Removing all children from a Sprite

lyrenbrown

I have this function:

function clear(sprite) for i = 1,sprite:getNumChildren() do sprite:removeChildAt(i) end end

And I'm getting this error on the removeChildAt line:

main.lua:72: The supplied index is out of bounds. stack traceback:

I didn't think that was possible. Are the child index values not always contiguous? If not, then how can I do this? I tried adding a guard but I get the same error:

function clear(sprite) for i = 1,sprite:getNumChildren() do if sprite:getChildAt(i) then sprite:removeChildAt(i) end end end

Ghando

Does reversing the "for" statement give you the same error? eg

for i = #sprite:getNumChildren, 1, -1 do sprite:removeChildAt(i) end

lyrenbrown

@Ghando Wow, thanks! That seems to fix it. Now I have this:

function clear(sprite) for i = sprite:getNumChildren(), 1, -1 do sprite:removeChildAt(i) end end

Why does that work?

Ghando

This blog post by Oz-Apps explains it better than I can:
http://howto.oz-apps.com/2011/09/tower-of-babel-no-honoi-maybe.html

lyrenbrown

Ah! So, the way I was doing it was forcing the whole array to be renumbered every iteration. That would also explain how the index values are always kept contiguous. Many thanks!

hnim

just change self:removeChildAt(i) to self:removeChildAt(1)

lyrenbrown

@hnim That's a good point, but I wonder if it's less efficient that way since the children will have to renumbered for every iteration? Seems O(n^2)'ish, no?

atilim

@lyrenbrown, Children are stored as a hash table on Lua and as an ordered array on C++. Therefore, @hnim's solution is O(n) on Lua side and O(n^2) on C++ side. (But the time spent on C++ is negligible compared to Lua while removing a child)

This is another solution (most probably the slowest one )

function clear(sprite) while sprite:getNumChildren() > 0 do sprite:removeChildAt(1) end end

Caroline

I assume that to remove the children of children you still have to do recursive removal?

(I was confused in Objective C, because when releasing a view, it would then release child views that were retained by it, so I didn't have to do recursive removal, but it's different in this case?)

ndoss

I assume that to remove the children of children you still have to do recursive removal?

If the deleted child is the only source of references to it's children, then I don't believe you have to recursively delete the children.

Said another way ... if the children of the deleted child don't have any references to them, I believe they will be deleted by lua's garbage collector.