I have a rather complex page that dynamically builds user controls inside of a repeater. This repeater must be bound during the Init page event before ViewState is initialized or the dynamically created user controls will not retain their state.
This creates an interesting Catch-22 because the object I bind the repeater to needs to be created on initial page load, and then persisted in memory until the user opts to leave or save.
Because I cannot use ViewState to store this object, yet have it available during Init, I have been forced to store it in Session.
This also has issues, because I have to explicitly null the session value during non postbacks in order to emulate how ViewState works.
There has to be a better way to state management in this scenario. Any ideas?
Edit: Some good suggestions about using LoadViewState, but I'm still having issues with state not being restored when I do that.
Here is somewhat if the page structure
Page --> UserControl --> Repeater --> N amount of UserControls Dynamicly Created.
I put the overridden LoadViewState in the parent UserControl, as it is designed to be completely encapsulated and independent of the page it is on. I am wondering if that is where the problem is.
The LoadViewState method on the page is definitely the answer. Here's the general idea:
protected override void LoadViewState( object savedState ) {
var savedStateArray = (object[])savedState;
// Get repeaterData from view state before the normal view state restoration occurs.
repeaterData = savedStateArray[ 0 ];
// Bind your repeater control to repeaterData here.
// Instruct ASP.NET to perform the normal restoration of view state.
// This will restore state to your dynamically created controls.
base.LoadViewState( savedStateArray[ 1 ] );
}
SaveViewState needs to create the savedState array that we are using above:
protected override object SaveViewState() {
var stateToSave = new List<object> { repeaterData, base.SaveViewState() };
return stateToSave.ToArray();
}
Don't forget to also bind the repeater in Init or Load using code like this:
if( !IsPostBack ) {
// Bind your repeater here.
}
This also has issues, because I have to explicitly null the session value during non postbacks in order to emulate how ViewState works.
Why do you have to explicitly null out the value (aside from memory management, etc)? Is it not an option to check Page.IsPostback, and either do something with the Session variable or not?
I have always recreated my dynamic controls in the LoadViewState event. You can store the number of controls needed to be created in the viewstate and then dynamically create that many of them using the LoadControl method inside the LoadViewState event. In this event you have access to the ViewState but it has not been restored to the controls on the page yet.
1) there's probably a way to get it to work... you just have to make sure to add your controls to the tree at the right moment. Too soon and you don't get ViewState. Too late and you don't get ViewState.
2) If you can't figure it out, maybe you can turn off viewstate for the hole page and then rely only on querystring for state changes? Any link that was previously a postback would be a link to another URL (or a postback-redirect).
This can really reduce the weight of the page and make it easier to avoid issues with ViewState.
protected override void LoadViewState(object savedState)
{
// Put your code here before base is called
base.LoadViewState(savedState);
}
Is that what you meant? Or did you mean in what order are the controls processed? I think the answer to that is it quasi-random.
Also, why can't you load the objects you bind to before Page_Load? It's ok to call your business layer at any time during the page lifecycle if you have to, with the exception of pre-render and anything after.
When creating dynamic controls ... I only populate them on the initial load. Afterwords I recreate the controls on postback in the page load event, and the viewstate seems to handle the repopulating of the values with no problems.
I have to explicitly null the session
value during non postbacks in order to
emulate how ViewState works.
I'm still foggy as to why you can't store whatever object(s) you are binding against in session. If you could store that object in session the following should work:
On first load bind your top user control to the object during OnPreInit. Store the object in session. Viewstate will automatically be stored for those controls. If you have to bind the control the first time on Page_Load that is ok, but you'll end up having two events that call bind if you follow the next step.
On postback, rebind your top user user control in the OnPreInit method against the object you stored in session. All of your controls should be recreated before the viewstate load. Then when viewstate is restored, the values will be set to whatever is in viewstate. The only caveat here is that when you bind again on the postback, you have to make 100% sure that the same number of controls are created again. The key to using Repeaters, Gridviews etc... with dynamic controls inside of them is that they have to be rebound on every postback before the viewstate is loaded. OnPreInit is typically the best place to do this. There is no technical constraint in the framework that dictates that you must do all your work in Page_Load on the first load.
This should work. However, if you can't use session for some reason, then you'll have to take a slightly different approach such as storing whatever you are binding against in the database after you bind your control, then pulling it out of the database and rebinding again on every postback.
Am I missing some obvious detail about your situation? I know it can be very tricky to explain the subtleties of the situation without posting code.
EDIT: I changed all references to OnInit to OnPreInit in this solution. I forgot that MS introduced this new event in ASP.NET 2.0. According to their page lifecycle documentation, OnPreInit is where dynamic controls should be created/recreated.
Related
I have a user control that I need to pass an object to prior to the onInit call of the user control so that the object can be used to build the user control's content prior to loading of the viewState.
