UIAlertView子类中的UITextView

这是我的子类的代码

- (id)initNarrationViewWithImage:(UIImage *)image text:(NSString *)text{
    if (self = [super init]) {
        self.backgroundImage = image;  
        UITextView * textView = [[UITextView alloc] initWithFrame:CGRectZero];
        self.textualNarrationView = textView;
        [textView release];
        self.textualNarrationView.text = text;
        self.textualNarrationView.backgroundColor = [UIColor colorWithRed:0.0 green:1.0 blue:0.0 alpha:1.0];
        self.textualNarrationView.opaque = YES;
        [self addSubview:textualNarrationView];
    }
    return self;
}

- (void)drawRect:(CGRect)rect {

    NSLog(@"DRAU");
    CGSize imageSize = self.backgroundImage.size;
    [self.backgroundImage drawInRect:CGRectMake(0, 0, imageSize.width, imageSize.height)];
}

- (void)layoutSubviews {


    CGSize imageSize = self.backgroundImage.size;
    self.textualNarrationView.bounds = CGRectMake(0, 0, imageSize.width - 20, imageSize.height - 20);
    self.textualNarrationView.center = CGPointMake(320/2, 480/2);


}

- (void)show{
    [super show];
    NSLog(@"SCIO");
    CGSize imageSize = self.backgroundImage.size;
    self.bounds = CGRectMake(0, 0, imageSize.width, imageSize.height);
    self.center = CGPointMake(320/2, 480/2);
}

这是我第一次对UIView进行子类化,我肯定遗漏了一些东西,因为背景图像显示正确,UITextview也出现了,但几秒钟后,它就跳了起来(似乎是个坐标问题)。