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CHAPTER 6 ASP.NET STATE MANAGEMENT
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Then, as the final step in the page lifecycle, comes the FriendsBase.Render() method, which besides repositioning controls as you ve seen, checks the status of the cookies, both from Response and Request. Based on that, it creates an enclosing <div> element, where it places the whole form, to get the background color displayed: Page.Controls.Remove(form) Page.Controls.AddAt(1, div) div.Controls.Add(form) The process for relocating the form is exactly the same as you used to position the other controls. Because all this code is placed in the base class for all your pages, they all gain this feature immediately. The home page (Default.aspx) might look like Figure 6-13 after you changed the color preference and expanded the preferences panel at the bottom.
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Figure 6-13. The Default page with color preference choices
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CHAPTER 6 ASP.NET STATE MANAGEMENT
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Caution You can reposition controls on the page collection only during the Rendering phase, when the viewstate for all of them has already been saved. If you do so earlier in the page lifecycle, the viewstate may stop working, as well as the firing of events that depend on it.
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If you restart your browser right now, you ll lose your preference. That s because the cookie expires with the browser session by default. Let s fix this now. Try It Out: Survive Browser Restarts In order to change the default behavior, so the cookie is retained after the browser session ends, add the following line to the FriendsBase.Render() method: div.Controls.Add(form) Response.Cookies("backcolor").Expires = DateTime.Now.AddYears(1) How It Works You have set the expiration for the cookie that holds the color preference to one year from the current time on the server. You should avoid setting the expiration to a higher value, because some browsers will distrust it and may ignore it altogether.
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Passing Data with Query Strings
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A query string is everything appended to a URL after the name of the page plus a question mark, such as in the following: http://localhost/ViewUser.aspx UserID=99 Some developers consider that passing data in the query string between pages is a way to keep state, too. You actually have used this form of state in your work with the Friends Reunion application; for example, you used it when you provided a link to view details about a user in News.aspx, which rendered very similarly to the preceding example to pass the user ID to the ViewUser.aspx page. It comes in especially handy when you use it in conjunction with data binding, as you did. You retrieved this value simply with: userID = Request.QueryString("UserID") Note that the key used is the value before the =, and the value you get is the string following it. The QueryString property is of type NameValueCollection, containing all the key/value pairs found in the URL. If multiple values are needed, they must be separated by an & sign.
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Caution Remember to always check input parameters you receive before using them, to ensure they
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are valid and within the expected values.
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CHAPTER 6 ASP.NET STATE MANAGEMENT
Except for this and similar cases where small sets of data need to be passed, this mechanism is not recommended. HTTP 1.0 web servers usually don t handle more than 255 characters in a query string, nor do older browsers. HTTP 1.1 solved this, but older browsers still won t work.
Passing Data with Hidden Form Fields
Using hidden form fields to pass data between the client and the server, as well as between pages, is also possible. You can simply add an HTML input field to the form, set its type to hidden, and then set values on it, either on the client side using a script or on the server. This is an example of a hidden field: <body ms_positioning="FlowLayout"> <form name="Default" method="post" action="Default.aspx" id="Default"> <input type="hidden" name="myField" value="myValue" /> On the client side, you can easily set the value of the field using JavaScript, ready to be passed to the server: document.getElementById(myField).value = "the value to pass!!"; You can get the values in a hidden form field using the Request.Form property on the server side: string value = Request.Form("myField") You already know that the viewstate uses this technique, so you can easily imagine your own applications for it, and they don t necessarily need to be simple, as you can see! This concludes our journey through the exciting ASP .NET state management features.
Summary
Equally important as dynamic features for web applications is the ability to retain state in its different forms. ASP .NET offers a wide range of possibilities, filling all the gaps of the past and creating new and improved state-handling approaches, such as the viewstate and transient state. We have covered each of the features it offers. We also discussed the performance and scalability trade-offs among them, and offered some hints on where to use each one. We ve taken advantage of the session state to allow the user to perform refining searches for fellow users. We have taken into account scalability problems that may arise and placed a limit on maximum search results, and also discussed different storage locations for this state data. We then used the viewstate to further increase our application s responsiveness by reducing the HTML payload as a result of enabling it only when needed. We also reduced server-side processing by avoiding reloading data, letting the viewstate take care of reconstructing UI elements, such as combo boxes. We then moved to more advanced uses of the viewstate as a store for arbitrary data, and added the possibility to select users from the search results and stored those selections in the viewstate. Next, we learned about a new feature available in ASP .NET, transient state, which allowed us to pass this selection data between pages without consuming server resources.
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