监测 UITextField 输入

• textField 添加target 触发条件 UIControlEventEditingChanged ,响应 textField 编辑时的内容改变事件

[self.textFeild addTarget:self action:@selector(passConTextChange:) forControlEvents:UIControlEventEditingChanged];

• 处理响应

- (void)textFieldEditingChanged:(UITextField *)textField{
    NSUInteger textLength = textField.text.length;
    if (!textLength) {
        textField.font = [UIFont systemFontOfSize:12.0f];
    }else {
        textField.font = [UIFont systemFontOfSize:20.0f];
    }      
}