If I set a StackPanel or what ever bound area I have as 'Collapsed', does the data load?
Will it trigger its Loaded event?
Yes:
Your data bindings will be evaluated,
Your Initialized events will fire, and
Your Loaded events will fire
But in the section of the visual tree under the Visibility="Collapsed" element:
Your controls will not be measured or arranged
Your controls' Templates will not be applied
So the bottom line is, if you want to avoid loading data for invisible sections of your UI, don't load your data and set your DataContext until after the control is first measured. Also consider putting any complexity inside a template.
Related
For efficiency, it appears WPF only loads visual elements when necessary. But what if you need them to load when their parent loads?
For example, I have a split-button control with a drop-down CheckListBox. Each CheckListBox item is bound to an IsSelected property and a Select command. Changing IsSelected in the ViewModel has the same effect as clicking a row in the control, but only once the drop-down portion of the control is loaded. Before that the select command doesn't react to changes in the IsSelected property (presumably) because the binding and command don't yet exist. Is there a clean way to force the split-button's contents to load at the same time it's parent loads, or otherwise connect it's bindings at initialization rather than when first entering the visual tree?
I am creating a resource-intensive dashboard application that will have many areas of data visualization. I am thinking that it would be best to use a frame and load the pages needed one at a time using WPF pages. These pages will also have different data contexts, security restrictions, etc. But, another developer says I can accomplish the same thing using a TabControl.
Does a TabControl load all the items in all the tabs at once, on application startup? Or, can I load them lazily as needed like with WPF pages (page only loads content when navigated to)? Also, can you have different data contexts per each item in a TabControl?
In WPF you can use UI Virtualization which means that only the visible controls are initialized and rendered. As far as I know, the TabControl does not support UI Virtualization by default but maybe you can add it manually or use another control. Maybe you want to have a look at the following article which presents some performance tips. There is also mentioned that there is a difference between UI and Data Virtualization. Not showing the controls does not mean that the underlying data are not in memory. All your binding targets will be loaded, but the controls will not be rendered.
To your second question: Yes, every TabItem can have its own DataContext. If you use a TabControls ItemsSource to bind a list of items, the DataContext for every TabItem will be one item of the list. If you manually add TabItems, you can set the DataContext like that:
<TabControl>
<TabItem DataContext="{Binding Context1}" />
<TabItem DataContext="{Binding Context2}" />
</TabControl>
It is more complex than you would guess. If you bind to Tab Collection (think MVVM) then the tab only get created when it is selected. And with a Collection if you leave a tab and come back it gets built AGAIN. If you create the tabs in XAML then the tabs are all built when the windows loads. Yes you can have different DataContext for each tab. What I do for lazy loads is bind to the TabItem property IsSelected and if it is false all the Properties in the class just return a (fast) static type compliant value. If IsSelected is changed to true then I load the real values and call NotifyPropertyChanged (and I save the real values).
I use the heck out of this were I load a big objects and one tab is a summary. Tabs do not virtualize but if you have big lists then for sure use virtualization in the tab. You can use BackgroundWorker to create properites but once it returns and you bind that returned value the UI locked until the UI control is rendered. For me reuse of a single frame versus tabs is a UI thing. Just to break up code I typically load a tab with a frame and a page (and I typically pass data to the page in the ctor to load dynamic content).
A while I go, I made a demo application with Expression Blend.
My first screen is a big selections of Buttons, so when user click on any of button, it goes to the MainView.
Then in the MainView, I have a list of Menu items that user can click and shows up its corresponing DisplayView. (Appointment Menu Item will shows up AppointmentView etc).
Everything is good, I can click the MenuItem, the Views shows up with animation and transition effects.
But the thing is, with creating in Expression Blend, the MainView, Menu, AppointmentView etc every thing is predefined in the XAML. So when user load the first screen has to load everything into memory.
Now thinking of it, shouldn't the MainView etc be dynamically add into the screen?
How do I do it with Expression Blend? Or the only way to do is just....do it in code-behind myself (writting StoryBoard etc for the dynamic add/remove controls?)
If there is any example/tutorial of doing it, it will be great.
I guess you have very limited possibilities to conditionally load or unload controls exclusively in Blend without writing code-behind.
In general an opening tag in XAML is equivalent to a parameter-less constructor of some class object. As soon as you write the tags your are instantiating an object but that doesn't mean that it's visual appearance is loaded into memory. This only happens when the control is actually shown on the screen.
In my opinion the leanest way to control the appearance of some control is to use a single-child control. Take a Border control for example and add the user control you want to conditionally load to its child property, so you can decide for example whether to load or unload a control.
But unfortunately I think you have to do this in code as well. Take this easy code snippet:
// either instantiate in code or use from markuup
Border myBorder = new Border();
// the control you want to conditionally appear and disappear
UserControl myUserControl = new UserControl();
myBorder.Child.Add(myUserControl);
Of course a much more sophisticated approach is to use Grids. Here you have to use attached properties to add or remove child elements:
// either instantiate in code or use from markuup
Grid myGrid = new Grid();
// the control you want to conditionally appear and disappear
UserControl myUserControl = new UserControl();
// set the target position inside the Grid via the Grids attached properties
Grid.setRow(myUserControl, 1);
Grid.setColumn(myUserControl, 0);
// actually add the control
Grid.Children.Add(myUserControl);
Although I am pretty sure you were aware of all of that I am hoping it helped a bit :)
I have just discovered that UserControls in a TabControl will not load until the parent TabPage is selected. Is there a way to prevent this delay loading? I need to initialize the UserControls when the main form loads.
The TabControl does not treat its controls specially, in fact it is normal under all circumstances for the Load event on a UserControl to occur immediately before the control is displayed for the first time. The TabPage is responsible for showing the control, therefore it will only be 'loaded' when first selected.
To overcome this (perfectly normal) Windows Forms behaviour, you could move your initialisation code to a separate method and call it when the Form loads, or you could just place your initialisation code in the UserControl's constructor instead. Either way, you can perform your initialisation immediately.
You can invoke Tabcontrol's SelectTab() method for the tabs in your Form's Load event handler.
I was just searching how to achieve this default behaviour you describe. An application I support was not delaying the load of the tabs. Turns out the tabs were getting initialised in the load event instead of the constructor.
So if you add the tabs to the tabcontrol in the form load event all the controls in tabs will have their load events fired as a part of the TabPages.AddRange call
I have a custom text box control which raises a routed event when its TEXT property changes. This text property is data bound to a property on our view-model object.
When I place this control on a TabControl page or Expander control, it appears as if data binding only occurs when the control becomes visible for the first time, therefore I never receive any of the routed events until I swap to the tab the control is on or expand the expander.
Is there any way I can force data binding to occur before the control is shown?
Sounds like you relying on the data binding to genreate the routed event is the wrong approach. Instead you need to have your Model or ViewModel generate an event when the text is modified and then you watch this event from an appropriate place in your View.
Not very likely. WPF is a fairly efficient framework and won't do any work that it doesn't absolutely have to. This includes scenarios like data binding. Why bother exercising a collection for a control that might not ever be shown?