Ideally I need to establish the object somewhere on the page's loading process, then target that userControl's object property to pass the established object to it. This all must occur prior to the OnInit call of the userControl.
I have tried a lot of different things, but here are the most obvious ways that have failed:
In the Page's Page_Init() I Create object and pass to the constructed user control, then I try and perform the necessary userControl content binding logic on the Page_InitComplete(). This fails because the userControl doesn't seem to call Page_InitComplete and Page_Init of the UserControl occurs before Page_Init of the Page (so weird)
I've attempted to put the UserControl binding logic in the Page_Load() call which does occur after the Page's Page_Init(). However this is too late and viewState has already failed to prepopulate because the proper databinding doesn't exist.
I've tried to populate the UserControl's object property in the Page_PreInit. However the UserControl hasn't yet been constructed so the reference to the UserControl on the page is null.
I recognize that I could try and use details found in session or application storage for the user control binding logic but this seems unnecessary. What if someday I got rid of that session or application variables?
Because this is a complex object I'm certain that I can't pass it in to the user control on the UI like <UC:CustomUC propertyName=new ComplexObject()>
What am I missing.
The main thing I was missing is that the main purpose of Page_Init() is to (re)generate DYNAMICALLY CREATED CONTROLS. Because my control were not dynamically generated I didn't need to rely on Page_Init.
As such, all of the items of the control are kept via viewstate so I didn't need to repopulate it which was what I was concerned about needing to do. SO this post is more a learning opportunity for me than anyone else.
I want to assign values to server control properties in my code behind to "initialize" a form. I don't need/want these values to be added to the viewstate. (I know I can disable viewstate for a specific control, but I need viewstate as some values may change and need to be persisted during postbacks.)
At the moment for me it seems its not possible to load these values in code without having them added to the viewstate.
As I understand the following happens:
Page: PreInit
I could add values to SOME controls (its working for example with a literal control to set the text value) here BUT since control's init methods are executed later I cannot for example find a RegisterUser control and its child controls at this stage yet, they are still null. > so no solution in this specific case, there's more cases
Control: Init
I cannot reach this point from within my page, this can only be used inside the user control code for example when you write your own usercontrol and want to initialize its values, I ofcourse dont want to create my own control for each control I need to initialize with executing some logic first.
Control: TrackViewState
Viewstate Tracking is enabled so from here on after anything I set will be marked as 'dirty' and therefore added to the viewstate.
Page:Init
Page:TrackViewState
So am I right in saying:
In code behind, without disabling a controls viewstate property..
You can initialize "1st level" child control properties from the page control in the PreIinit method without having the value being added to the viewstate.
You cannot initialize Child control properties from controls that are "1st level" child controls of the page without having the value added to the viewstate.
?
Then,
You can initialize control properties declaratively by using resources, but this does not circumvent the scenario where you need to execute logic
You can execute logic by using databinding syntax but you would have to manually call databind on the control having the same issues as above
Is there no way to iterate a set of controls/child controls and initiate values in code behind withouth having these values added to the viewstate?
With no reactions here and some further research, I am now at the point to conlude that you can indeed NOT iterate a set of controls/childcontrols and initiate values in code behind without having these values added to the viewstate, because of reasons mentioned in the question.
There are some partial solutions however as made clear here: http://weblogs.asp.net/infinitiesloop/archive/2006/08/03/Truly-Understanding-Viewstate.aspx
One possibility is to hook into the init event of the control itself declaratively, you would have to do that for each control. In code behind, inside the event handler you can set the property and since viewstate is not tracking yet it will not be saved in viewstate.
There are more options for different scenario's for example initializing dynamically added child controls inside a custom control by overriding the CreateChildControls method. Definately worth reading the above mentioned 'Truly Understanding Viewstate' link.
I have a checkboxlist and textbox controls on my asp.net page and the are dynamically created and added to the page. When I populate the values and submit the form, the values are empty by the time it hits the server. Any help?
They are empty because they are being re-created too late in the page lifecycle.
Without knowing the precise point in the ASP.NET Page Lifecycle you're adding your controls, (though I'd guess it's either Page_Load or an event handler), it goes something like this:
Build control tree
Add dynamic controls
Render
(Postback)
Build control tree
Reconstitute viewstate & bind to Post values
Add dynamic controls
Render
To solve this, you need to make sure your controls are created early enough in the lifecycle. Standard practice is to break the "control creation" into a separate method, and during CreateChildControls, check to see if they need to be created:
override CreateChildControls()
{
if(IsPostBack)
{
EnsureDynamicControlsAreAdded();
}
}
This way, if they do need to be initially added by something as far late in the lifecycle as an event handler (Button_Click, for example), you can call the same EnsureDynamicControlsAreAdded method from there as well, and on the next round-trip they'll be created much earlier.
