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The Overloaded CreateChildControls
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The overloaded version of CreateChildControls that is defined on the CompositeDataBoundControl class is shown here:
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protected abstract int CreateChildControls( IEnumerable dataSource, bool dataBinding);
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The first parameter is the collection of bound data. The second parameter indicates whether the control is being bound to fresh data (that is, it is working in binding mode)
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or is being refreshed after a postback. The return value indicates the number of items added to the control tree. This value will then be stored in the view state during the call to PerformDataBinding. The following code snippet shows an excerpt from the source code of PerformDataBinding on the CompositeDataBoundControl class:
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protected internal override void PerformDataBinding(IEnumerable data) { base.PerformDataBinding(data); Controls.Clear(); base.ClearChildViewState(); TrackViewState(); int numOfItems = CreateChildControls(data, true); base.ChildControlsCreated = true; ViewState["_!ItemCount"] = numOfItems; }
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Note that PerformDataBinding calls into the new overload of CreateChildControls and passes it true as the second argument, indicating that a binding operation is taking place. This makes sense because executing PerformDataBinding, by definition, means you are performing a binding operation. What kind of code should you place in the overloaded CreateChildControls Basically, you call your own control builder method (typically, CreateControlHierarchy) and return its return value. I ll return to this point later when discussing the sample BarChart control. The overloaded CreateChildControls method is invoked in binding mode from within PerformDataBinding, and it s invoked in nonbinding mode from within the other CreateChildControls method:
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// o is the value read from ViewState int numOfItems = (int) o; object[] items = new object[numOfItems]; CreateChildControls(items, false);
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In this case, the bound data passed to the method is an empty array of objects of a well-known size. The goal of this array is to force the control builder method (typically, CreateControlHierarchy) to loop the right number of times and build an outermost container with the right configuration for example, a table with the right number of rows and columns. As you ll see in detail for the sample BarChart control, a composite data-bound control neatly separates hierarchy from data. If the Boolean parameter of CreateChildControls is false, no data is added to the hierarchy. How can the control show up as it did the last time The ASP.NET postback mechanism guarantees that child controls are restored with all their values. In other words, if a composite data-bound control displays bound data through, say, a Label control, after a postback the composite control doesn t restore its bound data directly. However, it asks any child control, including the Label, to restore itself from the view state. In doing so, the Label restores the bound data from its Text property.
12 Custom Controls
The bottom line is that the amount of extra data that flows in the view state for a composite control is limited to the number of constituent items, and the control refreshes correctly after a postback. (Of course, child controls put in the view state the usual amount of data.)
The Control Item
It should be clear from the previous discussion that the ASP.NET team had excellent arguments to dictate that composite data-bound controls get their data exclusively from the data-binding process. This fact eliminates the need of having a kind of Items property on composite data-bound controls that works like the Items property of list controls. This said, feel free to add support for data item objects and collections to your composite controls if you need to. Most composite controls feature a collection of items, but not a collection of data items. Each item represents a control item that is, a logical building block of the control s user interface. For a GridView, it is a GridViewRow object that represents a table row. For a sample BarChart control that displays a bar chart, the control item will be a class derived from TableRow that contains all the information needed to handle a single bar. The number of items that composite controls store in the view state is exactly the number of control items. Let s see how these concepts apply to a sample composite data-bound control such as BarChart.
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