Develop Reference

Demand load web based images on background thread?

I have an application that loads some objects and displays them in a grid. One of those objects is a preview image which could be either a local image file or an image file on a web location. Right now it is working but the performance isn't very good when scrolling because whenever a new item scrolls into view it has to load the preview which takes a second or two.
What I am trying to accomplish is that the box for the preview would show up and start loading the preview on a background thread when it is scrolled into view. When the image is loaded it would notify (via property change event in the view model) and the image would come into place. This way the UI remains responsive and the preview is just blank until it loads and then it shows up. I have the getter for the bound property starting the backgroundWorker if the image isn't already cached, and the worker routine is as follows (_previewCache is the backing property for the bound preview):
void bw_LoadPreview_DoWork(object sender, DoWorkEventArgs e)
{
BitmapImage _tempCache = new BitmapImage(new Uri(PreviewImagePath, UriKind.RelativeOrAbsolute));
if (_tempCache.CanFreeze)
{
_tempCache.Freeze();
Dispatcher.CurrentDispatcher.Invoke((Action)(delegate { _previewCache = _tempCache; }));
}
}
This seems to work pretty well on local files, though it takes a little longer than I would expect for the images to appear, but the UI remains responsive. However when the image is a web path, it always ends up as _tempCache.CanFreeze = false so it skips it and doesn't load. If I try to do the invoke without freezing, it says it can't use the object because it's owned by a different thread.
What am I missing? How do you load a web based image this way? Or is there a better way to approach this issue that I'm not seeing? Any help is greatly appreciated.

WinApp Form loading too much data during the Load event

I have a form that is loading quite an amount of data from SQL server. Below is the code that will provide a good hint:
private void BranchCenter_Load(object sender, EventArgs e) {
//Combo boxes:
LoadCities();
LoadCoordinators();
LoadComputerSystems();
//Actual entity
LoadBranch();
}
private void LoadCities() {
//LINQ2SQL to load data. ~5000 records.
}
private void LoadCoordinators() {
//LINQ2SQL to load data. ~50 records.
}
private void LoadComputerSystems() {
//LINQ2SQL to load data. ~550 records.
}
private void LoadBranch() {
LoadBranchInit();
LoadBranchDetails();
LoadBranchTimings();
LoadBranchServices();
LoadBranchLocumsHistory();
LoadBranchJobs();
LoadBranchNotes();
}
private void LoadBranchInit() {
//LINQ2SQL to load the Branch object based upon the Branch ID
}
private void LoadBranchDetails() {
//LINQ2SQL to load basic branch stuff. 38 fields. Mixed editors.
}
private void LoadBranchTimings() {
//LINQ2SQL to load timings info into 80 date-time controls
}
private void LoadBranchServices() {
//LINQ2SQL to load services offered at branch info into 20 check-boxes controls
}
private void LoadBranchLocumsHistory() {
//LINQ2SQL to load branch history info into grid control. Always increasing # of rows :(
}
private void LoadBranchJobs() {
//LINQ2SQL to load branch jobs info into grid control. Always increasing # of rows :(
}
private void LoadBranchNotes() {
//LINQ2SQL to load branch notes info into grid control
}
The UI is a form with a tab controls. each detail from above goes to a tab page. I need to load and show the form to user as fast as possible. once the form is shown, i need to launch a series of background workers to get the data for each page.
I have been trying to mess with the background worker but unable to understand it's usage. I end up getting message of "different thread attempted to access control on your main thread... or something like that..."
The ideal situation would be to have a progress bar loading the data on every tab and the tab becoming inter-actable once the respective background worker finishes.
Any strategy or advice? Thanks for the read.

If each type of data is displayed on just one page, I would move the code so that each corresponding type of data is loaded the first time each tab page is loaded. That way the data loading work will be distributed over a longer time, and perhaps avoided altogether if the user does not navigate to all tabs during a session.
When loading a page, you could use a BackgroundWorker or any other asynchronous mechanism. In the tab page you can have two panel controls. One that contains the UI for the page, and that has Visible = false when the form is loaded, and one containing a label with a text like "Loading, please wait...". When the data is loaded for the page, and the UI is updated, toggle visibility on the two panels to show the UI. That way the form will still be responsive while the data is loading, and since you load data for only one page at a time, load times should be fairly short.

the ideal solution is that you do not really load data in the form load event.
Let the form load properly and entirely so the UI render completes.
After that you could display a progress bar wherever you want and you could execute the actual data loading, either synchronous or asynchronous.
if you don't need everything loaded at the same time I would also consider loading data only the first time a tab is accessed, so if the user never clicks on the last tab, for example, you didn't load anything for it.

