如何将不可变的Swift属性初始化为计算值？

Question

When I have this initializer:

let channels : [TVChannel]

required init?(json : NSObject)
{
    if let x = json as? [NSObject]
    {
        self.channels = x.map { TVChannel(json:$0)! }
    }
}

The compiler gives me the error:

Error:(12, 14) constant 'self.channels' used before being initialized

Why is this ? And how do I initialize the property my mapped array of TVChannel ?

Answer 1

The compiler needs to know how to initialize self.channels when the control flow doesn't enter the if let condition.

A solution could be to provide an else branch:

if let x = json as? [NSObject] {
    self.channels = x.map { TVChannel(json:$0)! }
} else {
    self.channels = []
}

Answer 2

You need to handle both cases where json is and isn't an array of NSObjects. Alternatively you could define a default value to the property and make it a var

eg

required init?(json : NSObject) {

  if let x = json as? [NSObject] {
    channels = x.map { TVChannel(json: $0) }
  } else {
    channels = [ ]
  }

}

or

var channels: [TVChannel] = [ ]

or

var channels = [TVChannel]()