Swift Combine: prepend() operator

In this tutorial, you are going to look at prepend() operator. Just as the name implies, the prepend() operator is going to append element in your sequence.

A publisher

let numbers = (1...5).publisher
numbers.sink {
		print($0) // 1,2,3,4,5
}

I created a range from 1 to 5 and made it a publisher. You can see 1,2,3,4,5 are printed out. But what if I want to prepend a value before the sequence happens?

prepend()

let numbers = (1...5).publisher
numbers
	.prepend(100)	
	.sink {
		print($0) // 100, 1,2,3,4,5
}
numbers
	.prepend(100, 101)	
	.sink {
		print($0) // 100, 101, 1,2,3,4,5
}
numbers
	.prepend(100, 101)	
	.prepend(-1, -2) 
	.sink {
		print($0) // -1, -2, 100, 101, 1,2,3,4,5
}
numbers
	.prepend([45, 47])
	.sink {
	  print($0) // 45, 47, 1, 2, 3, 4, 5
}

When you use this .prepend() operator, you can prepend some element. You can actually pass in a number or numbers to prepend.

Since you are using combination operators and the reactive way of writing your code, you can also chain different operator.

Not only you can provide a variadic parameter (sending multiple values in the argument), you can pass in an array.

Combining Publisher with prepend()

let numbers = (1...5).publisher
let publisher2 = (500...510).publisher
numbers
	.prened(publisher2)
	.sink {
		print($0) //500, 501, 502, 503, 504, 505, ... 510, 1, 2, 3, 4, 5
}

Besides numbers, I created another publisher from 500 to 510. You can pass in a publisher. You can check the result.

Conclusion

• prepend() allows you to add elements before the sequence.
• You can pass in variadic parameters , arrays and publisher as a parameter.