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Basic JavaScript Mapping for Classes

A Slice class is mapped to a JavaScript class with the same name. For each Slice data member, the JavaScript instance contains a corresponding property (just as for structures and exceptions). Consider the following class definition:

class TimeOfDay
{
    short hour;         // 0 - 23
    short minute;       // 0 - 59
    short second;       // 0 - 59
}

The Slice compiler generates the following code for this definition:

class TimeOfDay extends Ice.Value
{
    constructor(hour = 0, minute = 0, second = 0)
    {
        super();
        this.hour = hour;
        this.minute = minute;
        this.second = second;
    }
}

There are a number of things to note about the generated code:

1. The generated TimeOfDay class inherits from Ice.Value. Note that Ice.Value is not the same as Ice.ObjectPrx. In other words, you cannot pass a class where a proxy is expected and vice versa.
2. The generated class provides a constructor that accepts a value for each data member.
3. The generated class defines a property for each Slice data member.

There is quite a bit to discuss here, so we will look at each item in turn.

Inheritance from Ice.Value in JavaScript

As for Slice interfaces, the generated type for a Slice class implicitly inherits from a common base type. However, the type inherits from Ice.Value instead of Ice.ObjectPrx (which is at the base of the inheritance hierarchy for proxies). As a result, you cannot pass a class where a proxy is expected (and vice versa) because the base types for classes and proxies are not compatible.

Ice.Value defines a number of member functions:

class Ice.Value
{
    static ice_staticId() {}
    ice_id() {}
    ice_preMarshal() {}
    ice_postUnmarshal() {}
    ice_getSlicedData() {}
}

The member functions of Ice.Value behave as follows:

• ice_staticId
  This function returns the static type ID of a class.

• ice_id
  This function returns the actual run-time type ID for a class. If you call ice_id through a reference to a base instance, the returned type ID is the actual (possibly more derived) type ID of the instance.

• ice_preMarshal
  The Ice run time invokes this function prior to marshaling the object's state, providing the opportunity for a subtype to validate its declared data members.

• ice_postUnmarshal
  The Ice run time invokes this function after unmarshaling an object's state. A subtype typically overrides this function when it needs to perform additional initialization using the values of its declared data members.
• ice_getSlicedData
  This functions returns the SlicedData object if the value has been sliced during un-marshaling or null otherwise.

Class Constructors in JavaScript

The type generated for a Slice class provides a constructor that initializes each data member to a default value appropriate for its type:

If you wish to ensure that data members of primitive and enumerated types are initialized to specific values, you can declare default values in your Slice definition. The constructor initializes each of these data members to its declared value instead.

The constructor accepts one argument for each member of the class. This allows you to create and initialize an instance in a single statement, for example:

let tod = new TimeOfDayI(14, 45, 00); // 14:45pm

For derived classes, the constructor requires an argument for every member of the class, including inherited members. For example, consider the the definition from Class Inheritance once more:

class TimeOfDay
{
    short hour;         // 0 - 23
    short minute;       // 0 - 59
    short second;       // 0 - 59
}

class DateTime extends TimeOfDay
{
    short day;          // 1 - 31
    short month;        // 1 - 12
    short year;         // 1753 onwards
}

The constructors generated for these classes are similar to the following:

class TimeOfDay extends Ice.Value
{
    constructor(hour = 0, minute = 0, second = 0)
    {
        this.hour = hour;
        this.minute = minute;
        this.second = second;
    }
}

class DateTime extends TimeOfDay
{
    constructor(hour, minute, second, day = 0, month = 0, year = 0)
    {
        super(hour, minute, second);
        this.day = day;
        this.month = month;
        this.year = year;
    }
}

Pass undefined as the value of any optional data member that you wish to remain unset.

Class Data Members in JavaScript

By default, data members of classes are mapped exactly as for structures and exceptions: for each data member in the Slice definition, the generated type defines a corresponding property.

Optional data members use the same mapping as required data members, but an optional data member can also be set to undefined to indicate that the member is unset. A well-behaved program must compare an optional data member to undefined before using the member's value:

let v = ...
if(v.optionalMember === undefined)
{
    console.log("optionalMember is unset")
}
else
{
    console.log("optionalMember =", v.optionalMember)
}

Class Operations in JavaScript

With the JavaScript mapping, operations in classes are not mapped at all into the corresponding JavaScript class. The generated JavaScript class is the same whether the Slice class has operations or not.

The Slice to JavaScript compiler also generates a separate <class-name>Disp class, which can be used to implement an Ice object with these operations. For example:

class FormattedTimeOfDay
{
    short hour;         // 0 - 23
    short minute;       // 0 - 59
    short second;       // 0 - 59
    string tz;
    string format();
}

class FormattedTimeOfDay extends Ice.Value
{
     // ... operation format() not mapped at all here
}

// Disp class for servant implementation
class FormattedTimeOfDayDisp extends Ice.Object
{
     // ...
}

Value Factories in JavaScript

Value factories allow you to create classes derived from the JavaScript class generated by the Slice compiler, and tell the Ice run time to create instances of these classes when unmarshaling. For example, with the following simple interface:

interface Time
{
    TimeOfDay get();
}

The Ice run time will by default create and return a plain TimeOfDay instance.

If you wish, you can create your own custom derived class, and tell Ice to create and return these instances instead. For example:

var communicator = ...;
communicator.getValueFactoryManager().add(
  type => {
    if(type === TimeOfDay.ice_staticId())
      return new TimeOfDayI();
    return null;
  }, TimeOfDay.ice_staticId());

Now, whenever the Ice run time needs to instantiate an object with the type ID "::M::TimeOfDay", it calls the registered factory, which returns a TimeOfDayI instance to the Ice run time.

See Also

• Classes
• Class Inheritance
• Type IDs
• Value Factories