View source for Callbacks

==Introduction==

A callback is a special action 2 that the patch "calls" in order to modify various features, an example being the visual effect of train vehicles. &nbsp;See the [[CallbacksTut|callback tutorial]] on how to use them.

In the final action 2 of a chain of VarAction2s, the second byte of the matching entry has to have bit 7 set to identify it as a callback result. (Previously, it was required that the second byte be FF, but now only bit 7 needs to be set.)

For example, 04 00 is not a callback result (but it may be the ID of another chained action 2), whereas 04 80 is a callback result with a value of 4. &nbsp;For some callbacks, 15 bit results are possible, in which case the upper 7 bits are stored in the second byte. &nbsp;For example, 54 C2 is a callback result with a value of 4254=16980, where again the second byte has bit 7 set, changing the 42 into C2.

For compatibility with earlier patch versions, FF in the high byte is taken to mean the same thing as 80, so 04 FF also has a callback result of 4. &nbsp;Note that if your grf file needs to be compatible with versions before TTDPatch 2.0.1 alpha 40, you must set the high byte to FF, and so can use only 8 bit results.

== List of callbacks ==

The following callbacks are defined by the patch. &nbsp;The number of the callback is what will be stored in variable 0C so that you can return the right results if you have several callbacks. &nbsp;All callbacks are explained in more detail below.

'''PLEASE NOTE:''' Since TTDPatch 2.5 beta 5, the size of variable 0C has changed, and now there are word-sized callback numbers as well. To maintain backwards compatibility, only callbacks 10..3F can be checked using a byte-sized check. Callbacks 140 and above, on the other hand, must be checked using a word or dword sized check, to make sure they are identified uniquely.

{| |-
!Number!!Feature(s)!!Meaning
|-
|10||Vehicles||[[Callback: Visual effect and wagon power|Visual effect and wagon power]]
|-
|11||Vehicles||[[Callback: Wagon length|Wagon length]]
|-
|12||Vehicles||[[Callback: Load amount|Load amount]]
|-
|13||Stations||[[Callback: Station availability|Station availability]]
|-
|14||Stations||[[Callback: Station sprite layout|Select sprite layout]]
|-
|15||Vehicles||[[Callback: Refitted capacity callback|Refitted capacity callback]]
|-
|16||Trains||[[Callback: Articulated engine|Build articulated engines]]
|-
|17||Houses||[[Callback: House construction check|House construction check]]
|-
|18||Several <ref name="generic">Generic callbacks are called for the feature, not any specific vehicle ID. Read the [[Action3#n-id|Action 3]] entry on how to define these.</ref>||[[Callback: AI construction/purchase selection|AI construction/purchase selection]]
|-
|19||Vehicles||[[Callback: Cargo Subtype Display|Cargo sub-type display]]
|-
|1A||Houses||Next animation frame
|-
|1B||Houses||Animation control
|-
|1C||Houses||Construction stage changes
|-
|1D||Trains||Can wagon be attached
|-
|1E||Houses||Decide colour of building
|-
|1F||Houses||Cargo acceptance
|-
|20||Houses||Length of animation frame
|-
|21||Houses||Trigger building destruction
|-
|22||Industries||Availability
|-
|23||Vehicles||Additional text in purchase screen
|-
|24||Stations||Custom station layout
|-
|25||Industry tiles||Animation control
|-
|26||Industry tiles||Next animation frame
|-
|27||Industry tiles||Length of animation frame
|-
|28||Industries||Industry location permissibility
|-
|29||Industries||Random production change
|-
|2A||Houses||Get accepted cargo types
|-
|2B||Industry tiles||Decide cargo acceptance
|-
|2C||Industry tiles||Get accepted cargo types
|-
|2D||Vehicles||Select colour mapping for vehicle
|-
|2E||Houses||Custom cargo production
|-
|2F||Industry tiles||Custom shape check
|-
|30||Industry tiles||Decide drawing default foundations
|-
|31||Vehicles||Start/stop check
|-
|32||Vehicles||32-day callback
|-
|33||Several||Sound effect callback
|-
|34||Vehicles <ref name="generic"/>||Autoreplace vehicle selection
|-
|35||Industries||Monthly random production change
|-
|36||Vehicles||Change vehicle Properties
|-
|37||Industries||Cargo sub-type display
|-
|38||Industries||Show additional text in fund window
|-
|39||Cargoes||Custom profit calculation
|-
|3A||Industries||Show additional text in industry window
|-
|3B||Industries||Control special effects
|-
|3C||Industry tiles||Disable autosloping
|-
|3D||Industries||Opt out of accepting cargo
|-
|40..13F|| ||--Reserved--
|-
|140||Stations||Animation control
|-
|141||Stations||Next animation frame
|-
|142||Stations||Length of animation frame
|-
|143||Houses||Protect building conditionally
|-
|144||Sounds <ref name="generic"/>||Ambient sound effects
|-
|145||Cargoes||Custom station rating calculation
|-
|146||New Signals <ref name="generic"/>||New signals sprite drawing
|-
|147||Canals||Add sprite offset
|-
|148||Houses||Watched cargo accepted
|-
|149||Stations||Land slope check
|-
|14A||Industries||Decide industry color
|-
|14B||Industries||Decide input cargo types
|-
|14C||Industries||Decide output cargo types
|-
|14D||Houses||Customized building name
|-
|14E||Houses||Decide drawing default foundations
|-
|14F||Houses||Disable autosloping
|-
|150||Airport tiles||{{ottdp|1.1|no|ottdrev=r19197}} Decide drawing default foundations
|-
|152||Airport tiles||{{ottdp|1.1|no|ottdrev=r19197}} Animation control
|-
|153||Airport tiles||{{ottdp|1.1|no|ottdrev=r19197}} Next Animation frame
|-
|154||Airport tiles||{{ottdp|1.1|no|ottdrev=r19197}} Length of animation frame
|-
|155||Airports||{{ottdp|1.1|no|ottdrev=r20373}} Extra information about airport layout in the build gui
|-
|156||Airports||{{ottdp|1.1|no|ottdrev=r20373}} Airport layout name
|-
|157||Objects||Land slope check
|-
|158||Objects||Next animation frame
|-
|159||Objects||Animation control
|-
|15A||Objects||Length of animation frame
|-
|15B||Objects||Decide colour of building
|-
|15C||Objects||Show additional text in building window
|-
|15D||Objects||Disable autosloping
|-
|15E||Vehicles||{{ottdp|1.2|no|ottdrev=r23086}} Refit cost factor
|}
<references/>

