What is the difference between #available and @available

#available and @available are features related to the availability of APIs.

They are tools that mean to use together. Let's learn the difference and when to use them.

What is the difference between #available and @available

An availability condition (#available) is used as a condition of an if, while, and guard statement to query the availability of APIs at runtime.

An availability condition has the following form:

if #available(iOS 15, *) {
    // statements to execute if the APIs are available
} else {
    // fallback statements to execute if the APIs are unavailable
}

An @available is a Declaration attribute that you apply to a class or method declaration to specified platforms that those classes or methods support.

Here is an example of @available.

@available(iOS 16, *)
func newMethod() {
    // A method that available on iOS 16 forward.
}

@available(iOS 16, *)
class NewClass {
    // A class that available on iOS 16 forward.
}

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Summary

#available is a tool for API consumer. It helps us checking for the availability of features based on the platforms. This helps us cope with breaking changes in an API.

@available is a tool for API creator. It is a tool that lets you specify availability to the class/method, so the consumer of that API knows how to use it.

Example

Let's say I have a class, OldClass, with an instance method, oldMethod().

class OldClass {
    func oldMethod() {
        
    }
}

With the release of iOS 16, I want to add a new method that utilizes the iOS 16 feature. So, I create a new method, newMethod().

As, a creator, I mark my method with @available(iOS 16, *).

class OldClass {
    @available(iOS 16, *)
    func newMethod() {
        // Method that utilize iOS 16 features.
    }
    
    func oldMethod() {
        
    }
}

For users of my class, when they try to use newMethod() on iOS lower than 16, they will get the following error.

let oldClass = OldClass()
        
oldClass.newMethod()
// 'newMethod()' is only available in iOS 16 or newer

They can handle this using #available.

let oldClass = OldClass()

if #available(iOS 16, *) {
    oldClass.newMethod()
} else {
    // Fallback on earlier versions
}