Further to Rex M's answer, you could try creating the controls in the "Page_Init" event - this is one of the first events in the page lifecycle, and is where I would normally create controls in a viewstateless page (NB: If you do this, do not surround the content of the Page_Init handler with an "if (!IsPostback)" - this will prevent it from working as intended).
In my ASP.NET application, I am loading an .ascx dynamically using LoadControl, using the following pattern:
var ctrl = LoadControl("/path/to/control.ascx");
((ControlType)ctrl).SomeProperty = someData;
placeholder.Controls.Add(ctrl);
The control that I add saves the SomeProperty property value directly to ViewState, as follows:
public int? SomeProperty
{
get { return (int?)ViewState["SomeProperty"]; }
set { ViewState["SomeProperty"] = value; }
}
After that, the ascx control lives a life on its own and all is well until postback occurs. When the page posts back, suddenly the view state is empty! I suspect this happens because I manipulate the ViewState before I add the instantiated ascx to my page. Also, I can prevent the ViewState from getting lost by adding the following line in the Page_Load() method of my ascx control:
SomeProperty = SomeProperty;
I have to do the above for each and every property to ensure that the ViewState is preserved. Now, is there a prettier way of doing this? Manipulating the ViewState after the instantiated .ascx has been added to the page is not an option - I need the contents of the ViewState in the Page_Init() and Page_Load() methods, which are triggered the instant I add the .ascx to my page.
Thanks.
Take a look at the ASP.NET Page Life Cycle and Understanding View State. View State gets loaded after Initialization, so you won't be able to access it in Page_Init. You'd be better off using a hidden field.
If you are dead set on using View State, the earliest you can get to it would be by overriding the LoadViewState method (Remember to call base.LoadViewState before trying to access it though).
You also need to add the control to the controls collection BEFORE you set the property. ViewState does not get recorded until after it is added to the controls collection.
placeholder.Controls.Add(ctrl);
((ControlType)ctrl).SomeProperty = someData;
Keep track of the ID of the UserControl before postback then on postback re-create the control and assign the ID back and it should automatically load the ViewState back in.
I am adding some user controls dynamically to a PlaceHolder server control. My user control consists of some labels and some textbox controls.
When I submit the form and try to view the contents of the textboxes (within each user control) on the server, they are empty.
When the postback completes, the textboxes have the data that I entered prior to postback. This tells me that the text in the boxes are being retained through ViewState. I just don't know why I can't find them when I'm debugging.
Can someone please tell me why I would not be seeing the data the user entered on the server?
Thanks for any help.
This is based on .NET v1 event sequence, but it should give you the idea:
Initialize (Init event)
Begin Tracking View State (checks if postback)
Load View State (if postback)
Load Postback Data (if postback)
Load (Load event)
Raise Changed Events (if postback)
Raise Postback Events (if postback)
PreRender (PreRender event)
Save View State
Render
Unload (Unload event)
Dispose
As you can see, the loading of ViewState data back to the controls happen before the Load event. So in order for your dynamically-added controls to "retain" those values, they have to be present for the ASP.NET page to reload the values in the first place. You would have to re-create those controls at the Init stage, before Load View State occurs.
I figured out yesterday that you can actually make your app work like normal by loading the control tree right after the loadviewstateevent is fired. if you override the loadviewstate event, call mybase.loadviewstate and then put your own code to regenerate the controls right after it, the values for those controls will be available on page load. In one of my apps I use a viewstate field to hold the ID or the array info that can be used to recreate those controls.
Protected Overrides Sub LoadViewState(ByVal savedState As Object)
MyBase.LoadViewState(savedState)
If IsPostBack Then
CreateMyControls()
End If
End Sub
I believe you'll need to add the UserControl to the PlaceHolder during the Init phase of the page life cycle, in order to get the ViewState to be filled in by the Load phase to read those values. Is this the order in which you're loading those?
Ensure you are defining your dynamic controls at the class level and adding them to the ASP container:
Private dynControl As ASP.MyNamespace_MyControl_ascx
And when you instantiate the control, ensure you call LoadControl so the object is added properly:
dynControl = CType(LoadControl("~/MyNamespace/MyControl/MyControl.ascx"), ASP.MyNamespace_MyControl_ascx)
You have to create your controls in the Page_PreInit event handler. The ASP.NET server control model is tricky; you have to fully understand the page lifecycle to do it right.
As others have said, any form of control manipulation must be done before viewstate is created.
Here is a good link on the page lifecycle to help you out:
http://msdn.microsoft.com/en-us/library/ms178472.aspx
We have experienced the same thing and have handled it by using ghost controls on page_load that have the exact same .ID and then the post back picks up the events and the data. As others said it's the dynamic adding of the control after the init stages that the state is built already and controls added after aren't stored.
Hope this helps a bit.
I also want to add that I've seen user controls work the way that you'd expect them to just by setting the Control.ID property at run time. If you do not set the ID, items may get built in a different order and work oddly.