Swift 3: Converting Data to String returns a nil value

Question

Converting Data to String returns a nil value.

Code:

// thus unwraps the image
if let image = image{
        print("Saving image data")

    // don't unwrap here
        if let data = UIImagePNGRepresentation(image){ 
            let str = String(data: data, encoding: .utf8)

            print(str)

        }
    }

I don't know the reason. Also, how do I convert the String back to Data?

What are you actually trying to do? You shouldn't ever need to convert image data to a string — Fogmeister, Dec 12 '16 at 23:36
I am using EVReflection. I want to be able to json my classes. But data is creating a problem. — , Dec 12 '16 at 23:37
Basically, I just want an easy way to store my classes into SQLite. Any things close to Gson for swift? — , Dec 12 '16 at 23:38
You should be able to store binary data into sqlite. Shouldn't need to convert it to string. — Fogmeister, Dec 12 '16 at 23:40
I really don't want to write a whole scheme. I really want to do this : object.toJsonString() using EVReflection. Then simply save it. — , Dec 12 '16 at 23:42
@BillyJeans You should likely use the `NSCoding` api for serialization in swift/objc. — Joe Daniels, Dec 12 '16 at 23:44
Do the answers help at: http://stackoverflow.com/questions/29365145/how-to-encode-string-to-base64-in-swift — Son of a Beach, Dec 12 '16 at 23:58

Joe Daniels · Accepted Answer · 2016-12-12T23:42:43.577

6

This doesn't work because when you interpret the bytes of the Image as a String, the string is invalid. Not every jumble of data is a valid utf8 string. i.e. not every collection of n bits (8, sometimes 16) are a valid utf8 code point. The Swift String api loops through the data object you pass it to validate that it is a valid string. In your case, theres no reason to think that this Data is a valid string, so it doesn't work.

A good read on utf8: https://www.objc.io/issues/9-strings/unicode/

edited Dec 12 '16 at 23:42

answered Dec 12 '16 at 23:37

Joe Daniels

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You better use `base64` encoding to convert `Data` to `String`. – Ryan Dec 12 '16 at 23:49
How do I use base64 @Ryan – Dec 12 '16 at 23:50
3

base 64 is a bad solution for this kind of thing. – Joe Daniels Dec 12 '16 at 23:55
Strings store readable text, where readable text is defined as a data conforming to a specification/encoding (like utf8/16, ascii, etc). Your image data obviously doesn't fit that definition. – Joe Daniels Dec 12 '16 at 23:58
images also do not belong in sqlite. base64 blows up the size by quite a bit leading to significant inefficiency, and databases (esp. sqlite) aren't designed to store blobs of unstructured data, they're designed to store tabular data. – Joe Daniels Dec 13 '16 at 00:00
It is a bad solution. But this app isn't for commercial reasons. It is for my homework. The professor does not care how I do it. – Dec 13 '16 at 00:01
haha, understood... but the string api does care. I've tried this before, and I don't think there's any string encoding you can choose that will work in all cases. – Joe Daniels Dec 13 '16 at 00:03
1

Now That I have been developing for a while, I should have taken Ryan answer. The assignment was for school and base64 always does the trick. I hate stackoverflow because everyone over complicated everything. – Jan 12 '18 at 13:35

Swift 3: Converting Data to String returns a nil value

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