Common use cases are file handles, network connections, and shared hardware intermediaries like CBCentral Manager. For example, if you have a type that represents a local database connection, the code that manages access to that database needs full control over the state of the database as viewed from your app. It's appropriate to use a class in this case, but be sure to limit which parts of your app get access to the shared database object.
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Use structures when you're modeling data that contains information about an entity with an identity that you don't control. In an app that consults a remote database, for example, an instance's identity may be fully owned by an external entity and communicated by an identifier. If the consistency of an app's models is stored on a server, you can model records as structures with identifiers.
In the example below, json Response contains an encoded Pen Pal Record instance from a server:. Local changes to model types like Pen Pal Record are useful. For example, an app might recommend multiple different penpals in response to user feedback.
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Because the Pen Pal Record structure doesn't control the identity of the underlying database records, there's no risk that the changes made to local Pen Pal Record instances accidentally change values in the database. If another part of the app changes my Nickname and submits a change request back to the server, the most recently rejected penpal recommendation won't be mistakenly picked up by the change. Because the my ID property is declared as a constant, it can't change locally. As a result, requests to the database won't accidentally change the wrong record.
Structures and classes both support a form of inheritance. Structures and protocols can only adopt protocols; they can't inherit from classes. However, the kinds of inheritance hierarchies you can build with class inheritance can be also modeled using protocol inheritance and structures. If you're building an inheritance relationship from scratch, prefer protocol inheritance. Protocols permit classes, structures, and enumerations to participate in inheritance, while class inheritance is only compatible with other classes.
When you're choosing how to model your data, try building the hierarchy of data types using protocol inheritance first, then adopt those protocols in your structures. Article Choosing Between Structures and Classes. Decide how to store data and model behavior. Now we reach the phase for establishing relationships between classes. The relationships define how objects work together to perform the work needed for the application to run.
There are many types of relationships in object-oriented programming. The first one we will look at is the collaboration relationship.
This is because you can think of one class using another class. The idea of composition is that an object can be composed of other objects. An Order has a Customer. An Order also has a OrderItem.
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Here we have a diagram of a collaboration type relationship between classes. In this code we can see how the CustomerRepository class uses a Customer class. Inside of the Retrieve method, we see how a new Customer object is created. That object is then populated with data.
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In our case it is simply hard-coded, but you could imagine this data coming from a database. The CustomerRepository uses a Customer object once again in the Save method. In order for that method to do its job, it needs to accept a Customer object as a parameter. Inside that method, the data from the Customer object would be used to persist to the database. The repository classes use an entity class to populate the entity, or serialize the entity depending on if it is being used in the Retrieve or Save methods.
This pattern holds true for the other repository classes we have been working with as well. You can spot a collaboration type relationship any time you see a class use an instance of another class to perform an operation in the application. Composition is another key relationship type in object-oriented programming. A composition relationship exists when an object from one class, is made up of or composed of one or more objects from another class.
In our CRM application this type of relationship exists between the Customer class and the Address class. In the diagram above we show some composition relationships. The Order object is also composed of other objects. So we can kind of see how a given object can be composed of other objects to make the application work. The Composition relationship can be accomplished using references in our code, which leverage class properties. In the below code, we have highlighted a specific property.
The Customer object is composed of one or more Address objects. Why one or more? Because there could be a home address or a work address.
So we use a property of type List to allow for one or more Address objects to help compose a Customer object. A property declaration in the Customer class establishes the composition relationship between the Customer class and the Address class. Also note that since this property is a List, it must be initialized in the constructor like we also see below. If it is not, it would cause a null value exception. The key takeaway is that in composition style relationships, an object from one class is constructed from objects of another class.
In this case the construction of a Customer object constructs a List of Address objects. Often times Composition is preferred over Inheritance. To populate the Customer object with data, we need some code to do the work of retrieving that data. This would best be placed in an AddressRepository class. It will be used to get one or more addresses for a Customer. Interestingly, what we can do now is to set up a collaborative relationship between the CustomerRepository and AddressRepository classes. To display the dependency on the AddressRepository in CustomerRepository, we define the relationship at the top of the class and create an instance in the constructor.
Additionally, the private property addressRepository now holds an AddressRepository object. That code is highlighted here. So what is the purpose of this code then?
Well now, when any code requests to Retrieve a Customer, it will now retrieve a Customer and its associated Addresses in one swoop. Coupling is minimized as direct references are removed. You can think of these as simple id relationships.
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In the code below, we are highlighting the composition relationship between an Order and the Customer, as well as the composition relationship between the Order and the Address. That is great, but we have a problem. They do not actually show us any information.