MDQueryGetResultAtIndex and UnsafeRawPointer in Swift 3 - swift

Question

I'm having trouble exploring the results of a Spotlight search in Swift 3 using MDQuery. I expect MDQueryGetResultAtIndex to yield an MDItem, and in C/Objective-C, that assumption works, and I can call MDItemCopyAttribute on it to explore the item. Here, for example, I successfully get the pathname of a found item:
MDItemRef item = (MDItemRef)MDQueryGetResultAtIndex(q,i);
CFStringRef path = MDItemCopyAttribute(item,kMDItemPath);
But in Swift 3, MDQueryGetResultAtIndex returns an UnsafeRawPointer! (it's a pointer-to-void in C). To get past that, I've tried, for example:
if let item = MDQueryGetResultAtIndex(q, 0) {
let ptr = item.bindMemory(to: MDItem.self, capacity: 1)
let path = MDItemCopyAttribute(ptr.pointee, kMDItemPath)
}
but that crashes, and logging shows that ptr.pointee is an NSAtom. It is quite evident that my personal UnsafeRawPointer mojo is not working (and to be frank I've always found this confusing).
How would I transform this UnsafeRawPointer into something I can successfully call MDItemCopyAttribute on?
Alternatives
I can get over this hump by putting my Objective-C code into an Objective-C helper object and calling it from Swift; but I'd like to write a pure Swift solution.
Similarly, I could probably rewrite my code to use the higher-level NSMetadataQuery, and I may well do so; but my original Objective-C code using the lower-level MDQueryRef works fine, so now I'm curious how to turn it directly into Swift.
Complete code for those who would like to try this at home:
let s = "kMDItemDisplayName == \"test\"" // you probably have one somewhere
let q = MDQueryCreate(nil, s as CFString, nil, nil)
MDQueryExecute(q, CFOptionFlags(kMDQuerySynchronous.rawValue))
let ct = MDQueryGetResultCount(q)
if ct > 0 {
if let item = MDQueryGetResultAtIndex(q, 0) {
// ...
}
}

Answer 1

The problem in your code
if let item = MDQueryGetResultAtIndex(q, 0) {
let ptr = item.bindMemory(to: MDItem.self, capacity: 1)
let path = MDItemCopyAttribute(ptr.pointee, kMDItemPath)
}
is that the UnsafeRawPointer is interpreted as a pointer to an
MDItem reference and then dereferenced in ptr.pointee, but
the raw pointer is the MDItem reference, so it is dereferenced
once too often.
The "shortest" method to convert the raw pointer to an MDItem reference
is unsafeBitCast:
let item = unsafeBitCast(rawPtr, to: MDItem.self)
which is the direct analogue of an (Objective-)C cast.
You can also use the Unmanaged methods
to convert the raw pointer to an unmanaged reference and from there
to a managed reference (compare How to cast self to UnsafeMutablePointer<Void> type in swift):
let item = Unmanaged<MDItem>.fromOpaque(rawPtr).takeUnretainedValue()
This looks a bit more complicated but perhaps expresses the intention
more clearly. The latter approach works also with (+1) retained
references (using takeRetainedValue()).
Self-contained example:
import CoreServices
let queryString = "kMDItemContentType = com.apple.application-bundle"
if let query = MDQueryCreate(kCFAllocatorDefault, queryString as CFString, nil, nil) {
MDQueryExecute(query, CFOptionFlags(kMDQuerySynchronous.rawValue))
for i in 0..<MDQueryGetResultCount(query) {
if let rawPtr = MDQueryGetResultAtIndex(query, i) {
let item = Unmanaged<MDItem>.fromOpaque(rawPtr).takeUnretainedValue()
if let path = MDItemCopyAttribute(item, kMDItemPath) as? String {
print(path)
}
}
}
}