Since this question seems to be asked a lot lately, here's a quick list of types of addons that will and will not work.

Castingbar replacements:
Will still work.
v2.0 actually contains added functionality, making it easier to get information about which spell the player is casting and more.

Unit frame replacements and Raid addons:
Will still work, but will require a rewrite by the author.
The rewrite is needed to support the new secure templates, so users can cast spells and select party/raid members via the unit frames.

Click casting addons:
Will still work, requires rewrite for new secure templates.
Click casting addons, where you cast different spells, depending on which mouse button you use, and which modifier keys you're holding down, will still work. The secure templates Blizzard provides for use in unit frames etc support this natively, making it even easier for addon developers to implement.

Actionbar replacements:
Will still work, but will require a rewrite by the author.
There are some limitations to actionbar replacements, due to the nature of the 2.0 patch.
* It's no longer possible to have actionbutton automatically show/hide/resize/move depending on combat conditions. For instance, you can no longer have a large Overpower button in the center of the screen pop up everytime your opponent dodges. It's is possible to dim the button, so it appears almost transparent, and then make it fully lit when Overpower can be used, but the button will be there all the time, not possible to fully hide it.
* It is still possible to make bars/buttons show or hide by the press of a button, or if you move the mouse over an area.
* It's no longer possible for the actionbars to automatically change bars depending on your distance to your target.
* It's possible to make actionbars automatically change on Prowl and Shadowform.

Bag frame replacements:
Will still work, but will require rewrites.

Movement addons:
Addons that move around with existing frames will still work in the expansion.
There are some minor restrictions, in that the secure frames (action buttons, unit frames etc - frames that can target or cast spells) are locked down during combat (can still be moved if dragged, but not by using SetPoint or similar functions), meaning you'll have to move them to their correct spots while out of combat.

Damage/healing/threat meters:
Will still work, with minor tweaks.
Damage and healing meters should work, without much trouble, since most of them just parse the combatlog, which will still be there.
Threat meters should also work, as soon as the authors have had time to test all the new spells, adjust for the new talents etc.

Information addons:
More or less all addons that do not cast spells, or do not change target, will work.
v2.0 even adds a lot of methods for retriving information, that previously required guesswork, combatlog parsing or another method.

Emergency Monitors:
In current form: will not work.
Due to the restriction, that you can't move or change attributes on secure frames while in combat, it's not possible to have an emergency monitor as it is now, where you can click it to select the person.
It's still possible to have a non-interactive list, and then you'll have to find the person yourself in the raid unit frames. It's even possible for the emergency monitor to mark the unit frame with a fat red border, if needed, but you'll still have to click the unit frame, and not the emergency list.

Since you cannot dynamically change targets anymore, macros such as this are no longer possible (CTRA used as example here):
/script CT_RA_Emergency_TargetMember(1)
/cast Flash Heal

item change addons:
Addons, that allow for a quick change of gear, will still work.

Warning, Timer etc addons:
Addons that show warnings for boss encounter in raids, show warnings about spells being cast, give timers for spells etc will all still work.

Auto select spell addons:
Addons like decursive, that automatically select a target and spell for you, will not work.
Addons, that automatically downrank for you, if the target doesn't need a full heal, will not work.
It's possible to set up some keybindings, that change the rank of your spells, but it's the player that has to initiate it.
For instance, possible to make an addon, where the +/- keys raises and lowers the rank, and then have a third keybinding that casts the spell.

Auction and Trade:
Addons that automatically place bids on auctions will no longer work. Placing a bid requires user interaction in 2.0.
Addons that automatically trade items to others will no longer work (for instance an automated water dispenser for mages). It's still possible for an addon to put items in the trade window, but user interaction is required to perform the actual trade.

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Q u o t e:
Great writeup! Would you mind also posting it on the 2.0 changes thread?

Thanks!

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Ok. (with some minor fixes for grammar, as posted above :)

I took it out since you made almost all the changes. I am pretty sure the end user can still drag secure frames in combat b(justr no addons can move them themselves), but I don't understand enough of the changes to prove it one way or the other. Hopefully someone can let me know if I'm misinformed or not.

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Q u o t e:
I took it out since you made almost all the changes. I am pretty sure the end user can still drag secure frames in combat b(justr no addons can move them themselves), but I don't understand enough of the changes to prove it one way or the other. Hopefully someone can let me know if I'm misinformed or not.

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Yes, you can drag secure frames. But a lot of movement addons include an ability to "nudge" the frame into place, that feature wont work while in combat.

Bump.

