Unwrapping multiple optionals in swift - ios

Question

I want to load a PDF that is in my application bundle into a CGPDFDocument.
Is there some way of calling a function that if any of the parameters that don't accept options have values that are nil, the function isn't called and nil is returned.
eg:
let pdfPath : String? = NSBundle.mainBundle().pathForResouce("nac_06", ofType:"pdf")
//I want to do this
let data : NSData? = NSData(contentsOfFile:pdfPath)
//I have to do this
let data : NSData? = pdfPath != nil ? NSData(contentsOfFile:pdfPath) : nil
let doc : CGPDFDocumentRef? = CGPDFDocumentCreateWithProvider(CGDataProviderCreateWithCFData(data));
//pageView.pdf is optional, nicely this function accepts the document as an optional
pageView.pdfPage = CGPDFDocumentGetPage(doc, 1);
Because NSData.init?(contentsOfFile path:String), doesn't define path as optional, even though it is has an optional return value, I have to check before and if the parameter is nil, return nil. Is there some syntactic sugar for the data assignment (instead of the ?: operator)?

Answer 1

Either use multiple optional bindings separated by commas
func loadPDF() -> CGPDFDocumentRef?
{
if let pdfPath = NSBundle.mainBundle().pathForResouce("nac_06", ofType:"pdf"),
data = NSData(contentsOfFile:pdfPath),
doc = GPDFDocumentCreateWithProvider(CGDataProviderCreateWithCFData(data)) {
return doc
} else {
return nil
}
}
or use the guard statement
func loadPDF() -> CGPDFDocumentRef?
{
guard let pdfPath = NSBundle.mainBundle().pathForResouce("nac_06", ofType:"pdf") else { return nil }
guard let data = NSData(contentsOfFile:pdfPath) else { return nil }
return GPDFDocumentCreateWithProvider(CGDataProviderCreateWithCFData(data))
}
All explicit type annotations are syntactic sugar and not needed.
Edit:
In your particular case you need only to check if the file exists and even this – the file is missing – is very unlikely in iOS. Another benefit is to be able to return a non-optional PDFDocument.
func loadPDF() -> CGPDFDocumentRef
{
guard let pdfPath = NSBundle.mainBundle().pathForResource("nac_06", ofType:"pdf") else {
fatalError("file nac_06.pdf does not exist")
}
let data = NSData(contentsOfFile:pdfPath)
return CGPDFDocumentCreateWithProvider(CGDataProviderCreateWithCFData(data!))!
}

Answer 2

I assume that you also don't want to continue with the execution of the function if pdfPath or data is nil. In this case, guard would be the best choice:
guard let pdfPath = NSBundle.mainBundle().pathForResouce("nac_06", ofType:"pdf") else {
// eventually also report some error
return
}
guard let data = NSData(contentsOfFile:pdfPath) else {
// eventually also report some error
return
}
// at this point you have a valid data object
You could also combine this into a single guard statement, to reduce the code duplication, you'll loose however in this case the possibility to know which of the two failed.
guard let pdfPath = NSBundle.mainBundle().pathForResource("nac_06", ofType:"pdf"),
data = NSData(contentsOfFile:pdfPath) else {
// eventually also report some error
return
}

Answer 3

There is two ways to achieve this.
Extend NSData class and create your own convenience init? method
Use the guard statement
I prefer the second method:
func getPDF(path : String?) -> CGPDFDocumentRef?
{
guard let filePath = path,
data = NSData(contentsOfFile: filePath),
pdf = GPDFDocumentCreateWithProvider(CGDataProviderCreateWithCFData(data)) else
{
return nil
}
return pdf
}
Call the method like:
let doc = getPDF(path : NSBundle.mainBundle().pathForResouce("nac_06", ofType:"pdf"))

Answer 4

You could do something fancy by defining a custom operator to deal with this situation. For example:
infix operator ^> {associativity left precedence 150}
func ^><T, U>(arg: T?, f: T->U?) -> U?{
if let arg = arg {
return f(arg)
} else {
return nil
}
}
The operator takes an optional left-side argument and a function that takes a non-optional and returns another optional as a right-side argument.
You could then write your code like this:
let pdfPath = NSBundle.mainBundle().pathForResource("nac_06", ofType:"pdf")
//the line below needs a NSData extension
let data = pdfPath ^> NSData.fileContents
let doc = data ^> CGDataProviderCreateWithCFData ^> CGPDFDocumentCreateWithProvider
//pageView.pdf is optional, nicely this function accepts the document as an optional
pageView.pdfPage = CGPDFDocumentGetPage(doc, 1)
Note that for this to work you need to add an extension to NSData, as you cannot map the init(contentsOfFile:) initializer to a generic function that can be passed to ^>.
extension NSData {
class func fileContents(path: String) -> NSData? {
return NSData(contentsOfFile: path)
}
}
The usage of the ^> operator reverts however the order you write the function names, if you prefer having the function names in the same order as the original code, you can add a reversed operator that does the same thing:
infix operator ^< {associativity right precedence 150}
func ^< <T, U>(f: T->U?, arg: T?) -> U?{
if let arg = arg {
return f(arg)
} else {
return nil
}
}
let pdfPath = NSBundle.mainBundle().pathForResource("nac_06", ofType:"pdf")
let data = NSData.fileContents ^< pdfPath
let doc = CGPDFDocumentCreateWithProvider ^< CGDataProviderCreateWithCFData ^< data
//pageView.pdf is optional, nicely this function accepts the document as an optional
pageView.pdfPage = CGPDFDocumentGetPage(doc, 1)