What does [urlString componentsSeperatedByString: #"?"] [0]? mean in Swift? - objective-c

Question

I'm not entirely sure what
NSString * fileName = [self cachedFileNameForKey:[urlString componentsSeparatedByString:#"?"][0]];
Means from this code. I am primely writing in Swift so this notation is a bit confusing for me. What does the double [ ] notation mean? 3d array?
+ (AVPlayerItem *)localDownloadedVideoFromUrl:(NSURL *)url {
NSString * urlString = url.absoluteString;
NSString * fileName = [self cachedFileNameForKey:[urlString componentsSeparatedByString:#"?"][0]];
TWRDownloadManager * manager = [TWRDownloadManager sharedManager];
if ([manager fileExistsWithName:fileName]) {
AVPlayerItem * item = [AVPlayerItem playerItemWithURL:[NSURL fileURLWithPath:[manager localPathForFile:fileName]]];
return item;
}
return [NSNull null];
}

Answer 1

[] in objective-c is not only used to access arrays, but also used to call methods. Surprising, isn't it?
In general,
[xxx someMethod];
is equivalent to:
xxx.someMethod()
in swift.
So here:
[urlString componentsSeparatedByString:#"?"][0]
means
urlString.components(separatedBy: "?")[0] // "[0]" can also be replaced by ".first", which is safer.
This is then passed to the cachedFileNameForKey as a parameter.

Answer 2

In Swift
var urlString: String = "Know someone who can answer? Share a link to this" // Example string or url.absoluteString
let fileName = urlString.components(separatedBy: "?")
print(fileName)
let string1 = fileName[0]
let string2 = fileName[1]
Hope will helpful to you

Answer 3

Ahh so mistake on my part is that I didn't see what the method -cacheFileNameForKey: was doing.
+ (nullable NSString *)cachedFileNameForKey:(nullable NSString *)key {
const char *str = key.UTF8String;
if (str == NULL) {
str = "";
}
unsigned char r[CC_MD5_DIGEST_LENGTH];
CC_MD5(str, (CC_LONG)strlen(str), r);
NSString *filename = [NSString stringWithFormat:#"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%#",
r[0], r[1], r[2], r[3], r[4], r[5], r[6], r[7], r[8], r[9], r[10],
r[11], r[12], r[13], r[14], r[15], [key.pathExtension isEqualToString:#""] ? #"" : [NSString stringWithFormat:#".%#", key.pathExtension]];
return filename;
}
I guess its used for a function to generate a file name for a temporary movie file.