corona - Difference between tables and Metatables in lua -


what difference between tables , metatables in corona.what types of metatables . how use , can use it? main purpose of using tables , metatables?

lua (which language corona based on) uses metatables different purposes.

the relevant entry in manual section 2.8. nice tutorial can found here or here.

a metatable table other, set metatable on table (which i'll call base table further on, make difference between 2 tables).

a metatable can contain anything, special keys (starting double underscore) interesting ones. values set keys in table called on special occasions. occasion depends on key. interesting are:

  • __index: used whenever key in base table looked up, not exist. can either contain table, in key looked instead, or function, passed original table , key. can used implementing methods on tables (oop style), redirection, fall through cases, setting defaults, etc etc
  • __newindex: used whenever new key assigned in table (which nil). if it's table, key assigned in table. if it's function, function passed original table, key , value. can used controlling access table, preprocessing data, redirection of assignments.
  • __call: enables set function called if use eg. table().
  • __add,__sub,__mul,__div,__mod used implement binary operations,
  • __unm used implement unary operations,
  • __concat used implementing concatenation (the .. operator)
  • __len used implementing length operator (#)
  • __eq,__lt,__le used implementing comparisons

a small thing know when using __index & co.: in methods, should use rawget , rawset in order prevent calling metamethod each time again, causing loop. small example:

t={1,2,3}  -- basetable mt={} -- metatable mt.__index=function(t,k)     print("__index event "..tostring(t).." key "..k)     return "currently unavailable" end mt.__newindex=function(t,k,v)     print("__newindex event "..tostring(t).." key: "..k.." value: "..v)     if type(k)=="string"         rawset(t,k,v:reverse())     else         rawset(t,k,v)     end end mt.__call=function(t,...)     print("call table "..tostring(t).." arguments: ".. table.concat({...},','))     print("all elements of table:")     k,v in pairs(t) print(k,v) end end setmetatable(t,mt)  t[4]="foo" -- run __newindex method print(t[5]) -- run __index method t("foo","bar") -- multiple fall through example: t={} mt={} mt2={} setmetatable(t,mt)  -- metatable on base table setmetatable(mt,mt2) -- second layer of metatable mt.__index=function(t,k) print('key '..k..' not found in '..namelookup[t]) return getmetatable(t)[k] end -- tries looking nonexistant indexes in mt. mt2.__index=mt.__index -- function written portably, reuse it.  t[1]='a' mt[2]='b' mt2[3]='c' namelookup={[t]="t",[mt]="mt",[mt2]="mt2"} print(t[1],t[2],t[3],t[4])

now these silly examples, can more complex stuff. take @ examples, take @ relevant chapters in programming in lua, , experiment. , try not confused ;)


Comments

Post a Comment

Popular posts from this blog

java - Play! framework 2.0: How to display multiple image? -

i need display gallery of photos. here template: @(photos: list[photo]) @title = { <bold>gallery</bold> } @main(title,"photo"){ <ul class="thumbnails"> @for(photo <- photos) { <li class="span3"> <a href="#" class="thumbnail"> <img src="@photo.path" alt=""> </a> </li> } </ul> } and here controller method: public static result getphotos() { return ok(views.html.photo.gallery.render(photo.get())); } and here photo bean : @entity public class photo extends model { @id public long id; @required public string label; public string path; public photo(string path, string label) { this.path = path; this.label = label; } private static finder<long, photo> find = new finder<long, photo>( long.class, photo.class); public static list<phot...
Read more

gmail - Is there any documentation for read-only access to the Google Contacts API? -

i want read-only access user's google contacts. there stackoverflow member wanted this, , received following answer : yes, there is, use https://www.googleapis.com/auth/contacts.readonly scope , "view contacts". this exchange reference read-only access google contacts can find using google. simply using access token obtained attempt read usual google contacts api (i.e. https://www.google.com/m8/feeds ) fails insufficient permissions. similarly, using google contacts api on new target fails 404 errors (for example https://www.googleapis.com/auth/contacts.readonly/default/full ). is there documentation anywhere on how use api, or permissible way access google contacts api full read-write access?
Read more

php - Controller/JToolBar not working in Joomla 2.5 -

relevant code: jtoolbarhelper::custom('savecategories', 'save', '', 'save', false, false); ... <input type="hidden" name="controller" value="easyblogcontroller"> according can dig docs , own previous questions, should call savecategories() function in easyblogcontroller . i've tried setting value easyblog , easyblog.php (file name) current easyblogcontroller (class name). clicking on save button refreshes page. not redirect, echo or var_dump test code put in savecategories() function. var_dump ing jrequest::getvar('controller') , 'task' return correct values. creating controller object , using $controller->execute('task'); works. the values going depend on location of controller trying call, , have couple options here. typically there controller in base folder of component (likely components/com_easyblog) in file called controller.php , class name inside ...
Read more