Animation Library, Scene Library, and Game Library

After repeating a lot of code, I began to recognize some emerging patterns.

Animation Library

Previously, an animation was a reference to an SDL_Texture containing an animation sprite image data and an array of SDL_Rects to store each frame’s coordinates in the sprite.

I consolidated all the metadata of an animation into a new struct called animation_data and defined a new type AnimationData.

I combined all animation rendering functions into a single render_animation function, which accepts an SDL_Renderer*, an AnimationData*, and an SDL_Point, rendering a single frame of the animation on the screen.

Due to my inexperience with C, I faced an issue with the memory management of new AnimationData values. When you create a new struct value in a function (on the stack), its memory gets deallocated after the function returns. In my case, I was creating new AnimationData values in the init function and encountered errors when trying to reference them in the render_animation function. The solution was to allocate memory on the heap and return a pointer. I created a make_animation_data constructor function to encapsulate the logic for creating a new AnimationData value.

lldb Commands

I debugged the AnimationData memory issue a bit and found this list of lldb commands useful, especially print: lldb.llvm.org/use/map.html.

struct and typedef

I’m still unsure why struct animation_data can’t automatically define a new animation_data type without also adding typedef struct animation_data AnimationData. In any case, I decided to use the convention I’ve seen used in SDL, which is prepending typedef on struct definitions like this:

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typedef struct foo {
  int bar;
} Foo;

Scene Library (Function Pointers, Basic Interface)

All scenes must implement the same set of functions, so I decided to create a new Scene struct that models a game scene.

All the fields in the struct are function pointers, so the struct acts as an interface.

Scene headers declare an external scene variable; for instance, intro.h declares: extern Scene intro_scene;, then intro.c initializes the variable with the correct functions.

One issue with this approach is that if I add a new function to the Scene struct, the compiler doesn’t throw an error if the scene instances don’t initialize it.

Game Library

I extracted some logic from main.c into a new game.c/game.h library.

For now, I just rearranged the code, so the actual utility of this refactoring is limited.

game.h defines most of the functions of a Scene. I think the game library could implement the Scene interface, but then Scene would become the wrong kind of abstraction.

I defined set_active_scene, a function to transition from the current scene to a new scene. An on_scene_inactive callback is executed on the current scene to perform some cleanup (stop music, etc.). An on_scene_active callback is executed on the new scene to perform some initialization (start music, etc.).

Checkpoint

At this stage, I had the two initial scenes (intro and playground entrance) with placeholder images: