Combine publishers merge, zip, and combineLatest on iOS

While working on an app where I needed to subscribe to multiple Combine publishers, I got confused about if I should use merge, zip or combineLatest. These publishers are quite similar with subtle differences. For making sure I never get confused about it, I am going to present examples in this week’s blog post.

Merge

Merge publisher just re-publishes any values received from any of the publisher. Useful when there are multiple sources of data we would like to combine into a single flow of updates.

	@Published var state1 = "0"
	@Published var state2 = "a"
	func mergeExample() {
	$state1.merge(with: $state2)
	.sink { value in
	print("sink", value)
	}
	.store(in: &cancellables)
	print("will change state1 to 1")
	state1 = "1"
	print("will change state1 to 2")
	state1 = "2"
	print("will change state2 to b")
	state2 = "b"
	print("will change state1 to 3")
	state1 = "3"
	print("will change state2 to c")
	state2 = "c"
	}
	/* output:
	sink: 0
	sink: a
	will change state1 to 1
	sink: 1
	will change state1 to
	sink: 2
	will change state2 to b
	sink: b
	will change state1 to 3
	sink: 3
	will change state2 to c
	sink: c
	*/

view raw Merge.swift hosted with ❤ by GitHub

Zip

Zip waits until it has received at least one element from each of the underlying publisher, and then delivers the value as a tuple. If one of the publisher publishes multiple values, then the first received value is part of the tuple and other values are part of next tuples after that.

	@Published var state1 = "0"
	@Published var state2 = "a"
	func zipExample() {
	$state1.zip($state2)
	.sink { value in
	print("zip", value)
	}
	.store(in: &cancellables)
	print("will change state1 to 1")
	state1 = "1"
	print("will change state1 to 2")
	state1 = "2"
	print("will change state2 to b")
	state2 = "b"
	print("will change state1 to 3")
	state1 = "3"
	print("will change state2 to c")
	state2 = "c"
	}
	/* output
	sink ("0", "a")
	will change state1 to 1
	will change state1 to 2
	will change state2 to b
	sink ("1", "b")
	will change state1 to 3
	will change state2 to c
	sink ("2", "c")
	*/

view raw Zip.swift hosted with ❤ by GitHub

CombineLatest

CombineLatest publishes a tuple whenever any of the underlying publishers emits an element. The tuple contains the latest value from each of the publisher.

	@Published var state1 = "0"
	@Published var state2 = "a"
	func combineLatestExample() {
	$state1.combineLatest($state2)
	.sink { value in
	print("sink", value)
	}
	.store(in: &cancellables)
	print("will change state1 to 1")
	state1 = "1"
	print("will change state1 to 2")
	state1 = "2"
	print("will change state2 to b")
	state2 = "b"
	print("will change state1 to 3")
	state1 = "3"
	print("will change state2 to c")
	state2 = "c"
	}
	/* example
	sink ("0", "a")
	will change state1 to 1
	sink ("1", "a")
	will change state1 to 2
	sink ("2", "a")
	will change state2 to b
	sink ("2", "b")
	will change state1 to 3
	sink ("3", "b")
	will change state2 to c
	sink ("3", "c")
	*/

view raw CombineLatest.swift hosted with ❤ by GitHub

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