Lua ignores scope variable
This may be a silly question, however, I don't know what's going on.
I have a simple script that picks google time and I need to set it to a global variable time
. This way, inside the event, receive
I print the selected time and it works correctly.
The problem is that the variable is time
always empty when called outside the event. Here is the code:
-- test.lua
time = ""
function getTime()
conn = net.createConnection(net.TCP, 0)
conn:connect(80,'google.com')
conn:on("connection", function(conn, payload)
conn:send("HEAD / HTTP/1.1\r\n"..
"Host: google.com\r\n"..
"Accept: */*\r\n"..
"User-Agent: Mozilla/4.0 (compatible; esp8266 Lua;)"..
"\r\n\r\n"
)
end)
conn:on("receive", function(conn, payload)
conn:close()
time = string.sub(payload,string.find(payload,"Date: ")
+6,string.find(payload,"Date: ")+35)
end)
print("testing: " .. time) -- WORKS!
end
getTime()
print("variable: ".. time)
This is how I call the function (using terminal nodemcu-uploader):
➜ test nu terminal
--- Miniterm on /dev/cu.wchusbserial1410 115200,8,N,1 ---
--- Quit: Ctrl+] | Menu: Ctrl+T | Help: Ctrl+T followed by Ctrl+H ---
> dofile('lib/test.lua')
variable:
> testing: Sat, 20 May 2017 01:37:35 GMT
Any help would be really appreciated! Thanks to
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The NodeMCU programming model is similar to Node.js, only in Lua. It is asynchronous and event driven. Therefore, many functions have parameters for callback functions.
Source: https://github.com/nodemcu/nodemcu-firmware/#programming-model
This means that functions that take callback functions as parameters are not blocked. What it returns means that you can't just read a piece of code in turn and expect it to be executed in that order.
So, the sequence of events in the original program is something like this:
-
getTime
runs but does not block. Performed -
print("variable: ".. time)
...time
is still empty at this point. - A connection to google.com has been established.
- The HEAD request is sent to google.com.
- The response is a receive handler for the receive and receive event (i.e. an anonymous callback function).
-
time
is filled.
I see two obvious fixes, one using your global variable time
and one without it. Both are based on the pass-call-function-as-parameter pattern.
Note that you should always set up event listeners ( conn:on
in your case) prior to firing these events ( conn:connect
) to avoid some events. Your code
conn:connect(80,'google.com')
conn:on("connection"...
works only because it is conn:connect
not blocked and because it takes some time until the connection is established. By the time this happens, an on-connection event handler has been registered.
Saving a global variable
time = ""
function getTime(cb)
conn = net.createConnection(net.TCP, 0)
conn:on("connection", function(socket, payload)
socket:send("HEAD / HTTP/1.1\r\n" ..
"Host: google.com\r\n" ..
"Accept: */*\r\n" ..
"User-Agent: Mozilla/4.0 (compatible; esp8266 Lua;)" ..
"\r\n\r\n")
end)
conn:on("receive", function(socket, payload)
socket:close()
time = string.sub(payload, string.find(payload, "Date: ")
+ 6, string.find(payload, "Date: ") + 35)
print("time inside on-receive: " .. time)
cb()
end)
conn:connect(80, 'google.com')
end
function do_something_with_time()
print("time inside callback: " .. time)
end
getTime(do_something_with_time)
Without a global variable
function getTime(cb)
conn = net.createConnection(net.TCP, 0)
conn:on("connection", function(socket, payload)
socket:send("HEAD / HTTP/1.1\r\n" ..
"Host: google.com\r\n" ..
"Accept: */*\r\n" ..
"User-Agent: Mozilla/4.0 (compatible; esp8266 Lua;)" ..
"\r\n\r\n")
end)
conn:on("receive", function(socket, payload)
socket:close()
local time = string.sub(payload, string.find(payload, "Date: ")
+ 6, string.find(payload, "Date: ") + 35)
print("time inside on-receive: " .. time)
cb(time)
end)
conn:connect(80, 'google.com')
end
function do_something_with_time(time)
print("time inside callback: " .. time)
end
getTime(do_something_with_time)
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