Future Perfect Continuous
The Future Perfect Continuous (or sometimes called the Future Perfect Progresive)
subject + will + have + been + ing form
subject + will not/won't + have + been + ing form
Will + subject + have + been + ing form
We can shall in formal contexts instead of will. Find more information on Will & Shall here
The Future Perfect Continuous is used to emphasise the duration of an action or activity ongoing at a specific moment in the future
Come September I will have been living in Spain for ten years
When we describe an action in the future perfect continuous tense, we are talki g in the present but our point of narrative is in the future looking back at the duration of on activity. The activity will have begun sometime in the past from that point of narrative and is expected to continue in the future.
This is a way of looking into the past from the future, but the activity we are looking at is in the future compared to the moment of speaking. That is to say, it is a way of talking about the future, the activity is in the future compared to the moment of speaking but in the past in comparison to the point of narrative. The activity happens between those two points
This is different to the grammar of future in the past, which is the reverse of this idea. Future in the past is a way of talking about the past, but the activity in the sentence is in the future from the point of narrative, but in the past from the moment of speaking. The activity is between those two points.