Context Manager

The context manager module provides a python-like context manager syntax in Lua with a few limitations. Specifically, using break or return within the context manager is not supported due to the way Lua’s GC works and the lack of any concrete way to trigger some cleanup code to run when a variable loses scope.

Context managers can either be called standalone (as shown in the example below) in which case the for loop block will be executed exactly once, with the enter and exit functions being called before and after the loop respectively, or they can wrap another iterator, in which case the for loop variables are passed through and the enter and exit functions are called before the first iteration and after the last iteration respectively.

Example

A common situation where context managers are helpful is to measure the execution time of a block of code. LibTSMUtil provides the ExecutionTime module for this purpose, but below is a simplified implemention to demonstration how the context manager module can be used.

local MyModule = select(2, ...).MyModule
local ContextManager = MyModule:From("LibTSMUtil"):Include("BaseType.ContextManager")

local function EnterFunc(arg)
   print(format("Started block (%s)", arg))
   return LibTSMUtil.GetTime()
end
local function ExitFunc(arg, startTime)
   local elapsed = LibTSMUtil.GetTime() - startTime
   print(format("Completed block (%s) after %d seconds", arg, elapsed))
end
local measurementContextObj = ContextManager.Create(EnterFunc, ExitFunc)

local sum = 0
for _, value in measurementContextObj:With("CALCULATE_SUM", ipairs({1, 2, 3, 4, 5})) do
   sum = sum + value
end

API

class BaseType.ContextManager: LibTSMModule

staticmethod Create( enterFunc: fun(arg: <A>): <E>, exitFunc: fun(arg: <A>, enterValue: <E>) ): ContextManagerObject

Creates a context manager.

Parameters:

enterFunc (fun(arg: <A>): <E>) – The enter function which is called when the managed code block starts
exitFunc (fun(arg: <A>, enterValue: <E>)) – The exit function which is called when the managed code block completes

class ContextManagerObject

With( self: ContextManagerObject, arg: any, func?: fun(obj: any, key?: any): ...unknown, obj: any, key: any ): function, any, any

Returns an iterator which executes to completion with the context manager.

NOTE: The iterator must not be interrupted (i.e. with a break or return).

Parameters:

arg (any) – A value to pass as the first argument to the enter and exit functions (must be non-nil)
func? (fun(obj: any, key?: any): ...unknown) – The iterator function or nil to iterate exactly once
obj (any) – The object to pass to the iterator function
key (any) – The initial key to pass to the iterator function