---
title: "6. Subclassing oce objects"
author: "Dan Kelley (https://orcid.org/0000-0001-7808-5911)"
date: "`r Sys.Date()`"
output:
  rmarkdown::html_vignette:
    toc: true
    number_sections: true
    fig_caption: yes
    dev.args: list(pointsize=11)
vignette: >
  %\VignetteIndexEntry{6. Subclassing oce objects}
  %\VignetteEngine{knitr::rmarkdown}
  \usepackage[utf8]{inputenc}
---

<!-- HOW TO BUILD THE VIGNETTE. -->
<!-- Look in oce.Rmd for instructions. -->


```{r, echo=FALSE}
knitr::opts_chunk$set(collapse=TRUE, comment="#>")
```

**Abstract.** This vignette explains how to create new object classes, using
the low-level `oce` class as a foundation. This scheme is beneficial, because
the newly-formed objects automatically gain the key properties of oce objects,
in terms of operators such as `[[` and `[[<-`, functions such as `subset()` and
`summary()`, schemes for handling units, etc. The `setMethod()` function is
central to the process of creating new classes, as will be illustrated here in
a practical way, using drifting-buoy data for example.


# A drifting buoy

Data from buoy number 4201703 were downloaded from
`http://www.coriolis.eu.org/Data-Products/Data-selection` on
2022-01-09, by deselecting all data types except for "Drifting buoy", selecting
a data window south of Nova Scotia and of area similar to that province,
expanding the tarball created by the website and then focussing on the
downloaded file named `"drifting-buoys-4201703.csv`".

There are many data in the source data file, but only a subset is used here:
starting with 300 samples of just five of the data fields: time (called `t` for
brevity), longitude (`lon`), latitude (`lat`), temperature (`Tatm`) and
atmospheric pressure (`Patm`). This set was further trimmed by removing data
ensembles in which temperature was not reported.  The result was 146
observations of 5 variables. (Note that they are *not* spaced uniformly in
time, a feature that is of importance only at the end of this document.)

The data were combined into a data frame named `db` and packaged in an RDA
file, an overview of which is provided as follows.
```{r}
library(oce)
load(system.file("extdata", "drifter.rda", package="oce"))
str(db)
```

Before showing how to create a new subclass, it will first be shown that such
data could also be stored in the foundational class of the package.

# Using the base oce class

The base class is named `"oce"`, and so a new object of that type can be created with
```{r}
o <- new("oce")
```

At this point, the `summary()` method does not reveal much:
```{r}
summary(o)
```
but if we populate the object with some data and metadata
```{r}
o <- oceSetData(o, "time", db$t)
o <- oceSetData(o, "longitude", db$lon)
o <- oceSetData(o, "latitude", db$lat)
o <- oceSetMetadata(o, "ID", 4201703)
```
then the summary is more informative
```{r}
summary(o)
```

Note, however, that the `ID` is not listed by `summary()`.  This is because the
general form of that function is set up to focus on the contents of the `data`
slot, not the `metadata` slot. (Extending `summary()` will described later.)

Objects of `oce-class` automatically have other methods, besides
`summary()`.  For example, the accessing operator `[[` works for the
object that was just created:
```{r}
str(o[["latitude"]])
```

We can also use the default `plot()` function to show the interdependence of
entries in the `data` slot.
```{r fig.cap="**Figure 1.** Demonstration of base-level plot().", fig.width=4, fig.height=4, dpi=72, dev.args=list(pointsize=10)}
plot(o, pch=20, cex=0.5)
```

Although this plot provides a useful overview of the data, it is not something
an analyst would consider publishing.  To get more useful plot types, we must
create a new subclass that has a specialized `plot()` method.

# Creating a new subclass

A reasonable name for our new subclass might be `"drifter"`.  With this
choice, the class can be defined in a single line of code:
```{r}
drifter <- setClass(Class="drifter", contains="oce")
```

Once this has been executed, the user can create a new object of this subclass
with
```{r}
d <- new("drifter")
```

Although it is simple to add data and metadata as in the previous section, it
is more convenient to supply this information when `new()` is used.  To
accomplish this, we must set up an initialization function, as follows.

```{r}
setMethod(f="initialize",
  signature="drifter",
  definition=function(.Object, time, longitude, latitude, ID="unknown") {
      if (missing(time))
          stop("In new(drifter) : must provide 'time'", call.=FALSE)
      if (missing(longitude))
          stop("In new(drifter) : must provide 'longitude'", call.=FALSE)
      if (missing(latitude))
          stop("In new(drifter) : must provide 'latitude'", call.=FALSE)
      .Object@data$time <- time
      .Object@data$longitude <- longitude
      .Object@data$latitude <- latitude
      .Object@metadata$ID <- ID
      .Object@processingLog$time <- presentTime()
      .Object@processingLog$value <- "create 'drifter' object"
      return(.Object)
})
```

This method takes away the choice of where to put things (in `data` or in
`metadata`), saving users from having to make that decision, possibly breaking
other code.  Also, note that some error checking has been used here, so that an
attempt to use `new("drifter")` without providing time, etc., produces an error
message, as shown below.
```
d <- new("drifter")
Error: In new(drifter) : must provide 'time'
```


With this setup, we may now create an object that we can use for the rest of
this vignette:
```{r}
d <- new("drifter", time=db$t, longitude=db$lon, latitude=db$lat, ID=4201703)
```
and this will be used in the next three subsections, showing how to specialize
the `plot()` method, the `summary()` method, and the `[[` accessor. Note the
use of `setMethod()` in each instance.