This should be stickied, imo.

In the action bars segment :

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Q u o t e:
* It is still possible though, to make bars/buttons show or hide by the press of a button.

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Would you be able to show/hide bars by mousing over them? Haven't seen anything yet that addresses this issue. A lot of my bars are potions, pets, and crafting stuff that I have hidden unless I mouse over them.. kinda curious if this setup will still work for me while in combat.

Can you scale buttons when in combat? ie, Have the button normal size, then double it's size when you hit 40% hp?

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Q u o t e:
Would you be able to show/hide bars by mousing over them?

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Yes.

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Q u o t e:
Can you scale buttons when in combat? ie, Have the button normal size, then double it's size when you hit 40% hp?

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No. Well, Yes and No. You can change the size in response to the same inputs that can show and hide them (eg, mouse click, mouseover, keypress), but not in response to combat conditions like your health or mana etc.

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Q u o t e:


Emergency Monitors:
In current form: will not work.
Due to the restriction, that you can't move or change attributes on secure frames while in combat, it's not possible to have an emergency monitor as it is now, where you can click it to select the person.
It's still possible to have a non-interactive list, and then you'll have to find the person yourself in the raid unit frames. It's even possible for the emergency monitor to mark the unit frame with a fat red border, if needed, but you'll still have to click the unit frame, and not the emergency list.

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From what i understand though, it is possible to make an addon that targets person with lowest health on click.

Kinda like /script CT_RA_Emergency_TargetMember(1); works nowadays

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Q u o t e:


From what i understand though, it is possible to make an addon that targets person with lowest health on click.

Kinda like /script CT_RA_Emergency_TargetMember(1); works nowadays

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Nope.

From what I understand mods cannot dynamically change targets for you. Such a mod would be impossible.

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Q u o t e:


Nope.

From what I understand mods cannot dynamically change targets for you. Such a mod would be impossible.

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hmmm ok wasnt aware of that change, assumed target switching would still be enabled for assist mods and stuff like that.

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Q u o t e:


hmmm ok wasnt aware of that change, assumed target switching would still be enabled for assist mods and stuff like that.

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You can make a simple macro like:
/assist Maldivia

You can also make assist mods etc, where you specify a name or a unit (before combat starts), that you then assist via a mouse click or a keybind.

A way for a semi dynamic assist macro, would be to set the unit to assist to "focus", and then manually change the focus, using the /focus command, if your MA changes/dies during combat.

Can I show and move a bar under the cursor while a key is down, and then hide it when the key is up, if the contents of that bar are static and set up out of combat?

This produces a kind of "contextual menu" effect that's a nice way to conserve screen real estate by hiding non-essential buttons.

From what I've read, it seems like there is a secure way to hide and show in response to a keypress (maybe I'm wrong on that, too), but I'm not sure if that keypress could also move the bar under the cursor (bars 'locked in combat?')

I suppose I could just anchor the popup menu in the middle of the screen as is often done on console games, but it's more elegant to move it under the cursor like a true contextual menu, and it doesn't really convey any advantage beyond good design.

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Q u o t e:


If you're planning to write an addon yourself, take a look at the code behind the SecureAnchor_____Templates.

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Thanks. I hadn't thought about writing a full addon (I'm more at the level of fixing existing code through tinkering), but it's good to know that this is available. I'll take a look at the code for SecureAnchor and see if I can sort it out. ;)

Small bump.

Can you still auto target Hunters after feign death?



Can you still cancel casting(via spamming a macro) if the target of your spell is under/above a certain health%?



Does click casting require attaining the target you want to heal or can you be targeting a boss while click casting the MT?



Will addons(such as those integrated to La Vendetta or CTRA) that warn you when you draw aggro still work?

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Q u o t e:
Can you still auto target Hunters after feign death?

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You can't auto anything. You can make a macro, that tries to target your previous target, if you loose it, something like:
/targetlasttarget [noexists]

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Q u o t e:
Can you still cancel casting(via spamming a macro) if the target of your spell is under/above a certain health%?

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Nope. Health, buff, debuffs and like are not valid conditions for the new macro system, and you can't use /script commands with SpellStopCasting() in 2.0

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Q u o t e:
Does click casting require attaining the target you want to heal or can you be targeting a boss while click casting the MT?

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The new API for click casting allows you to have any target (and keep that target), while casting spells on other people. (Well, probably doesn't work for melee attacks :)

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Q u o t e:
Will addons(such as those integrated to La Vendetta or CTRA) that warn you when you draw aggro still work?

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This is purely information, so should still work just fine.