UIPickerView – spinning multiple components

by Josh Highland on September 17, 2009

grey UIPickerView   spinning multiple components

I found an interesting issues with the iPhone SDK and the UIPickerView

The didSelectRow method gets called after a users selects a new value on a picker wheel. The method knows what wheel (component) was spun and the index of the newly selected value. This works great for a picker with a single wheel. My picker had 5 wheels in it.

If I spun two components of the picker at the same time. I would only get one call to the didSelectRow method. More specifically, if I spin one component and move another component no call was received until the first component stops spinning, and so the second component move is never registered.

Long story short, I didn’t catch this problem when I submitted my app, “Ka-Ching”, to the iTunes store. As a result of not catching this, Apple rejected my app. I could argue that the iPhone SDK should have so programmatic hooks that would only allow one component to be spun at a time, or even a method to return the state of all of components in the picker.

grey UIPickerView   spinning multiple components

Trying to figure out a solution to the problem I expanded on my idea of recording the state of all the components in the picker when the didSelectRow method was called. Realistically, by the time that didSelectRow gets processed it is possible that all 5 of the components had changed.

Looking through the SDK documentation for the UIPickerView I came across this:

- (NSInteger)selectedRowInComponent:(NSInteger)component

With this method you can determine the selected row index for any of the components in your picker. you don’t have to rely on the values passed into the didSelectRow method.

The wrong way to do it

- (void)pickerView:(UIPickerView *)thePickerView
 didSelectRow:(NSInteger)row
 inComponent:(NSInteger)component
{
	//get the value of the component that was changed
	if(component == 0)
	{
        	currency_value = [currency objectAtIndex:row];
	}
	else if(component == 1)
	{
        	hundereds_value = [hundereds objectAtIndex:row];
 
	}
	else if(component == 2)
	{
        	tens_value = [tens objectAtIndex:row];
	}
 
	else if(component == 3)
	{
        	ones_value = [ones objectAtIndex:row];
	}
 
	else if(component == 4)
	{
        	change_value = [cents objectAtIndex:row];
	}
 
	//some code here to save the values
 
}

The right way to do it

- (void)pickerView:(UIPickerView *)thePickerView
 didSelectRow:(NSInteger)row
 inComponent:(NSInteger)component
{
 
	//get the values
	NSInteger currency_NewRow = [thePickerView selectedRowInComponent:0];
	NSInteger hundereds_NewRow = [thePickerView selectedRowInComponent:1];
	NSInteger tens_NewRow = [thePickerView selectedRowInComponent:2];
	NSInteger ones_NewRow = [thePickerView selectedRowInComponent:3];
	NSInteger cents_NewRow = [thePickerView selectedRowInComponent:4];
 
	//some code to save the values 
 
}

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