DebuggingTechniques.md

Debugging Techniques

This document lists debugging techniques and infrastructure helpful for debugging ReactiveCocoa applications.

Unscrambling Swift compiler errors

Type inferrence can be a source of hard-to-debug compiler errors. There are two potential places to be wrong when type inferrence used:

1. Definition of type inferred variable
2. Consumption of type inferred variable

In both cases errors are related to incorrect assumptions about type. Such issues are common for ReactiveCocoa applications as it is all about operations over data and related types. The current state of the Swift compiler can cause misleading type errors, especially when error happens in the middle of a signal chain.

Below is an example of type-error scenario:

SignalProducer<Int, NoError>(value:42)
    .on(next: { answer in
        return _
    })
    .startWithCompleted {
        print("Completed.")
    }

The code above will not compile with the following error on a print call error: ambiguous reference to member 'print' print("Completed."). To find the actual compile error, the chain needs to be broken apart. Add explicit definitions of closure types on each of the steps:

let initialProducer = SignalProducer<Int, NoError>.init(value:42)
let sideEffectProducer = initialProducer.on(next: { (answer: Int) in
    return _
})
let disposable = sideEffectProducer.startWithCompleted {
    print("Completed.")
}

The code above will not compile too, but with the error error: cannot convert value of type '(_) -> _' to expected argument type '(Int -> ())?' on definition of on closure. This gives enough of information to locate unexpected return _ since on closure should not have any return value.

Binding DynamicProperty with <~ operator

Using the <~ operator to bind a Signal or a SignalProducer to a DynamicProperty can result in unexpected compiler errors.

Below is an example of this scenario:

let label = UILabel()
let property = MutableProperty<String>("")

DynamicProperty(object: label, keyPath: "text") <~ property.producer

This will often result in a compiler error:

error: binary operator '<' cannot be applied to operands of type 'DynamicProperty' and 'SignalProducer<String, NoError>' DynamicProperty(object: label, keyPath: "text") < property.producer

The reason is a limitation in the swift type checker - A DynamicProperty always has a type of AnyObject?, but the <~ operator requires the values of both sides to have the same type, so the right side value would have to be AnyObject? as well, but usually a more concrete type is used (in this example String).

Usually, the fix is as easy as adding a .map{ $0 }.

DynamicProperty(object: label, keyPath: "text") <~ property.producer.map { $0 }

This allows the type checker to infer that String can be converted to AnyProperty? and thus, the binding succeeds.

Debugging event streams

As mentioned in the README, stream debugging can be quite difficut and tedious, so we provide the logEvents operator. In its simplest form:

let searchString = textField.rac_textSignal()
    .toSignalProducer()
    .map { text in text as! String }
    .throttle(0.5, onScheduler: QueueScheduler.mainQueueScheduler)
    .logEvents()

This will print to the standard output the events. For most use cases, this is enough and will greatly help you understand your flow. The biggest problem with this approach, is that it will continue to ouput in Release mode. This leaves with you with two options:

1. Comment out the operator: //.logEvents(). This is the simpleste approach, but it's error prone, since you will eventually forget to do this.
2. Pass your own function and manipulate the output as you see fit. This is the recommended approach.

Let's see how this would look like if we didn't want to print in Release mode:

func debugLog(identifier: String, event: String, fileName: String, functionName: String, lineNumber: Int) {
   // Don't forget to set up the DEBUG symbol (http://stackoverflow.com/a/24112024/491239)
   #if DEBUG
      print(event)
   #endif
}

You would then:

let searchString = textField.rac_textSignal()
    .toSignalProducer()
    .map { text in text as! String }
    .throttle(0.5, onScheduler: QueueScheduler.mainQueueScheduler)
    .logEvents(logger: debugLog)

We also provide the identifier parameter. This is useful when you are debugging multiple streams and you don't want to get lost:

let searchString = textField.rac_textSignal()
    .toSignalProducer()
    .map { text in text as! String }
    .throttle(0.5, onScheduler: QueueScheduler.mainQueueScheduler)
    .logEvents(identifier: "✨My awesome stream ✨")

There also cases, specially with [hot signals][[Signals]], when there is simply too much output. For those, you can specify which events you are interested in:

let searchString = textField.rac_textSignal()
    .toSignalProducer()
    .map { text in text as! String }
    .throttle(0.5, onScheduler: QueueScheduler.mainQueueScheduler)
    .logEvents(events:[.Disposed]) // This will happen when the `UITextField` is released