{{:Callback: Visual effect and wagon power}}
{{:Callback: Wagon length}}
{{:Callback: Load amount}}
{{:Callback: Station availability}}
{{:Callback: Station sprite layout}}
{{:Callback: Refitted capacity callback}}
{{:Callback: Articulated engine}}
{{:Callback: House construction check}}
{{:Callback: AI construction/purchase selection}}
{{:Callback: Cargo Subtype Display}}
{{:Callback: Next animation frame}}
{{:Callback: Animation control}}
{{:Callback: Construction stage changes}}
{{:Callback: Can wagon be attached}}
{{:Callback: Building colour}}
{{:Callback: Cargo acceptance}}
{{:Callback: Length of animation frame}}
{{:Callback: Trigger building destruction}}
{{:Callback: Industry availability}}
{{:Callback: Additional text in purchase screen}}
{{:Callback: Custom station layout}}
{{:Callback: Industry location permissibility}}
{{:Callback: Random production change}}
{{:Callback: Get accepted cargo types}}
{{:Callback: Decide cargo acceptance}}
{{:Callback: Select colour mapping for vehicle}}
{{:Callback: Custom cargo production}}
{{:Callback: Custom shape check}}
{{:Callback: Decide drawing default foundations}}
{{:Callback: Vehicle Start/stop check}}
{{:Callback: 32-day callback}}
{{:Callback: Sound effect}}
{{:Callback: Autoreplace vehicle selection}}
{{:Callback: Monthly random production change}}
{{:Callback: Change vehicle properties}}
{{:Callback: Cargo sub-type display for industries}}
{{:Callback: Show additional text in fund/building window}}
{{:Callback: Custom profit calculation for cargoes}}
{{:Callback: Show additional text in industry window}}
{{:Callback: Control special industry effects}}
{{:Callback: Disable autosloping}}
{{:Callback: Opt out of accepting cargo}}
{{:Callback: Protect building conditionally}}
{{:Callback: Ambient sound effects}}
{{:Callback: Custom station rating calculation}}
{{:Callback: New signals sprite drawing callback}}
{{:Callback: Add sprite offset callback}}
{{:Callback: Watched cargo accepted}}
{{:Callback: Station slope check}}
{{:Callback: Decide industry colour}}
{{:Callback: Decide input and output cargo types}}
{{:Callback: Customized building name}}
{{:Callback: Extra information about layout in the build gui}}
{{:Callback: Layout name}}
{{:Callback: Object slope check}}
{{:Callback: Decide object colour}}
{{:Callback: Refit cost factor}}