Archive for the ‘heavyduty’ Category

The UI Implications of Having a Big Finger

January 30, 2010

Among the settings that a user of my Twitter app, ThrededTweet, can adjust are the number of tweets that should be loaded at one time, and the time interval that should pass between reloads.  For the former, the allowed values are between 20 and 200, while for the latter it’s between 0 and 60 minutes.  These values are adjusted with a slider on the “Settings” page.

Currently (version 1.1.2 and earlier), the sliders behave in a purely linear manner. That is, the distance you have to slide the slider is the same to go from 5 minutes to 6 minutes, as it is to go from 55 minutes to 56 minutes. While this works well in the iPhone simulator, where user interface elements are manipulated with a mouse, on an actual iPhone I’ve discovered that it’s a different story. I’m 6’5″, with hands to match, and I’ve found it difficult to exercise the fine amount of control needed to hit a value precisely. If I want to reload every 5 minutes, sometimes I’ll end up with 4 minutes or 6 minutes.

In version 1.2.0, the sliders are still there, but after having a realization about usage, I’ve modified how they’re handled in a way that improves usability quite a bit. The realization is that, most of the time, people will be more concerned about hitting small values precisely than they will about hitting large values. Many people will want to select between a reload every minute vs. every two minutes, while few if any will be concerned about accurately selecting between 55 minutes and 56 minutes.

So as of 1.2.0, slider values are interpreted with an exponential function, rather than a linear function. This gives a lot more sensitivity for small values, at the cost of sensitivity for large values.

If we assume that a slider all the way left has value 0.0, and one all the way to the right has value 1.0, then the reload time can be defined like so:

reloadTime = 10^sliderValue * 6.666666 – 6.666666

For sliderValue=0, this gives a reload time of 0 minutes, while for sliderValue=1, this gives 60 minutes. In Objective-C, this is:

    int reloadTime = round(pow(10.0,reloadTimeSlider.value)*6.6666666-6.6666666);

Going the other way:

    reloadTimeSlider.value = log10((reloadTime+6.6666666)/6.6666666);

For the number of tweets to load, since the range is between 10^0*20 and 10^1*20 (20-200), the formula is simpler:

    int numTweetsToLoad = round(pow(10.0,feedCountSlider.value)*20.0);
    feedCountSlider.value = log10(numTweetsToLoad/20.0);

After making this change, I’ve found these sliders much easier to manipulate accurately on the actual phone.


Handling Ampersand Codes in ThrededTweet

December 21, 2009

One of the architectural issues I’ve been having with ThrededTweet is handling HTML ampersand codes, such as & , ⁁ , etc. ThrededTweet uses a UIWebView instance to display whole tweets, and that handles ampersand codes automatically. But the summaries of tweets that are displayed, such as on the main Feed page, use a UILabel.

For release 1.0.0, I manually added handling of what I assumed were common cases, but then immediately found an example of a tweet with an M-dash (—), which I didn’t handle. Version 1.0.1 expanded the set of ampersand codes that are handled, but as you can probably guess, as soon as 1.0.1 came out, I found example of another code that wasn’t handled. So I needed a comprehensive solution.

For verson  1.0.2, I decided to use a program called gperf to create code to automatically handle all ampersand codes. gperf is the GNU perfect hash function generator:

For a given list of strings, [gperf] produces a hash function and hash table, in form of C or C++ code, for looking up a value depending on the input string. The hash function is perfect, which means that the hash table has no collisions, and the hash table lookup needs a single string comparison only.

The first step was to get a list of all HTML ampersand codes, along with their corresponding Unicode code points, which is available on the “List of XML and HTML character entity reference” page on Wikipedia. I cut-and-pasted the code list, separated out the parts I wanted with a one-line Perl script, and stored the results in a file called codelist:


The default behavior of gperf is to create a function that just verifies whether or not a given string is in the hash table, but I needed it to return the Unicode code point as well. So I added the definition of the struct that would be stored in the hash table at the start of the codelist file:

struct code { char *name, int val; };

I then fed this to gperf (the “-t” means that I’m providing my own struct definition):

gperf –t codelist >amp.c

That created a file I could import into Xcode. The important function is that file is:

struct code *
in_word_set (str, len)
register const char *str;
register unsigned int len;

Feed it a string, and its length, and it will return the instance of “code” that corresponds to that string, or NULL if none was found.  If the pointer was not NULL, I still needed to compare code->str to the string I was looking for, in order to validate that we found the correct entry. Note that, for this case, the only reason that it would not match is there was a garbage ampersand code (e.g. “&iowfdna;”).

I then did some cleanup. “in_word_set()” became “is_amp_code()”, and I changed the function to something more modern than ye olde K&R style generated by gperf:

struct AmpCode *
is_amp_code (const char* str, unsigned int len)

The structure definition changed from this:

struct code { char *name, int val; };

to this:

typedef struct AmpCode { char *name, int val; } AmpCode_t;

Finally, “amp.c” became “Amp.m” (perhaps not strictly necessary), and I created an “Amp.h” that contained the struct definition and the prototype for is_amp_code().

I then imported Amp.m and Amp.h into Xcode, removed most of my old ampersand-code parsing code (keeping the part that found substrings that started with “&” and ended with “;”), and put a call to is_amp_code() in its place.

Voila! An ampersand-parser that handled anything I threw at it. If you were following my Twitter test account, you would have seen a string of tweets with all sorts of random ampersand codes, some legal, some not.