>\n\n\n\n
    \/\/ToCurrency As Currency<\/mark>
    Try<\/mark>
      CurrencyValue = Currency<\/mark>.FromString(Self<\/mark>.Text, Locale.Current)
    Catch<\/mark> error As<\/mark> RuntimeException
      MessageBox(\"Error \"+error.Message)
    End Try<\/mark>
    Return<\/mark> CurrencyValue<\/pre>\n\n\n\n
    In the following paragraphs, I use a more practical example to illustrate adding events<\/span><\/font> to a subclass.<\/p>\n\n\n\n
    How to add events to a (sub)class?<\/h2>\n\n\n\n
    \"Accounts<\/figure>\n\n\n\n
    For the following example, I have two ‘special’ bank accounts: a retirement account (RetirementAccount<\/mark>) and a savings account (SavingsAccount<\/mark>). Both accounts are slightly different: for a retirement account, each deposit is limited to 1000.0 (MaximumAmount<\/mark>); for a savings account, on the other hand, the total account balance is limited to 5000.0 (MaximumBalance<\/mark>).<\/p>\n\n\n
    \n
    \"klassen\"<\/figure>\n<\/div>\n\n\n
    To solve this problem and illustrate the use of events, I created the class Account<\/mark> and two subclasses LimitAmountAccount<\/mark> and LimitBalanceAccount<\/mark>. <\/p>\n\n\n\n
    <\/div>\n\n\n\n
    REMARK<\/mark>
    There are several possible solutions to this problem. This solution was chosen to best illustrate the use of events.<\/pre>\n\n\n\n
    Account<\/h3>\n\n\n\n
    The class Account<\/mark> has a method named Deposit<\/mark>(scope Protected<\/mark>):<\/p>\n\n\n\n
    \/\/Deposit(amount As Double)<\/mark>
    AccountBalance = AccountBalance + amount<\/pre>\n\n\n\n
    The class Account<\/mark> also has a property AccountBalance<\/mark> (Type Double<\/mark> – Scope Protected<\/mark>).<\/p>\n\n\n\n
    Protected<\/mark> means that the property or method can be used in the class itself and by its subclasses.<\/pre>\n\n\n\n
    LimitAmountAccount<\/h3>\n\n\n\n
    LimitAmountAccount<\/mark> is a subclass of Account<\/mark>: Account<\/mark> is the superclass (Super<\/mark> Account<\/mark>). <\/p>\n\n\n\n
    Because a special rule applies to this subclass (the deposited amount cannot exceed 1000.00), a special method Deposit<\/mark> was created for this subclass. The following method has been added:<\/p>\n\n\n\n
    \/\/Deposit(amount As Double)
    \/\/ Calling the overridden superclass method.<\/mark>
    Super<\/mark>.Deposit(amount)
    If<\/mark> amount > 1000.0 Then<\/mark>
      RaiseEvent<\/mark> MaximumAmount
      AccountBalance = AccountBalance - amount
    End If<\/mark>
    MessageBox(Str(AccountBalance))<\/pre>\n\n\n\n
    Super.Deposit(amount)<\/h4>\n\n\n\n
    The first statement (Super.Deposit(amount)<\/mark> ) is not difficult to understand: execute the Deposit<\/mark> method from the superclass (Account<\/mark>). In this case, the property AccountBalance<\/mark> is first incremented by the inserted value (amount<\/mark>). This statement is automatically added by Xojo if you refer to an existing method of the superclass (you want to override the existing method).<\/p>\n\n\n\n
    RaiseEvent MaximumAmount<\/h4>\n\n\n\n
    The RaiseEvent MaximumAmount<\/mark> statement refers to an event<\/mark> named MaximumAmount<\/mark>.<\/p>\n\n\n
    \n
    \"Event<\/figure>\n<\/div>\n\n\n
    The use of events is easy to understand if you think of an event as a special method<\/mark> called when a particular action is performed. An event (the ‘special’ method) must be defined in an Event Definition<\/mark>.<\/p>\n\n\n\n
