Callback function scenario

Callback Function

顧名思義就是回去呼叫某個函式執行，至於什麼時候回去呼叫？如何知道呼叫哪個函式？

Sequential function call the function directly

int compute() {
    // Do something
}
void main() {
    int result = compute();
    printf("compute result: %d", result);
}

Callback function is event-driven

當特定事件發生時(例如觸控發生，或某手勢被偵測)才會被呼叫：

• Step 1: Register the callback function (function pointer and function args as parameters).
• Step 2: Wait to be called.

舉個例子

Secnario: 如果有 A, B 兩個 programs，希望某個 event 發生的時候 B 可以通知 A
Solution: A 必須傳一個 function pointer 給 B 當 event 發生的時候要執行什麼

實務上的例子

a Unix program might not want to terminate immediately when it receives SIGTERM, so to make sure that its termination is handled properly, it would register the cleanup function as a callback.

// Nachos source code
void CallOnUserAbort(void (*func)(int)) {
    (void)signal(SIGTERM, func);
}
static void Cleanup(int x) {
    cerr << "Cleaning up after signal " << x << "\n";
    delete kernel;
}
int main() {
    kernel = new Kernel(argc, argv);
    kernel->Initialize();
    
    // if user hits Ctrl-C, delete kernel
    CallOnUserAbort(Cleanup);
    kernel->ExecAll();
}