## Specializing the plot() function

The `f`, `signature` and `definition` arguments follow the same pattern as in
the previous example, and readers ought to focus on last of these. Here, three
plot types are provided, as selected by an argument called `which`. The first
two give time-series plots of drifter longitude and latitude, and the third
shows the trajectory.

```{r}
setMethod(f="plot",
    signature=signature("drifter"),
    definition=function(x, which=1, ...) {
        lonlab <- expression("Longitude ["*degree*"E]")
        latlab <- expression("Latitude ["*degree*"N]")
        if (which == 1) {
            oce.plot.ts(x[["time"]], x[["longitude"]],
                ylab=lonlab, ...)
        } else if (which == 2) {
            oce.plot.ts(x[["time"]], x[["latitude"]],
                ylab=latlab, ...)
        } else if (which == 3) {
            asp <- 1 / cos(mean(range(x[["latitude"]]) * pi / 180))
            plot(x[["longitude"]], x[["latitude"]],
                asp=asp, xlab=lonlab, ylab=latlab, ...)
        } else {
            stop("In plot,drifter-method : try which=1, 2 or 3", call.=FALSE)
        }
    })
```

The three plot types are fairly rudimentary, but the `...` argument can be used
for customization, as in the following example.

```{r fig.cap="**Figure 2.** Demonstration of specialized plot()."}
par(mar=c(3.3, 3.3, 1, 1), mgp=c(2, 0.7, 0))
layout(matrix(c(1, 3, 2, 3), nrow=2, byrow=TRUE))
plot(d, which=1, drawTimeRange=FALSE)
plot(d, which=2, drawTimeRange=FALSE)
plot(d, which=3)
```


## Specializing the summary() function

Here, our sole purpose is to add an indication of the ID of the drifter. The
rest of the information provided by the base-level `summary()` function will
suffice to summarize the rest.

So, with
```{r}
setMethod(f="summary",
    signature="drifter",
    definition=function(object, ...) {
        cat("CTD Summary\n-----------\n\n")
        cat("* ID:          ", object[["ID"]], "\n", sep="")
        invisible(callNextMethod())
    })
```

An example follows.
```{r}
summary(d)
```



## Specializing the `[[` accessor

The variation of drifter position provides information on ocean velocity, and
this is of sufficient interest that it might be worth adding to `[[`.  This may
be done as follows.

```{r}
setMethod(f="[[",
    signature(x="drifter", i="ANY", j="ANY"),
    definition=function(x, i, j, ...) {
        if (i == "velocity") {
            D <- function(x) {
                dx <- diff(x)
                c(dx[1], dx)
            }
            lat <- x[["latitude"]]
            lon <- x[["longitude"]]
            dt <- D(as.numeric(x[["time"]])) # seconds
            scalex <- 111.12e3         # m per degree latitude
            scaley <- scalex * cos(lat*pi/180)
            u <- scalex * D(lon) / dt
            v <- scaley * D(lat) / dt
            list(u=u, v=v)
        } else {
            callNextMethod()
        }
    })
```

As a test, we can plot the velocity components.
```{r fig.cap="**Figure 3.** Velocities inferred from drifter motion.", fig.width=5, fig.height=3, dpi=72, dev.args=list(pointsize=10)}
uv <- d[["velocity"]]
par(mfrow=c(2, 1))
oce.plot.ts(d[["time"]], uv$u, ylab="Eastward velo. [m/s]", grid=TRUE)
oce.plot.ts(d[["time"]], uv$v, ylab="Northward vel. [m/s]", grid=TRUE)
```

The `acf()` function, which computes lagged autocorrelation, can shed some
light on the periodicity of the velocity signals (but see Exercise 3), as
follows.

```{r fig.cap="**Figure 4.** Autocorrelation analysis of drifter velocities, showing also the M2 period.", fig.width=5, fig.height=3, dpi=72, dev.args=list(pointsize=10)}
data(tidedata)
M2period <- 1 / with(tidedata$const, freq[[which(name=="M2")]])
uv <- d[["velocity"]]
par(mfrow=c(2, 1), mar=c(3, 3, 1, 1), mgp=c(2, 0.7, 0))
acf(uv$u, main="", ylab="u ACF")
abline(v=M2period, col=2)
acf(uv$v, main="", ylab="v ACF")
abline(v=M2period, col=2)
```


# Summary

As has been illustrated above, the `setMethod()` function is the key to
altering generic functions such as `plot()`, `summary()` and `[[`. The details
of how this works are explained in numerous manuals, books, and websites, but
impatient readers can save some time by simply copying and expanding on the
code provided in the previous sections.

# Appendix: exercises for the reader

1. Add atmospheric temperature and pressure (both contained in the source RDA
   file) to the `drifter` object initialization function.  Be sure to check
   that they were provided or, if you choose, let them be optional.

2. Think of other useful plot types, and add these to the `plot()` method.

3. For the final lagged-autocorrelation function, use `approx()` to get new
   time-series with a constant sampling rate.

4. Again for the lagged-autocorrelation function, add a line for local Coriolis
   period.

5. As an alternative to the lagged-autocorrelation method, test for periodicity
   of velocity by using `nls()`, supplied with a sinusoidal function that has
   adjustable amplitude, phase, and frequency.
