R: Callback classes

callback {readr} | R Documentation

## Callback classes

### Description

These classes are used to define callback behaviors.

### Details

ChunkCallback

  Callback interface definition, all callback functions should inherit from this class.

SideEffectChunkCallback

  Callback function that is used only for side effects, no results are returned.

DataFrameCallback

  Callback function that combines each result together at the end.

### See Also

Other chunked: `read_delim_chunked`, `read_lines_chunked`

### Examples

    ## If given a regular function it is converted to a SideEffectChunkCallback

    # view structure of each chunk
    read_lines_chunked(readr_example("mtcars.csv"), str, chunk_size = 5)

    # Print starting line of each chunk
    f <- function(x, pos) print(pos)
    read_lines_chunked(readr_example("mtcars.csv"), SideEffectChunkCallback$new(f), chunk_size = 5)

    # If combined results are desired you can use the DataFrameCallback

    # Cars with 3 gears
    f <- function(x, pos) subset(x, gear == 3)
    read_csv_chunked(readr_example("mtcars.csv"), DataFrameCallback$new(f), chunk_size = 5)

    # The ListCallback can be used for more flexible output
    f <- function(x, pos) x$mpg[x$hp > 100]
    read_csv_chunked(readr_example("mtcars.csv"), ListCallback$new(f), chunk_size = 5)

[Package readr version 1.1.0 Index]