I'm working in windows phone 8 application with MVVM pattern. I need to preserve page state when my app going to suspend. So I'm using State dictionary to store my viewmodel its working fine, But when I try to store viewmodel with parameter wise constructor following error will be occurred.
Type 'ViewModel' cannot be serialized. Consider marking it with the DataContractAttribute attribute, and marking all of its members you want serialized with the DataMemberAttribute attribute. Alternatively, you can ensure that the type is public and has a parameterless constructor - all public members of the type will then be serialized, and no attributes will be required.
If you pass an object to the State dictionary it will be serialized with the [Xml]DataContractSerializer and that requires a public, parameterless constructor.
If you're not able, or unwilling, to add such a constructor then you'll need to handle the serialization and deserialization of the object yourself and add the serialized data (the string) to the dictionary instead.
Related
I got an error ...(no Creators, like default constructor, exist): cannot deserialize from Object value (no delegate- or property-based Creator) which indicates that I need a property-based creator. I have a few constructors with different parameters, but no default.
My solution was to add a default constructor SomeClass() {}. My question is: why does this happen? Also, what is a delegate/property-based constructor?
If there is no constructors that is annotated with #JsonCreator, by default, Jackson needs a default no-args constructor in order to parse the JSON into POJO or bean classes. That is why when you add a default constructor, it will work fine.
And if you don't actually need the usage of the default constructor, just add it for Jackson only, you can set it to private, protected or package-protected. Jackson is still able to fill all the fields via reflection.
Regarding the no delegate- or property-based Creator, they are constructors that is annotated with #JsonCreator. In Jackson, there are 2 types of Creator/JsonCreator which are delegate-based Creator and property-based Creator.
Delegate-based creators take just one argument, which is NOT annotated with #JsonProperty. Type of that property is used by Jackson to bind the whole JSON value (JSON Object, array or scalar value), to be passed as value of that one argument.
Property-based creators take one or more arguments; all of which MUST be annotated with #JsonProperty, to specify JSON name used for property. They can only be used to bind data from JSON Objects; and each parameter represents one property of the JSON Object; type of property being used for binding data to be passed as that parameter when calling creator.
You can read for more details about these 2 creators in the article below.
http://www.cowtowncoder.com/blog/archives/2011/07/entry_457.html
Is it possible to give Spring MVC's form binding some kind of type hint to indicate what class to instantiate and populate with form data?
I've got some quite unusual requirements to try and build a dynamic form, that represents a collection of different objects. The user can add objects from a selection of types to this collection, and then set the properties for that object type using form elements. I can figure out how to create the form using jQuery, but I'm not sure how to get Spring to handle a load of POST data when it won't know what types to bind to in advance.
One way that I can think of doing this is to write your own custom HandlerMethodArgumentResolver , which is responsible for translating the request into the argument values of the controller methods. You should be able to create a custom annotation that will indicate to Spring MVC, that your custom handler method argument resolver will be resolving the specific annotated method arguments(say #CustomType Object argument).
Once the call comes into the handler resolver, you can probably determine the type that the json request should map to, and call the json mapper with the actual type.
You can register a custom argument resolver this way:
<annotation-driven>
<argument-resolvers>
<beans:bean class="..CustomArgumentResolver"/>
</argument-resolvers>
</annotation-driven>
I.e. I only want a nice way to retrieve existing objects from my Model (mostly some SessionAttributes).
I don't want new objects to be created and I especially don't want objects to be instantiated from request parameters and put into the model. This just sounds like a back door to me.
It would also be great if an Exception can be thrown if no matching parameter is in the model.
I got the answer to this by reading the source code. According to the implementation of org.springframework.web.bind.annotation.support.HandlerMethodInvoker.resolveModelAttribute, a new bind Object will not be created if the name of the model attribute is declared as a session attribute using the #SessionAttributes annotation.
If the attribute is not present in the session, an Exception will be thrown.
So it is relatively safe to bind session attributes this way.
I have a linq-to-sql class. I have a property "Password" for which I want to call the underlying ASP.NET Membership provider. Thus, I do not want this property written out directly but via my own code. I basically want to create a facade/proxy for this property such that I may use the underlying membership provider or a custom stored procedure.
I want to accomplish without modifying the LINQ-TO-SQL designer generated code, if at all possible.
It is possible. You can add your properties and methods to linq generated class using partial class mechanism. Linq generated classes are marked partial so you can add class members with:
public partial class YourLinqClass
{
// your methods and properties. refer linq properites and methods with "this."
// example:
public string Password
{
get
{
int id = this.UserId;
string password = // ... get password
return password;
}
set
{
// ...
}
}
}
You have to place the partial class in the same namespace as the rest of dbml.
The best option is to remove the property from the designer and write it in code, in the partial class, as described by PanJanek.
However, if you do it this way, you are pursuing a bad design. You're introducing a dependency into your entity class that breaks layer encapsulation. Entity classes shouldn't know about providers any more than they know about the DataContext that loads them. They aren't really meant to be anything more than containers for data going in and out of the database.
You should consider making a separate class that wraps the entity, the context, the username provider, and whatever other services you require, and in that class retrieve the username and do the required operations to your entity.
It looks like it might be possible to create a custom DataContext to handle this situation.
http://weblogs.asp.net/scottgu/archive/2007/07/11/linq-to-sql-part-4-updating-our-database.aspx
There are partial methods for the individual properties as, well and an OnValidate method.
In my case, I think the best solution is to throw an exception in the property changing method and add a public method for setting this individual property. This solution while not perfect will avoid touching the SQL generated code, where the property could be set to readonly or the setter removed.
Other suggestions welcome.
I use Spring MVC (via Spring Roo) to build a small web application for administering persons. In the page for creating a person the bean Person is used as form backing object (key "person" in the model map).
<form:form action="${form_url}" method="POST" modelAttribute="person">
I would like to add some attributes to the model map which can be altered by the user in the creation form. Basically, I try to add a Boolean, so that I can control which page is displayed next after the user presses the submit button.
I try to modify the Boolean (key "myBoolean" in the model map) using a simple checkbox:
<form:checkbox id="_myboolean_id" path="myBoolean"/>
However, as I am new to Spring MVC I have some difficulties here. The Boolean object is not an attribute of the form backing object. So if I try to access it the following exception is thrown (of course):
Invalid property 'myBoolean' of bean
class [de.cm.model.Person]: Bean
property 'myBoolean' is not readable or
has an invalid getter method: Does the
return type of the getter match the
parameter type of the setter?
Is there are way to access a value of the model map directly? The only solution I can imagine right now is a kind of wrapper object around the class Person and my additional attributes which is used as a new form backing object. However, this is more work for a IMHO simple task. Do you have a better solution?
You may create custom form field:
<input type="checkbox" name="myBoolean"/>
And specify additional parameter in Controller post method:
public ModelAndView savePerson(#ModelAttribute("person") Person person, #RequestParameter ("myBoolean") Boolean myBoolean)