STTextView可以设置placeHolder和字符个数，达到字符限制不能再输入；另外还添加了字符变化的block回调，方便开发者使用。

#define MAXLENGTH 100

STTextView *textView = [[STTextView alloc] initWithFrame:CGRectMake(20, 84, self.view.bounds.size.width - 40, 120)];
    textView.placeholder = [NSString stringWithFormat:@"写点什么吧，最多%@个字", @(MAXLENGTH)];
    textView.maxLength = MAXLENGTH;
    textView.font = [UIFont systemFontOfSize:15];
    [textView setTextDidChangeBlock:^(UITextView *textView) {
        _countLabel.text = [NSString stringWithFormat:@"还可以写%@个字", @(MAXLENGTH - textView.text.length)];
    }];
    
textView.layer.borderColor = [UIColor lightGrayColor].CGColor;
textView.layer.borderWidth = 1.0;
textView.layer.opacity = .7;

[self.view addSubview:textView];