Objective c MFMailComposeViewControllerDelegate设置消息正文问题

这是我的密码：

    if (!_mail) {
    _mail = [[MFMailComposeViewController alloc] init];
    _mail.mailComposeDelegate = self;

    CGRect mailRect = _mail.view.frame;
    mailRect.size.height = self.view.frame.size.height;
    mailRect.size.width = self.view.frame.size.width;
    _mail.view.frame = mailRect;
    _mail.view.backgroundColor = [UIColor whiteColor];
}

NSString *messageBody = @"text"

[_mail setSubject:@"Book Order"];
[_mail setMessageBody:messageBody isHTML:YES];

[self.view addSubview:_mail.view];

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此消息之后未设置正文。两者都不是主题。为什么？你能帮我吗？

我找到了答案。我必须删除_mail.view.frame=mailRect；线 如果我调整它的大小，为什么它不工作