Monitor memory warning

.m

    dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_MEMORYPRESSURE,
                                                      0,
                                                      0x1|0x2|0x4|0x8|0x10|0x20|0xf0000000,
                                                      NULL);
    dispatch_source_set_event_handler(source, ^{
        dispatch_source_memorypressure_flags_t pressureLevel = dispatch_source_get_data(source);
        NSLog(@"Received memory warning with pressure: %@", @(pressureLevel).description);
    });
    dispatch_resume(source);