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What the heck is “associatetype” in Swift?

12/24/2022

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An associative type in Swift is a type that is associated with a protocol. This means that the type can be specified as a placeholder for another type, and the actual type that is used will be determined when the protocol is implemented.

For example, consider a protocol called Container that defines a type called Item:

    
​Here, the Item type is an associative type, because it is associated with the Container protocol. This means that when we implement the Container protocol, we can specify the actual type that will be used for the Item type. For example, we could implement the Container protocol like this:

    
In this example, we have implemented the Container protocol for a stack of generic type. We have specified that the Item type will be Generic by using the typealias keyword. This means that wherever the Item type is used in the Container protocol, it will be replaced with the Generic type.
​
Using associative types in this way allows us to create generic protocols that can be implemented with different types. This can be useful for creating reusable components that can be used in different contexts, and it allows us to take advantage of the type safety features of Swift.

Let's see an example of the Int Implementation:

    
Using the same stack, let's see an implementation of a String Type:

    

Another example of how we can use an associatetype in a protocol.

​Imagine that we have a protocol called DataSource that defines a type called Data, and provides a function for retrieving data from the source:

    
We can implement this protocol in different ways depending on the type of data that we want to retrieve. For example, we could create a FileDataSource that retrieves data from a file on disk:

    
​In this implementation, we have specified that the Data type will be String, so the getData() function will return a string. We can also create other implementations of the DataSource protocol that use different types for the Data type. For example, we could create a NetworkDataSource that retrieves data from a network resource:

    
In this implementation, we have specified that the Data type will be a dictionary, so the getData() function will return a dictionary.

Using associative types in this way allows us to create protocols that are flexible and can be implemented in different ways, depending on the specific needs of the application. This can be useful for creating reusable components that can be used in different contexts, and it allows us to take advantage of the type safety features of Swift.
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