iOS CGRectInset

dx, dy为正

    UIView *redView = [[UIView alloc] initWithFrame:CGRectMake(200, 200, 500, 500)];
    redView.backgroundColor = [UIColor redColor];
    [self.view addSubview:redView];
    
    UIView *blueView = [[UIView alloc]init];
    blueView.frame = CGRectInset(redView.frame, 50, 50);
    blueView.backgroundColor = [UIColor blueColor];
    [self.view addSubview:blueView];

dx, dy为负

    UIView *redView = [[UIView alloc] initWithFrame:CGRectMake(200, 200, 500, 500)];
    redView.backgroundColor = [UIColor redColor];
    [self.view addSubview:redView];
    
    UIView *blueView = [[UIView alloc]init];
    blueView.frame = CGRectInset(redView.frame, -50, -50);
    blueView.backgroundColor = [UIColor colorWithRed:0.0 green:0.0 blue:1.0 alpha:0.5];
    [self.view addSubview:blueView];

注意blueView加了50%的透明，否则redView被完全覆盖