startleft | automation tool for generating Threat Models

A couple of observations:

1. This invalid context is a result that you’re initiating an asynchronous process, so by the time the dispatch_after block is called, the context supplied to drawRect no longer exists and your asynchronously called block has no context in which to stroke these lines.

   But the view shouldn’t be initiating this network request and parsing. Usually the view controller (or better, some other network controller or the like) should be initiating the network request and the parsing.

2. The drawRect is for rendering the view at a given moment in time. If there’s nothing to render yet, it should just return immediately. When the data is available, you supply the view the data necessary to do the rendering and initiate the setNeedsDisplay.

3. So, a common pattern would be to have a property in your view subclass, and have the setter for that property call setNeedsDisplay for you.

4. Rather than initiating the asynchronous request and trying to use the data in two seconds (or any arbitrary amount of time), you instead give your downloadData a completion handler block parameter, which it calls when the download is done, and trigger the updating immediately as soon as the download and parse is done. This avoids unnecessary delays (e.g. if you wait two seconds, but get data in 0.5 seconds, why wait longer than necessary; if you want two seconds, but get data in 2.1 seconds, you risk not having any data to show). Initiate the update of the view exactly when the download and parse is done.

5. This float * reference is a local variable and will never get populated. Your downloadData probably should return the necessary data in the aforementioned completion handler. Frankly, this notion of a pointer to a C array is not a pattern you should be using in Objective-C, anyway. If your network response really returns just two floats, that’s what you should be passing to this view, not a float *.

6. Note, I’ve replaced the CoreGraphics code with UIKit drawing. Personally, I’d be inclined to go further and move to CAShapeLayer and not have a drawRect at all. But I didn’t want to throw too much at you. But the general idea is to use the highest level of abstraction as you can and there’s no need to get into the weeds of CoreGraphics for something as simple as this.

This isn’t going to be quite right as I don’t really understand what your model data is, but let’s assume for a second it’s just returning a series of float values. So you might have something like: