Let's first start with some imperative swift code.
The purpose of example is to bind identifier c to a value calculated from a and b if some condition is satisfied.
Here is the imperative swift code that calculates the value of c:
// this is usual imperative code
var c: String
var a = 1 // this will only assign value `1` to `a` once
var b = 2 // this will only assign value `2` to `b` once
if a + b >= 0 {
c = "\(a + b) is positive" // this will only assign value to `c` once
}The value of c is now 3 is positive. But if we change the value of a to 4, c will still contain the old value.
a = 4 // c will still be equal "3 is positive" which is not good
// c should be equal to "6 is positive" because 4 + 2 = 6This is not the wanted behavior.
To integrate RxSwift framework into your project just include framework in your project and write import RxSwit.
This is the same logic using RxSwift.
let a /*: Observable<Int>*/ = Variable(1) // a = 1
let b /*: Observable<Int>*/ = Variable(2) // b = 2
// This will "bind" rx variable `c` to definition
// if a + b >= 0 {
// c = "\(a + b) is positive"
// }
let c = combineLatest(a, b) { $0 + $1 } // combines latest values of variables `a` and `b` using `+`
.filter { $0 >= 0 } // if `a + b >= 0` is true, `a + b` is passed to map operator
.map { "\($0) is positive" } // maps `a + b` to "\(a + b) is positive"
// Since initial values are a = 1, b = 2
// 1 + 2 = 3 which is >= 0, `c` is intially equal to "3 is positive"
// To pull values out of rx variable `c`, subscribe to values from `c`.
// `subscribeNext` means subscribe to next (fresh) values of variable `c`.
// That also includes the inital value "3 is positive".
c.subscribeNext { print($0) } // prints: "3 is positive"
// Now let's increase the value of `a`
// a = 4 is in RxSwift
a.value = 4 // prints: 6 is positive
// Sum of latest values is now `4 + 2`, `6` is >= 0, map operator
// produces "6 is positive" and that result is "assigned" to `c`.
// Since the value of `c` changed, `{ print($0) }` will get called,
// and "6 is positive" is printed.
// Now let's change the value of `b`
// b = -8 is in RxSwift
b.value = -8 // doesn't print anything
// Sum of latest values is `4 + (-8)`, `-4` is not >= 0, map doesn't
// get executed.
// That means that `c` still contains "6 is positive" and that's correct.
// Since `c` hasn't been updated, that means next value hasn't been produced,
// and `{ print($0) }` won't be called.
// ...- instead of binding to variables, let's bind to text field values (rx_text)
- next, parse that into an int and calculate if the number is prime using an async API (map)
- if text field value is changed before async call completes, new async call will be enqueued (concat)
- bind results to label (bindTo(resultLabel.rx_text))
let subscription/*: Disposable */ = primeTextField.rx_text // type is Observable<String>
.map { WolframAlphaIsPrime($0.toInt() ?? 0) } // type is Observable<Observable<Prime>>
.concat() // type is Observable<Prime>
.map { "number \($0.n) is prime? \($0.isPrime)" } // type is Observable<String>
.bindTo(resultLabel.rx_text) // return Disposable that can be used to unbind everything
// This will set resultLabel.text to "number 43 is prime? true" after
// server call completes.
primeTextField.text = "43"
// ...
// to unbind everything, just call
subscription.dispose()All of the operators used in this example are the same operators used in the first example with variables. Nothing special about it.
If you are new to Rx, next example will probably be a little overwhelming, but it's here to demonstrate how RxSwift code looks like in real world examples.
The third example is a real world, complex UI async validation logic, with progress notifications.
All operations are cancelled the moment disposeBag is deallocated.
Let's give it a shot.
// bind UI control values directly
// use username from `usernameOutlet` as username values source
self.usernameOutlet.rx_text
.map { username in
// synchronous validation, nothing special here
if count(username) == 0 {
// Convenience for constructing synchronous result.
// In case there is mixed synchronous and asychronous code inside the same
// method, this will construct an async result that is resolved immediatelly.
return just((valid: false, message: "Username can't be empty."))
}
...
// Every user interface probably shows some state while async operation
// is executing.
// Let's assume that we want to show "Checking availability" while waiting for result.
// valid parameter can be
// * true - is valid
// * false - not valid
// * nil - validation pending
let loadingValue = (valid: nil, message: "Checking availability ...")
// This will fire a server call to check if the username already exists.
// Guess what, its type is `Observable<ValidationResult>`
return API.usernameAvailable(username)
.map { available in
if available {
return (true, "Username available")
}
else {
return (false, "Username already taken")
}
}
// use `loadingValue` until server responds
.startWith(loadingValue)
}
// Since we now have `Observable<Observable<ValidationResult>>`
// we somehow need to return to normal `Observable` world.
// We could use `concat` operator from second example, but we really
// want to cancel pending asynchronous operation if new username is
// provided.
// That's what `switchLatest` does
.switchLatest()
// Now we need to bind that to the user interface somehow.
// Good old `subscribeNext` can do that
// That's the end of `Observable` chain.
// This will produce a `Disposable` object that can unbind everything and cancel
// pending async operations.
.subscribeNext { valid in
errorLabel.textColor = validationColor(valid)
errorLabel.text = valid.message
}
// Why would we do it manually, that's tedious,
// let's dispose everything automagically on view controller dealloc.
.addDisposableTo(disposeBag)Can't get any simpler than this. There are more examples in the repository, so feel free to check them out.
They include examples on how to use it in the context of MVVM pattern or without it.