Use nil coalescing in ChartDataSet's entryCount (Fixes #631)

[aarondaub]

The force unwrapping in entryCount caused a crash when yVals was nil. Now entryCount returns 0.
#631

[liuxuan30]

I'm not sure if we should initialize yVals in init()?

    public override required init()
    {
        super.init()
    }

It does not initialize yVals, so it will crash. Not sure which way is better.

BTW, _yVals?.count is Int?, while entryCount is Int, though seems swift will help unwrap it. Not sure if we should make the return type consistent

[aarondaub]

@liuxuan30 Having that initializer keep properties in the same state as other initializers would make sense.

I'm not sure I follow what you mean about making the return type consistent – making entryCount Int? requires changes throughout the rest of the codebase but having it fail safely (by using ??) only requires changing the getter itself.

[liuxuan30]

@aarondaub, I am no expert of the details of swift, so what I want to ask is, _yVals?.count will return Int? instead of Int, while entryCount is declared as Int type. Let's say the yVals has 5 entries, so it will return Optional(5). I tried the code, and swift will help unwrap, and entryCount is assigned as 5.
I am just not sure, if this is the right way to do so, or we should follow the style in documentation:

if let count = _yVals?.count 
{
   ...
} else {
   ...
}

I am just being cautious about ? :)

[aarondaub]

@liuxuan30 ah, let me clarify.

You're right that

yVals?.count

is of type Int? so it is not a valid return type for a function (in this case entryCount) that returns Int. In fact it wouldn't compile if the implementation was just:

public var entryCount: Int { return _yVals? }

To get around this, I used the nil coalescing operator ??. What ?? does is unwraps its left hand argument and return that if it is not nil, otherwise it returns its right hand argument.

So nil ?? 0 is 0, and Optional(1) ?? 0 is 1.

As far as style goes: I personally think the nil coalescing operator is fine, adding an explicit if-let/else would just be unnecessary bulk IMHO.

[danielgindi]

Seems fine to me :-) Thanks!