notifyDataChanged() will crash for dataset without entries

[whvandervelde]

More specifically
Setting CandleChartData with a dataset containing 0 entries and then calling data?.notifyDataChanged() will cause a crash.

I think this happens because it will try to reference entries using subscript index, so probably tries to get entries[0] (because count = 0) and that obviously won't work because that entries doesn't exist.

A workaround is to check if your dataset contains any entries and set it data to nil otherwise.

However, I think Charts could improve here by either using safe subscript referencing (so you can check entries[0] safely without crashing) or just checking if there are actual entries before trying to subscript access them.

[liuxuan30]

Well ah it used to check the count before rendering I remember, but I don't know its has been removed the check.
I checked that candle stick indeed crashes, however, bar chart doesn't, due to stride to/through difference.
Welcome file a PR to fix this :)

[whvandervelde]

added PR #1948

There are checks for returned entry type throughout Charts, so returning nil seemed best here.

[liuxuan30]

Thanks! Guess I need @danielgindi review it since I am not sure if I would miss anything. BTW, he's missing recently, so it might take a long time..

[cescobaz]

Same problem here, specially in real-time graph.
I get a lot of crashes for several reason during my development, but, at the end, I always found a workaround. This time I can't get the library working.

With the following example I will show up other bugs also.

func updateChart(value : double) {

    // not relevant code ...
    // ...    

    chartDataSet.addEntry(ChartDataEntry(x: Double(chartIndex), y: value ))
    
    // notify data change
    // if I comment the following line -> CRASH
    data.notifyDataChanged()

    // notify data change (again?)
    // if I comment the following line -> CRASH (again)
    chart.notifyDataSetChanged()

    // set the maximum visible range
    // why I do this fixed setup at every value update? Because if I set it at the moment of the chart creation it CRASHs
    chart.setVisibleXRangeMaximum(50)
    
    // calculate offset
    let offset = max (chartIndex - 50, 0 )

    // CRASH HERE (if I remove all datasets and re-add one or more -> crash)
    chart.moveViewToX(Double(offset))

    chartIndex += 1
}

I try to explain my scenario without more code because I can't copy all my code here, it is too long and not useful for your debug.
I load an empty graph and, when I start to get values from a stream I add values to the data set. To be precise I use more than one dataset. I have also several toggles that remove/add a dataset.

The crash happens when I switch off all the toggles, so no dataset in the chart.data exist, and I switch on one again (then adding again a dataset).

The errors I get are the following:

Error: CGAffineTransformInvert: singular matrix

fatal error: Double value cannot converted to Int because it is either infinite or NaN.

The above code works correcly until I remove all dataset and re-add at least one dataset. I tried to set chart.data = nill and then re-add data with/without dataset, it doesn't work.

I didn't read the source library code, but, in my opinion, the error is always the same, if you fix it, you will fix a lot of correlated bug.

Thanks!

[liuxuan30]

@cescobaz have you tried this PR, does it fix your crashes?

as much as I wanted to merge this, it will potentially impact many things, so I still need @danielgindi to review this not to missing anything :(

[cescobaz]

@liuxuan30 I just try this PR and it doesn't fix the bug.

The app logs many times the following error:

: Error this application or a library it uses, has passed an invalid numeric value (NaN, or not-a-number) to CoreGraphics API and this value is being ignored. Please fix the problem.

Then, after a a few seconds of the above error appears multiple times in console, the app crash with:

fatal error: Double value cannot be converted to Int because it is either infinite or NaN

The last call I can see before crash is on line 125 of file ../Renderers/AxisRendererBase.swift

[cescobaz]

Ok, I found a solution.

For some reason _xAxis._axisMinimum and _xAxis._axisMaximum are NaN when you set chart.data = nil or all the datasets are empty or there are no dataset in chart.data.
So I change my code to reset axisMinimum and axisMaximum every time. In the following the "new version" of the code above:

func updateChart(value : double) {

    // not relevant code ...
    // ...    

    chartDataSet.addEntry(ChartDataEntry(x: Double(chartIndex), y: value ))
    
    // calculate offset
    let xmax = 50
    let offset = max (chartIndex - xmax , 0 )

    // set max and min of xAxis because they are NaN when datasets are empty or chart.data == nil
    chart.xAxis.axisMinimum = Double(offset)
    chart.xAxis.axisMaximum = Double(offset + xmax)

    // notify data change
    data.notifyDataChanged()

    // notify data change
    chart.notifyDataSetChanged()

    // set the maximum visible range
    chart.setVisibleXRangeMaximum(Double(xmax))
    
    chart.moveViewToX(Double(offset))

    chartIndex += 1
}