    An Event Definition<\/mark> can be added by selecting the class and selecting the menu option Event Definition<\/mark> from the Insert<\/mark> menu.<\/p>\n\n\n\n
    <\/div>\n\n\n
    \n
    \"events\"<\/figure>\n<\/div>\n\n\n
    Just as you define an ordinary method, for events you must also specify the name of the event, optional parameters and an optional return type. No further information is required. <\/p>\n\n\n\n
    <\/div>\n\n\n\n
    <\/div>\n\n\n\n
    An Event Definition needs only an Event Name, optional Parameters and an optional Return Type. No code is added in the coding pane.<\/mark><\/pre>\n\n\n\n
    The MaximumAmount <\/mark>event is called by the statement RaiseEvent MaximumAmount<\/mark> if the deposit in an account of the LimitAmountAccount<\/mark> subclass exceeds 1000.00. In this case, the account balance is also corrected back (the superclass already added the deposited amount) with the instruction AccountBalance = AccountBalance – amount<\/mark>.<\/p>\n\n\n\n
    How to use the new event?<\/h4>\n\n\n\n
    Drag a class to the window<\/h5>\n\n\n\n
    A (sub)class is just a template to define real objects. To use our new construction we have to create an object. A quick way to do this is to drag the class LimitAmountAccount<\/mark> into the window and define the Name (RetirementAccount<\/mark>), the Superclass (LimitAmountAccount<\/mark>) and the Scope (Private<\/mark>). <\/p>\n\n\n
    \n
    \"RetirementAccount<\/figure>\n<\/div>\n\n\n
    The object RetirementAccount<\/mark> knows one event (MaximumAmount<\/mark>). The Event Handler (what must be done if the deposit is greater than 1000.00?) can be added to the RetirementAccount<\/mark>. The following screenshot shows the way how the Event Handler is made available.<\/p>\n\n\n\n
    <\/div>\n\n\n
    \n
    \"Event<\/figure>\n<\/div>\n\n\n
    <\/div>\n\n\n\n
    In this case, we chose to provide only an error message (“Amount > 1000.00!”) to indicate that this deposit will not be accepted. This piece of code can be added to the Event Handler:<\/p>\n\n\n\n
    \/\/MaximumAmount<\/mark>
    MessageBox(\"Amount > 1000.00!\")<\/pre>\n\n\n\n
    The same process can be used to create a new MaximumBalance<\/mark> event for the LimitBalanceAccount<\/mark> subclass.<\/p>\n\n\n\n
    The Deposit<\/mark> method for the LimitBalanceAccount<\/mark> subclass will be:<\/p>\n\n\n\n
    \/\/Deposit(amount As Double)
    \/\/ Calling the overridden superclass method.<\/mark>
    Super<\/mark>.Deposit(amount)
    If<\/mark> AccountBalance > 5000.00 Then<\/mark>
      RaiseEvent<\/mark> MaximumBalance
      AccountBalance = AccountBalance - amount
    End If<\/mark>
    MessageBox(Str(AccountBalance))<\/pre>\n\n\n\n
    An Event Definition must be added to the subclass for the new Event MaximumBalance<\/mark> (Name MaximumBalance<\/mark>, no parameters, no Return).<\/p>\n\n\n\n
    A new object can be added by dragging the LimitBalanceAccount<\/mark>subclass onto the Window. The new object is named SavingsAccount<\/mark>, Super is LimitBalanceAccount<\/mark> and Scope is Private<\/mark>.<\/p>\n\n\n\n
    The code for the Event Handler MaximumBalance<\/mark> could be:<\/p>\n\n\n\n
    \/\/MaximumBalance<\/mark>
    MessageBox(\"Balance Limit 5000.00!\")<\/pre>\n\n\n\n
    Create your class at runtime<\/h5>\n\n\n\n
    In the previous lesson, we created the accounts at runtime with the statements RetirementAccount = New LimitAmountAccount<\/mark> and SavingsAccount = New LimitBalanceAccount<\/mark>. RetirementAccount<\/mark> and SavingsAccount<\/mark> were two properties defined in AccountWindow<\/mark>. <\/p>\n\n\n\n
