ElvUI/Libraries/LibSimpleSticky/LibSimpleSticky.lua

--[[---------------------------------------------------------------------------------
  General Library providing an alternate StartMoving() that allows you to
  specify a number of frames to snap-to when moving the frame around

  Example Usage:

	<OnLoad>
		this:RegisterForDrag("LeftButton")
	</OnLoad>
	<OnDragStart>
		StickyFrames:StartMoving(this, {WatchDogFrame_player, WatchDogFrame_target, WatchDogFrame_party1, WatchDogFrame_party2, WatchDogFrame_party3, WatchDogFrame_party4},3,3,3,3)
	</OnDragStart>
	<OnDragStop>
		StickyFrames:StopMoving(this)
		StickyFrames:AnchorFrame(this)
	</OnDragStop>

------------------------------------------------------------------------------------
This is a modified version by Elv for ElvUI
------------------------------------------------------------------------------------]]

local MAJOR, MINOR = "LibSimpleSticky-1.0", 2
local StickyFrames, oldminor = LibStub:NewLibrary(MAJOR, MINOR)

if not StickyFrames then return end

--[[---------------------------------------------------------------------------------
  Class declaration, along with a temporary table to hold any existing OnUpdate
  scripts.
------------------------------------------------------------------------------------]]

StickyFrames.scripts = StickyFrames.scripts or {}
StickyFrames.rangeX = 15
StickyFrames.rangeY = 15
StickyFrames.sticky = StickyFrames.sticky or {}

--[[---------------------------------------------------------------------------------
  StickyFrames:StartMoving() - Sets a custom OnUpdate for the frame so it follows
  the mouse and snaps to the frames you specify

	frame:	 	The frame we want to move.  Is typically "this"

	frameList: 	A integer indexed list of frames that the given frame should try to
				stick to.  These don't have to have anything special done to them,
				and they don't really even need to exist.  You can inclue the
				moving frame in this list, it will be ignored.  This helps you
				if you have a number of frames, just make ONE list to pass.

				{WatchDogFrame_player, WatchDogFrame_party1, .. WatchDogFrame_party4}

	left:		If your frame has a tranparent border around the entire frame
				(think backdrops with borders).  This can be used to fine tune the
				edges when you're stickying groups.  Refers to any offset on the
				LEFT edge of the frame being moved.

	top:		same
	right:		same
	bottom:		same
------------------------------------------------------------------------------------]]

function StickyFrames:StartMoving(frame, frameList, left, top, right, bottom)
	local x,y = GetCursorPosition()
	local aX,aY = frame:GetCenter()
	local aS = frame:GetEffectiveScale()

	aX,aY = aX*aS,aY*aS
	local xoffset,yoffset = (aX - x),(aY - y)
	self.scripts[frame] = frame:GetScript("OnUpdate")
	frame:SetScript("OnUpdate", self:GetUpdateFunc(frame, frameList, xoffset, yoffset, left, top, right, bottom))
end

--[[---------------------------------------------------------------------------------
  This stops the OnUpdate, leaving the frame at its last position.  This will
  leave it anchored to UIParent.  You can call StickyFrames:AnchorFrame() to
  anchor it back "TOPLEFT" , "TOPLEFT" to the parent.
------------------------------------------------------------------------------------]]

function StickyFrames:StopMoving(frame)
	frame:SetScript("OnUpdate", self.scripts[frame])
	self.scripts[frame] = nil

	if StickyFrames.sticky[frame] then
		local sticky = StickyFrames.sticky[frame]
		StickyFrames.sticky[frame] = nil
		return true, sticky
	else
		return false, nil
	end
end

--[[---------------------------------------------------------------------------------
  This can be called in conjunction with StickyFrames:StopMoving() to anchor the
  frame right back to the parent, so you can manipulate its children as a group
  (This is useful in WatchDog)
------------------------------------------------------------------------------------]]

function StickyFrames:AnchorFrame(frame)
	local xA,yA = frame:GetCenter()
	local parent = frame:GetParent() or UIParent
	local xP,yP = parent:GetCenter()
	local sA,sP = frame:GetEffectiveScale(), parent:GetEffectiveScale()

	xP,yP = (xP*sP) / sA, (yP*sP) / sA

	local xo,yo = (xP - xA)*-1, (yP - yA)*-1

	frame:ClearAllPoints()
	frame:SetPoint("CENTER", parent, "CENTER", xo, yo)
end


--[[---------------------------------------------------------------------------------
  Internal Functions -- Do not call these.
------------------------------------------------------------------------------------]]



--[[---------------------------------------------------------------------------------
  Returns an anonymous OnUpdate function for the frame in question.  Need
  to provide the frame, frameList along with the x and y offset (difference between
  where the mouse picked up the frame, and the insets (left,top,right,bottom) in the
  case of borders, etc.w
------------------------------------------------------------------------------------]]

function StickyFrames:GetUpdateFunc(frame, frameList, xoffset, yoffset, left, top, right, bottom)
	return function()
		local x,y = GetCursorPosition()
		local s = frame:GetEffectiveScale()
		local sticky = nil

		x,y = x/s,y/s

		frame:ClearAllPoints()
		frame:SetPoint("CENTER", UIParent, "BOTTOMLEFT", x+xoffset, y+yoffset)

