Example ipc (#60)

* add blank example

* add ipc example code, need to change FURI API

* add ipc example code, need to change FURI API

* change core API, add context

* check handler at take

* fix important bugs in furi

* drawing example

* add posix mq

* fix unsigned demo counter

* create at open

* working local demo

* russian version of IPC example

* english version

* add gif

wiki/examples/IPC-example.md

@@ -0,0 +1,145 @@
+In this example we show how to interact data between different applications.
+As we already know, we can use FURI Record for this purpose: one application create named record and other application opens it by name.
+
+We will simulate the display. The first application (called `application_ipc_display`) will be display driver, and the second app (called `application_ipc_widget`) will be draw such simple demo on the screen.
+
+# Dsiplay definition
+
+For work with the display we create simple framebuffer and write some "pixels" into it.
+
+```C
+#define FB_WIDTH 10
+#define FB_HEIGHT 3
+#define FB_SIZE (FB_WIDTH * FB_HEIGHT)
+
+char _framebuffer[FB_SIZE];
+
+for(size_t i = 0; i < FB_SIZE; i++) {
+    _framebuffer[i] = ' ';
+}
+```
+
+On local target we just draw framebuffer content like this:
+```
+    +==========+
+    |          |
+    |          |
+    |          |
+    +==========+
+```
+
+```C
+fuprintf(log, "+==========+\n");
+for(uint8_t i = 0; i < FB_HEIGHT; i++) {
+    strncpy(row_buffer, &fb[FB_WIDTH * i], FB_WIDTH);
+    fuprintf(log, "|%s|\n", row_buffer);
+}
+fuprintf(log, "+==========+\n");
+```
+
+_Notice: after creating display emulator this example should be changed to work with real 128×64 display using u8g2_
+
+# Demo "widget" application
+
+The application opens record with framebuffer:
+
+```C
+FuriRecordSubscriber* fb_record = furi_open(
+    "test_fb", false, false, NULL, NULL, NULL
+);
+```
+
+Then it clear display and draw "pixel" every 120 ms, and pixel move across the screen.
+
+For do that we should tale framebuffer:
+
+`char* fb = (char*)furi_take(fb_record);`
+
+Write some data:
+
+```C
+if(fb == NULL) furiac_exit(NULL);
+
+for(size_t i = 0; i < FB_SIZE; i++) {
+    fb[i] = ' ';
+}
+
+fb[counter % FB_SIZE] = '#';
+```
+And give framebuffer (use `furi_commit` to notify another apps that record was changed):
+
+`furi_commit(fb_record);`
+
+`counter` is increasing on every iteration to make "pixel" moving.
+
+# Display driver
+
+The driver application creates framebuffer after start (see [Display definition](#Display-definition)) and creates new FURI record with framebuffer pointer:
+
+`furi_create("test_fb", (void*)_framebuffer, FB_SIZE)`
+
+Next it opens this record and subscribe to its changing:
+
+```
+FuriRecordSubscriber* fb_record = furi_open(
+    "test_fb", false, false, handle_fb_change, NULL, &ctx
+);
+```
+
+The handler is called any time when some app writes to framebuffer record (by calling `furi_commit`):
+
+```C
+static void handle_fb_change(const void* fb, size_t fb_size, void* raw_ctx) {
+    IpcCtx* ctx = (IpcCtx*)raw_ctx; // make right type
+
+    fuprintf(ctx->log, "[cb] framebuffer updated\n");
+
+    // send event to app thread
+    xSemaphoreGive(ctx->events);
+
+    // Attention! Please, do not make blocking operation like IO and waits inside callback
+    // Remember that callback execute in calling thread/context
+}
+```
+
+That callback execute in calling thread/context, so handler pass control flow to app thread by semaphore. App thread wait for semaphore and then do the "rendering":
+
+```C
+if(xSemaphoreTake(events, portMAX_DELAY) == pdTRUE) {
+    fuprintf(log, "[display] get fb update\n\n");
+
+    #ifdef HW_DISPLAY
+    // on Flipper target draw the screen
+    #else
+    // on local target just print
+    {
+        void* fb = furi_take(fb_record);
+        print_fb((char*)fb, log);
+        furi_give(fb_record);
+    }
+    #endif
+}
+```
+
+A structure containing the context is used so that the callback can access the semaphore. This structure is passed as an argument to `furi_open` and goes to the handler:
+
+```C
+typedef struct {
+    SemaphoreHandle_t events; // queue to pass events from callback to app thread
+    FuriRecordSubscriber* log; // app logger
+} IpcCtx;
+```
+
+We just have to create a semaphore and define a context:
+
+```C
+StaticSemaphore_t event_descriptor;
+// create stack-based counting semaphore
+SemaphoreHandle_t events = xSemaphoreCreateCountingStatic(255, 0, &event_descriptor);
+
+IpcCtx ctx = {.events = events, .log = log};
+```
+
+You can find full example code in `applications/examples/ipc.c`, and run it by `docker-compose exec dev make -C target_lo example_ipc`.
+
+![](https://github.com/Flipper-Zero/flipperzero-firmware-community/raw/master/wiki_static/application_examples/example_ipc.gif)