Location Matters

The location of a piece of code always matters. I was fiddling with the Charts library today and found out that if you set a chart’s data to nil, it renders some informative text by default. My data source doesn’t pass nil but an empty array around, though, so I had to convert empty arrays to nil to make use of this feature.

Take this simple code:

func drawLineGraph(dataPoints: [DateBasedDataPoint]) {

    guard !dataPoints.isEmpty else {
        chartView.data = nil
        return
    }

    chartView.data = lineChartData(dataPoints: dataPoints)
}

private func lineChartData(dataPoints dataPoints: [DateBasedDataPoint]) 
    -> LineChartData {
    /* ... */
}

That’s my API’s entry point. The logic is simple enough and it achieves the desired effect. Either set data to nil or transform the values.

Now take this other version after a simple refactoring:

func drawLineGraph(dataPoints: [DateBasedDataPoint]) {

    chartView.data = lineChartData(dataPoints: dataPoints)
}

private func lineChartData(dataPoints dataPoints: [DateBasedDataPoint]) 
    -> LineChartData? {
        
    guard !dataPoints.isEmpty else { return nil }
    
    /* ... */
}

There, the conversion function takes care of returning nil if the dataPoints array is empty.

What’s the gain?

The drawLineGraph method now only takes care of obtaining compatible values and assigning them to the chartView. It doesn’t know what else to do with the dataPoints. Before, the method had knowledge of doing 2 things: obtaining values or setting the chartView to an “empty” state, depending on knowledge about the dataPoints.

Even though the guard statement is quite similar, moving the decision to a place where intimate knowledge about creating chart view data is required makes more sense and simplifies the call site (drawLineGraph).

When I’m done fiddling around with this, I can promote the conversion from a private method into an object on its own. Then the converter will have all the knowledge required to create data that the Charts library understands.

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