In a short way:

let reduced = array.reduce(0) { $1.isAvailable ? 1.bars.count : $0 }

Now, let's explicit it:

The logic is quite simple:

• We set the initial value to 0
• In the closure, we have two parameters, the first one is the current value (at start it's the initial value), which we'll increment at each iteration, and the second one is the element of the array. We return then the new value (partial) that we incremented or not according to your condition (here is isAvailable)

Explicitly:

let reduced2 = array.reduce(0) { partial, current in
    if current.isAvailable {
        return partial + current.bars.count
    } else {
        return partial
    }
}

With a ternary if:

let reduced3 = array.reduce(0) { partial, current in
    return current.isAvailable ? partial + current.bars.count : partial
}

Getting rid of the return, see Functions With an Implicit Return in Functions of Swift or Implicit Returns from Single-Expression Closures of Closures of Swift.

let reduced4 = array.reduce(0) { partial, current in
    current.isAvailable ? partial + current.bars.count : partial
}

With Shorthand Argument Names on Closures of Swift.

let reduced5 = array.reduce(0) { $1.isAvailable ? 1.bars.count : $0 }