Sunday, July 2, 2017

iOS 11 beta 1,2 - NSString sizeWithAttributes - breaking change and potential bug

Edge behavior of [NSString sizeWithAttributes] seems to be changed in iOS 11 beta 1 and 2. When asked for the


float size around 0.0;
    
    NSString *testString = @"Some test string";
    NSString *fontName = @".SFUIText-Semibold";
    
    UIFont *targetFont = [UIFont fontWithName:fontName size:size];
    
    CGSize stringUISize = [testString sizeWithAttributes:@
                           {
                           NSFontAttributeName: targetFont
                           }];

You may get very different from iOS 7- 10 values. Here is the test:



While running it on iOS 7-10, everything converges to zero string UI width:

2017-07-02 10:11:45.996 speedometer[23301:7084973] attempted font size: 1, calculated string width: 12.157471, delta: -12
2017-07-02 10:11:45.997 speedometer[23301:7084973] attempted font size: 0, calculated string width: 4.052490, delta: -4

2017-07-02 10:11:46.005 speedometer[23301:7084973] attempted font size: 0, calculated string width: 0.000000, delta: 0

 while on iOS 11 beta 1 and 2 the same test case is "stuck" on value 97 something on iPhone 6 Plus:

2017-07-02 08:29:07.157511+0200 speedometer[36517:257340] attempted font size: 4, calculated string size: 36.437256, delta: -36
2017-07-02 08:29:07.158646+0200 speedometer[36517:257340] attempted font size: 3, calculated string size: 28.340088, delta: -28
2017-07-02 08:29:07.159723+0200 speedometer[36517:257340] attempted font size: 2, calculated string size: 20.242920, delta: -20
2017-07-02 08:29:07.160738+0200 speedometer[36517:257340] attempted font size: 1, calculated string size: 12.145752, delta: -12
2017-07-02 08:29:07.161953+0200 speedometer[36517:257340] attempted font size: 0, calculated string size: 4.048584, delta: -4
2017-07-02 08:29:07.166740+0200 speedometer[36517:257340] attempted font size: 0, calculated string size: 97.166016, delta: -97
2017-07-02 08:29:07.167756+0200 speedometer[36517:257340] attempted font size: -1, calculated string size: 97.166016, delta: -97

2017-07-02 08:29:07.168736+0200 speedometer[36517:257340] attempted font size: -2, calculated string size: 97.166016, delta: -97

Looks to me as a broken "contract" and invariant. One would really expect the consistent behavior as it was since iOS7 and having the width of the string calculated as zero for zero font size and zero it should be downwards? Also string width jumping from about approaching zero values to 97.166016 when value got a bit on below zero looks inconsistent. Probably a missing unit test :)!

For me it looks like a bug and I'm reporting it to Apple now. Hopefully it will be fixed to the same consistent behavior as it used to be on iOS 7 - 10.

Defensive coding is like an alloc/release for me, I try to make it right and upfront, but doing static analysis later I found myself being "mentally away" way too often.

Yours and wishing you happy coding!
Stan.
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