		StickyFrames.sticky[frame] = nil
		for i = 1, #frameList do
			local v = frameList[i]
			if frame ~= v and frame ~= v:GetParent() and not IsShiftKeyDown() and v:IsVisible() then
				if self:SnapFrame(frame, v, left, top, right, bottom) then
					StickyFrames.sticky[frame] = v
					break
				end
			end
		end
	end
end


--[[---------------------------------------------------------------------------------
  Internal debug function.
------------------------------------------------------------------------------------]]

function StickyFrames:debug(msg)
	DEFAULT_CHAT_FRAME:AddMessage("|cffffff00StickyFrames: |r"..tostring(msg))
end

--[[---------------------------------------------------------------------------------
  This is called when finding an overlap between two sticky frame.  If frameA is near
  a sticky edge of frameB, then it will snap to that edge and return true.  If there
  is no sticky edge collision, will return false so we can test other frames for
  stickyness.
------------------------------------------------------------------------------------]]
function StickyFrames:SnapFrame(frameA, frameB, left, top, right, bottom)
	local sA, sB = frameA:GetEffectiveScale(), frameB:GetEffectiveScale()
	local xA, yA = frameA:GetCenter()
	local xB, yB = frameB:GetCenter()
	local hA, hB = frameA:GetHeight() / 2, ((frameB:GetHeight() * sB) / sA) / 2
	local wA, wB = frameA:GetWidth() / 2, ((frameB:GetWidth() * sB) / sA) / 2

	local newX, newY = xA, yA

	if not left then left = 0 end
	if not top then top = 0 end
	if not right then right = 0 end
	if not bottom then bottom = 0 end

	-- Lets translate B's coords into A's scale
	if not xB or not yB or not sB or not sA or not sB then return end
	xB, yB = (xB*sB) / sA, (yB*sB) / sA

	local stickyAx, stickyAy = wA * 0.75, hA * 0.75
	local stickyBx, stickyBy = wB * 0.75, hB * 0.75

	-- Grab the edges of each frame, for easier comparison

	local lA, tA, rA, bA = frameA:GetLeft(), frameA:GetTop(), frameA:GetRight(), frameA:GetBottom()
	local lB, tB, rB, bB = frameB:GetLeft(), frameB:GetTop(), frameB:GetRight(), frameB:GetBottom()
	local snap = nil

	-- Translate into A's scale
	lB, tB, rB, bB = (lB * sB) / sA, (tB * sB) / sA, (rB * sB) / sA, (bB * sB) / sA

	if (bA <= tB and bB <= tA) then

		-- Horizontal Centers
		if xA <= (xB + StickyFrames.rangeX) and xA >= (xB - StickyFrames.rangeX) then
			newX = xB
			snap = true
		end

		-- Interior Left
		if lA <= (lB + StickyFrames.rangeX) and lA >= (lB - StickyFrames.rangeX) then
			newX = lB + wA
			if frameB == UIParent or frameB == WorldFrame or frameB == ElvUIParent then
				newX = newX + 4
			end
			snap = true
		end

		-- Interior Right
		if rA <= (rB + StickyFrames.rangeX) and rA >= (rB - StickyFrames.rangeX) then
			newX = rB - wA
			if frameB == UIParent or frameB == WorldFrame or frameB == ElvUIParent then
				newX = newX - 4
			end
			snap = true
		end

		-- Exterior Left to Right
		if lA <= (rB + StickyFrames.rangeX) and lA >= (rB - StickyFrames.rangeX) then
			newX = rB + (wA - left)

			snap = true
		end

		-- Exterior Right to Left
		if rA <= (lB + StickyFrames.rangeX) and rA >= (lB - StickyFrames.rangeX) then
			newX = lB - (wA - right)
			snap = true
		end

	end

	if (lA <= rB and lB <= rA) then

		-- Vertical Centers
		if yA <= (yB + StickyFrames.rangeY) and yA >= (yB - StickyFrames.rangeY) then
			newY = yB
			snap = true
		end

		-- Interior Top
		if tA <= (tB + StickyFrames.rangeY) and tA >= (tB - StickyFrames.rangeY) then
			newY = tB - hA
			if frameB == UIParent or frameB == WorldFrame or frameB == ElvUIParent then
				newY = newY - 4
			end
			snap = true
		end

		-- Interior Bottom
		if bA <= (bB + StickyFrames.rangeY) and bA >= (bB - StickyFrames.rangeY) then
			newY = bB + hA
			if frameB == UIParent or frameB == WorldFrame or frameB == ElvUIParent then
				newY = newY + 4
			end
			snap = true
		end

		-- Exterior Top to Bottom
		if tA <= (bB + StickyFrames.rangeY + bottom) and tA >= (bB - StickyFrames.rangeY + bottom) then
			newY = bB - (hA - top)
			snap = true
		end

		-- Exterior Bottom to Top
		if bA <= (tB + StickyFrames.rangeY - top) and bA >= (tB - StickyFrames.rangeY - top) then
			newY = tB + (hA - bottom)
			snap = true
		end

	end

	if snap then
		frameA:ClearAllPoints()
		frameA:SetPoint("CENTER", UIParent, "BOTTOMLEFT", newX, newY)
		return true
	end
end