Resistance is Useless

 I've been working may way through the UIKit components learning how they work and how to use them. Got a bit carried away with the UIPickerView and ended up building an entire app.

There's two tricks I've developed. The first is a simple means of getting the picker dials to wrap (by default the work like a list - once you reace the end you then have to scroll all the way back up to the beginning again). A number of the native apps, such as the alarm (but interestingly, not the timer) feature wrapping dials, and I thought it seemed like a good idea. The second trick was highlighting the selected row of each dial to make it more prominent and the overall look less like a kids toy.

Second one first. I have a class ResistorBand which all four band types inherit. The band objects are called upon by the controller (which acts as a delegate for the UIPickerView) to supply the views for their corresponding dial. I've subclassed UIView and overwritten  - (void)drawRect:(CGRect)rect which actually paints each square. If the view is not the selected view for that row, it has a white rectangle with a 0.6 alpha drawn over it. This has the effect of slightly muting the non-selected colours.

Getting the dials to wrap up being simple in the end, thanks to a suggestion from a friend of mine [thanks Matt]. 

To start, we need to tell the picker there's three times the number of rows than there really are:

- (NSInteger)pickerView:(UIPickerView *)pickerView

numberOfRowsInComponent:(NSInteger)component {

    return [[self bandForIndex:component] viewCount]* 3;

}

Next, when the picker asks for a particular view, we need to map the row it's asking for, with the actual views, ie just row mod viewCount:

return [views objectAtIndex:position % [self viewCount]];

Finally, when the user selects a row, as well as our normal logic, we need to make sure the picker row selected is always in the range [band viewCount] and [band viewCount]*2-1 (inclusive). This means that whenever the picker draws the selectd view, there will always be others above and below it (we'll never reach the end of the list):

- (void)pickerView:(UIPickerView *)pickerView

      didSelectRow:(NSInteger)row

       inComponent:(NSInteger)component {

    ResistorBand *band= [self bandForIndex:component];

    [band selectPosition:row % [band viewCount]];

    NSInteger updatedRow= row;

    if (row< [band viewCount]) {

        updatedRow= row+ [band viewCount];

    } else if (row>= 2* [band viewCount]) {

        updatedRow= row- [band viewCount];

    }

    [pickerView selectRow:updatedRow

               inComponnt:component

                 animated:NO];

    [self updateDisplay];

}

Hope this makes sense.