How can I define a variable which could initialise from two different classes?

Question

Sorry about the slightly vague question title but I found it very difficult to get it straight in my head myself.

The issue here is that I have two different dataSources I might be initialising and loading data from. Depending on the data changes which dataSource I need.

The problem I am having is how to define a variable of that dataSource when it could come from two different classes.

If I define them in my interface:

BColumnChartDataSource * chartDatasource = [[BColumnChartDataSource alloc] initWithExercise:_exercise];
BDoubleColumnChartDataSource * chartDatasource = [[BDoubleColumnChartDataSource alloc] initWithExercise:_exercise];

Then it obviously doesn't like them being called the same thing.

If I try to put them in an if statement then they aren't available outside the logic statement

if (_exercise.unitTypeLinks.count < 2) {
    BColumnChartDataSource * chartDatasource = [[BColumnChartDataSource alloc] initWithExercise:_exercise];
    }
else {
    BDoubleColumnChartDataSource * chartDatasource = [[BDoubleColumnChartDataSource alloc] initWithExercise:_exercise];
}

Eventually I want to put them into a statement like this so I could put an if statement into every one of these but it is an awfully verbose way which might take more time if I add more dataSources.

// Get the exercise event list for our clicked exercise
_exerciseEventList = [chartDatasource getExerciseEventList];

I think I must be missing something obvious here so thank you for any help you can give

Answer 1

You should create create a base class, and inherit both of those classes with the base class.

@interface BDataSource : NSObject
@end

@interface BColumnChartDataSource : BDataSource
//your custom implementation here
@end

@interface BDoubleColumnChartDataSource : BDataSource
//your custom implementation here
@end

After that you can initialise your datasource like this

BDataSource *dataSource = nil;

if (_exercise.unitTypeLinks.count < 2) {
    dataSource = [[BColumnChartDataSource alloc]     initWithExercise:_exercise];
}
else {
    dataSource = [[BDoubleColumnChartDataSource alloc] initWithExercise:_exercise];
}