最简单的方法设置UITextView placeholder

// _placeholderLabel
UILabel *placeHolderLabel = [[UILabel alloc] init];
placeHolderLabel.text = @"请输入内容";
placeHolderLabel.numberOfLines = 0;
placeHolderLabel.textColor = [UIColor lightGrayColor];
[placeHolderLabel sizeToFit];
[textView addSubview:placeHolderLabel];
[textView setValue:placeHolderLabel forKey:@"_placeholderLabel"];