NSString *selName = @"_performMemoryWarning";
SEL sel = NSSelectorFromString(selName);
if (sel && [[UIApplication sharedApplication]respondsToSelector:sel]) {
IMP methodImp = class_getMethodImplementation([UIApplication class], sel);
void (* func)(id, SEL) = (void *)methodImp;
func([UIApplication sharedApplication], sel);
}