examples: Added audio/simple-playback-callback.

examples/CMakeLists.txt

@@ -179,6 +179,7 @@ endmacro()
 add_sdl_example_executable(renderer-clear SOURCES renderer/01-clear/renderer-clear.c)
 add_sdl_example_executable(renderer-primitives SOURCES renderer/02-primitives/renderer-primitives.c)
 add_sdl_example_executable(audio-simple-playback SOURCES audio/01-simple-playback/simple-playback.c)
+add_sdl_example_executable(audio-simple-playback-callback SOURCES audio/02-simple-playback-callback/simple-playback-callback.c)
 
 
 if(PSP)

examples/audio/02-simple-playback-callback/simple-playback-callback.c

@@ -0,0 +1,114 @@
+/*
+ * This example code creates an simple audio stream for playing sound, and
+ * generates a sine wave sound effect for it to play as time goes on. Unlike
+ * the previous example, this uses a callback to generate sound.
+ *
+ * This might be the path of least resistance if you're moving an SDL2
+ * program's audio code to SDL3.
+ *
+ * This code is public domain. Feel free to use it for any purpose!
+ */
+
+#define SDL_MAIN_USE_CALLBACKS 1  /* use the callbacks instead of main() */
+#include <SDL3/SDL.h>
+#include <SDL3/SDL_main.h>
+
+/* We will use this renderer to draw into this window every frame. */
+static SDL_Window *window = NULL;
+static SDL_Renderer *renderer = NULL;
+static SDL_AudioStream *stream = NULL;
+static int total_samples_generated = 0;
+
+/* this function will be called (usually in a background thread) when the audio stream is consuming data. */
+static void SDLCALL FeedTheAudioStreamMore(void *userdata, SDL_AudioStream *astream, int additional_amount, int total_amount)
+{
+    /* total_amount is how much data the audio stream is eating right now, additional_amount is how much more it needs
+       than what it currently has queued (which might be zero!). You can supply any amount of data here; it will take what
+       it needs and use the extra later. If you don't give it enough, it will take everything and then feed silence to the
+       hardware for the rest. Ideally, though, we always give it what it needs and no extra, so we aren't buffering more
+       than necessary. */
+    additional_amount /= sizeof (float);  /* convert from bytes to samples */
+    while (additional_amount > 0) {
+        float samples[128];  /* this will feed 128 samples each iteration until we have enough. */
+        const int total = SDL_min(additional_amount, SDL_arraysize(samples));
+        int i;
+
+        for (i = 0; i < total; i++) {
+            /* You don't have to care about this math; we're just generating a simple sine wave as we go.
+               https://en.wikipedia.org/wiki/Sine_wave */
+            const float time = total_samples_generated / 8000.0f;
+            const int sine_freq = 500;   /* run the wave at 500Hz */
+            samples[i] = SDL_sinf(6.283185f * sine_freq * time);
+            total_samples_generated++;
+        }
+
+        /* feed the new data to the stream. It will queue at the end, and trickle out as the hardware needs more data. */
+        SDL_PutAudioStreamData(astream, samples, total * sizeof (float));
+        additional_amount -= total;  /* subtract what we've just fed the stream. */
+    }
+}
+
+/* This function runs once at startup. */
+int SDL_AppInit(void **appstate, int argc, char *argv[])
+{
+    SDL_AudioSpec spec;
+
+    if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO) == -1) {
+        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't initialize SDL!", SDL_GetError(), NULL);
+        return SDL_APP_FAILURE;
+    }
+
+    /* we don't _need_ a window for audio-only things but it's good policy to have one. */
+    if (SDL_CreateWindowAndRenderer("examples/audio/simple-playback-callback", 640, 480, 0, &window, &renderer) == -1) {
+        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't create window/renderer!", SDL_GetError(), NULL);
+        return SDL_APP_FAILURE;
+    }
+
+    /* We're just playing a single thing here, so we'll use the simplified option.
+       We are always going to feed audio in as mono, float32 data at 8000Hz.
+       The stream will convert it to whatever the hardware wants on the other side. */
+    spec.channels = 1;
+    spec.format = SDL_AUDIO_F32;
+    spec.freq = 8000;
+    stream = SDL_OpenAudioDeviceStream(SDL_AUDIO_DEVICE_DEFAULT_PLAYBACK, &spec, FeedTheAudioStreamMore, NULL);
+    if (!stream) {
+        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't create audio stream!", SDL_GetError(), window);
+        return SDL_APP_FAILURE;
+    }
+
+    /* SDL_OpenAudioDeviceStream starts the device paused. You have to tell it to start! */
+    SDL_ResumeAudioStreamDevice(stream);
+
+    /* (this is a web browser requirement, not an SDL thing.) */
+    SDL_Log("If you're running this in a web browser, you need to click the window before you'll hear anything.");
+
+    return SDL_APP_CONTINUE;  /* carry on with the program! */
+}
+
+/* This function runs when a new event (mouse input, keypresses, etc) occurs. */
+int SDL_AppEvent(void *appstate, const SDL_Event *event)
+{
+    if (event->type == SDL_EVENT_QUIT) {
+        return SDL_APP_SUCCESS;  /* end the program, reporting success to the OS. */
+    }
+    return SDL_APP_CONTINUE;  /* carry on with the program! */
+}
+
+/* This function runs once per frame, and is the heart of the program. */
+int SDL_AppIterate(void *appstate)
+{
+    /* we're not doing anything with the renderer, so just blank it out. */
+    SDL_RenderClear(renderer);
+    SDL_RenderPresent(renderer);
+
+    /* all the work of feeding the audio stream is happening in a callback in a background thread. */
+
+    return SDL_APP_CONTINUE;  /* carry on with the program! */
+}
+
+/* This function runs once at shutdown. */
+void SDL_AppQuit(void *appstate)
+{
+    /* SDL will clean up the window/renderer for us. */
+}
+