Custom assign operator that safely unwraps optionals and preserves existing value of the receiver (expression on the left side) unchanged, if the optional on the right side does not have a value (i.e. equal to nil).

Use this operator in these cases:

• to safely assign a regular optional value, only if it has non-nil value (otherwise operator will do nothing);
• to safely assign an implicitly unwrapped optional value, only if it has non-nil value (otherwise operator will do nothing).

BONUS: the value on the right will be automatically converted to receiver type before assign, so no need to cast explicitly.

Imagine you have a property that is required to always have a value, so you star with default value:

var title = 'Default value'

... and later you got a dictionary, that might have new value for the title:

let aDict: [String: AnyObject] = ... // maybe got from network?

This is how we can try to get new title value from the dictionary using standard Swift syntax:

if let newTitleValue = aDict["title"] as? String
{
    // yes it has a non-nil value
    title = newTitleValue
}

With optional assign operator you do the same with just this:

title ?= newTitleValue

See unit tests for more examples.

Starting from version 2.0, this library supports Swift 3. For compatibility with Swift 2.2 and Swift 2.3 use older version.