Saturday, September 29, 2012

Tutorial 18 - Saving and Loading Complicated Tables

18.1 Recap

Tutorial 18 brings together elements from a number of the previous Tutorials. It is an example of where asking the right question makes finding the answer much simpler. Our aim is to be able to load and save level data from our Tower Defence level generator. Most of the data we need is stored in a table. The tricky part is that Codea only allows us to save data as strings at the moment (some smart folks over at the Codea Forum have worked out how to jam data into image files and save those).
Tutorial 16 examined a technique for saving simple table structures like a one dimensional array. Initially we thought that saving a more complicated table wouldn't be that tough. All we needed was one of the existing Lua table to string parsers like Pickle / UnPickle. WRONG!

18.2 Limitations of Table Serialisation Parsers

We discussed Lua tables in Interlude 6. They are simple in concept but capable of representing very complicated structures. Tables can contain all the simple Lua types (booleans, strings, nil, and number), functions, other tables, closures, etc. The most complete table to string parser that we could find was Data Dumper. But even Data Dumper can not handle tables that contain:
  • Both light and full userdata
  • Coroutines
  • C functions
And of course our level data contains userdata (e.g. vec2 is represented using userdata). So where to from here?

18.3 The Modified Cell Class

This is where asking the right question makes all the difference. Looking at which data we actually need to save, it becomes apparent that we don't need to save the entire table (including functions) as this can all be reconstituted. All we really need from the Grid table is what is contained in each cell. So rather than trying to convert a two dimensional array of cell objects (which contain userdata) into a string, we extend the cell class to provides its contents (state) as a string. We also provide the inverse function which sets the cell state from a string so we can load the level back in. The extended cell class is shown below.
Once we can get the grid cell contents out as a string it is easy to write a function to create a simple two dimensional table holding each of these strings. Data Dumper makes short work of converting this table to a string which we can save to global data. Even better, Data Dumper saves the table in a format that loadstring can use to rebuild the original table. The updated Main class for dGenerator details these save and load functions.

We updated the File Manager class from Tutorial 17 to allow us to select the file to be loaded. As part of this update we got rid of the submenus because we didn't need them and they meant that the user had to do an extra tap to load. In addition, Delete was right next to Load which is poor design. One slip of the finger would be potentially disastrous (particularly because there is no confirmation for delete and no undo). The other improvement to this version of File Manager is we truncate keys and values which are larger than the ListScroll widths. You can see the upgraded version in the screenshot below. The About menu item doesn't do anything at this stage.

18.4 Loading & Saving Data

Tapping the Save button on the main screen of dGenerator will call the saveLevel() function in Main(). The function starts off by saving the simplified table data in saveGrid. It then generates the key which contains a header "dGen" (to indicate when loading if it is the right data type), the game name and level number, a boolean indicating if the start cell has been selected and its co-ordinates and then a boolean indicating if the end cell has been selected and its co-ordinates. Finally we use Data Dumper to convert saveGrid to a string and save the key and value to global data.
Loading a level is the reverse. Tapping the Load button on the dGenerator main screen will call the loadLevel() function in Main. This prints out some debug data but its only compulsory action is to set the App state to stateFileManager. This will draw the File Manager instead of the main screen and handle its touches.
Once the user has navigated to global data and selected an appropriate key containing level data, tapping load on the File Manager menu bar will call the loadFile() function in Main. This function splits the key back into its component parts using the explode() helper function and then uses loadstring() to create a function which rebuilds the data table. The createGrid() function was updated so that it can be loaded using this data table.
And that is all there is to it. You can use this link to download the entire code including the updated classes.
Next up we will look at adding creeps to your levels in a number of waves.