How do you add a Dictionary of items into another Dictionary

In Swift 4, one should use merging(_:uniquingKeysWith:):

Example:

let dictA = ["x" : 1, "y": 2, "z": 3]
let dictB = ["x" : 11, "y": 22, "w": 0]

let resultA = dictA.merging(dictB, uniquingKeysWith: { (first, _) in first })
let resultB = dictA.merging(dictB, uniquingKeysWith: { (_, last) in last })

print(resultA) // ["x": 1, "y": 2, "z": 3, "w": 0]
print(resultB) // ["x": 11, "y": 22, "z": 3, "w": 0]

Swift 4 provides merging(_:uniquingKeysWith:), so for your case:

let combinedDict = dict1.merging(dict2) { $1 }

The shorthand closure returns $1, therefore dict2's value will be used when there is a conflict with the keys.

You can define += operator for Dictionary, e.g.,

func += <K, V> (left: inout [K:V], right: [K:V]) { 
    for (k, v) in right { 
        left[k] = v
    } 
}