    \/\/Opening<\/mark>
    RetirementAccount = New<\/mark> LimitAmountAccount
    AddHandler<\/mark> RetirementAccount.MaximumAmount, AddressOf<\/mark> MaxAmountMethod

    SavingsAccount = New<\/mark> LimitBalanceAccount
    AddHandler<\/mark> SavingsAccount.MaximumBalance, AddressOf<\/mark> MaxBalanceMethod<\/pre>\n\n\n\n
    We have now placed the above piece of code in the Opening<\/mark> event of AccountWindow<\/mark>. The two accounts are created at runtime. Because these objects are created at runtime, it is not yet possible to handle the MaximumAmount<\/mark> and MaximumBalance<\/mark> events . AddHandler<\/mark> associates an event with a method that must be executed  when the event occurs.<\/p>\n\n\n\n
    The following syntax is used:<\/p>\n\n\n\n
    AddHandler<\/mark> eventName, delegateMethod<\/pre>\n\n\n\n
    A delegateMethod<\/mark> is a pointer to a named method.  AddressOf<\/mark> gets the address of the named method . This is the code of the referenced methods:<\/p>\n\n\n\n
    \/\/MaxAmountMethod(sender As LimitAmountAccount)<\/mark>
    MessageBox(\"Amount > 1000!\")<\/pre>\n\n\n\n
    \/\/MaxBalanceMethod(sender As LimitBalanceAccount)
    <\/mark>MessageBox(\"Balance > 5000!\")<\/pre>\n\n\n\n
    An additional parameter (sender As LimitAmountAccount<\/mark>in MaxAmountMethod , sender As LimitBalanceAccount<\/mark>in MaxBalanceMethod) is required for the methods that must be executed when the events occur. The first parameter of the referenced methods must always be of the type of  the original object.<\/p>\n\n\n\n
    The first parameter of the method must always be of the type of the original object.<\/pre>\n\n\n\n
    If you use Addhandler<\/mark>, you should always remove the handler when it is no longer needed. The handlers are removed in the Closing<\/mark> event of AccountWindow<\/mark>:<\/p>\n\n\n\n
    \/\/Closing<\/mark>
    RemoveHandler<\/mark>  RetirementAccount.MaximumAmount, AddressOf<\/mark> MaxAmountMethod
    RemoveHandler<\/mark>  SavingsAccount.MaximumBalance, AddressOf<\/mark> MaxBalanceMethod<\/pre>\n\n\n\n
    <\/p>\n","protected":false},"excerpt":{"rendered":"
    Introduction In the previous lessons, we discussed in detail how to define classes and how to use objects created from these classes. In this lesson, we will learn how to create and use subclasses. A subclass inherits properties and methods from a parent class, called the superclass. Using some small examples, we will illustrate the … Continue reading “Subclasses”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-615","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/xojo.itbib4you.be\/index.php\/wp-json\/wp\/v2\/pages\/615","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/xojo.itbib4you.be\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/xojo.itbib4you.be\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/xojo.itbib4you.be\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/xojo.itbib4you.be\/index.php\/wp-json\/wp\/v2\/comments?post=615"}],"version-history":[{"count":46,"href":"https:\/\/xojo.itbib4you.be\/index.php\/wp-json\/wp\/v2\/pages\/615\/revisions"}],"predecessor-version":[{"id":677,"href":"https:\/\/xojo.itbib4you.be\/index.php\/wp-json\/wp\/v2\/pages\/615\/revisions\/677"}],"wp:attachment":[{"href":"https:\/\/xojo.itbib4you.be\/index.php\/wp-json\/wp\/v2\/media?parent=615"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}