GCD on openFrameworks

ofApp.cpp

#include "ofMain.h"

class ofApp : public ofBaseApp {
public:
    string str = "";
    
    void setup() {
        ofBackground(0);
        ofSetFrameRate(60);
        ofSetVerticalSync(true);
    }

    void update() {
    }

    void draw() {
        ofDrawBitmapString(str, 15, 30);
    }
    
    void keyReleased(int key) {
        str = "";
        if (key == '1') {
            // run method from main thread
            dispatch_async(dispatch_get_main_queue(), ^{
                str = "hello from main thread";
            });
        }
        if (key == '2') {
            // run method from background thread
            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                str = "hello from background thread";
            });
        }
        if (key == '3') {
            // run method from main thread after delay
            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 3 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
                str = "hello from main thread";
            });
        }
        if (key == '4') {
            // run method from background thread after delay
            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 3 * NSEC_PER_SEC), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                str = "hello from background thread";
            });
        }
    }
};

int main() {
    ofSetupOpenGL(320, 240, OF_WINDOW);
    ofRunApp(new ofApp());
}