Create a BackgroundWorker for each of the tabs. In the Load event of the form, disable all tabs and call the RunWorkerAsync() method for each of the background workers.
In each DoWork event handler, load the data required for the associated tab page from the database into a data table and set the Result property of the DoWorkEventArgs to the data table.
Note: In the DoWork event handler you should NOT update any UI control since it is operating in a different thread. You should only retrieve the data from the database and set the Result property.
In the RunWorkerCompleted event handlers, you can access the data table, that was retrieved in the DoWork event handler, by getting the Result property of the RunWorkerCompletedEventArgs. Then you can set the properties of the UI controls accordingly and then enable the tab page associated with the current background worker.

The problem here is that the DoWork method of the BackgroundWorker cannot touch the GUI.
It has to use the BackgroundWorker.ReportProgress( int progress, object state) to invoke a method on the GUI thread that is free to update the GUI.
The important point here is that the GUI controls should only be updated from the GUI thread itself, else random exceptions will occur here and there in your program.

You can't mess around with the UI from any other thread apart from the UI thread; that's the source of your errors. If you want more information on how to properly update a UI element from a different thread, you'll need to use Invoke. This question on Stack Overflow might help you out.

Yes, BackgroundWorker is great for this.
I think that you do it wrong because you want to change your form in the DoWork event.
However, you should only collect results of this in e.Results of that event
Then in RunWorkerCompleted you use its e.Results to change the form.

The TabControl already does lazy-loading.
Do yourself a favor and place the contents of each tab on a UserControl.

To address just the problem you are encountering with the background worker thread, it sounds like you may be trying to access UI components from the thread itself. To do this from a non-UI thread you will need to make use of Control.Invoke rather than trying to access the components directly.

Okay, I face a new scenario. I shifted to Visual Studio 2010 and ran into Task.Factory.StartNew(() => myMethod());
This is what I have so far:
private void LoadLocum() {
var TaskInit = Task.Factory.StartNew(() => LoadLocumInit());
TaskInit.Wait();
var TaskDetails = Task.Factory.StartNew(() => LoadLocumDetails());
TaskDetails.Wait();
var TaskQualifications = Task.Factory.StartNew(() => LoadLocumQualifications());
//Enable Qualification TabPage automatically whenever TaskQualifications is complete
Parallel.Invoke(
() => LoadLocumComputerSystems(),
() => LoadLocumOtherInfo(),
() => LoadLocumEmergencyContacts(),
() => LoadLocumDistanceRates(),
() => LoadLocumDocuments(),
() => LoadLocumNotes(),
() => LoadLocumBannedStatus()
);
}
The first two steps (tasks) are critical. Now, the Tab Pages of the Tab are disabled. I need to enable them based upon their related task completion. I can find an event to subscribe to that indicates a certain task is completed or not.

Okay. Massive response in just about no time. Love this place. Jumping from link-2-link, I came face to face with Reactive Extensions for .NET (Rx). Can this be a good alternate as well? Can't find a tutorial on how to use Rx in my scenario.

C# Background Worker UI Update

I am trying to use a background worker in order to retrieve a large amount of data from the database without stalling the main thread. This seems to work well, except that when it comes to update the UI, the update freezes the screen. Relevant code as follows:
private void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
lvwTest.BeginUpdate();
lvwTest.Items.Clear();
// Populate the UI
foreach (TestItem ti in testData)
{
ListViewItem lvi = lvwTest.Items.Add(ti.Value1);
lvi.SubItems.Add(ti.Value2);
}
lvwTest.EndUpdate();
}
The update takes around 2 - 3 seconds, for which time the screen is locked. I realise that only the main thread can update the screen, but is it possible to load this data into memory in some way (in a background thread or another instance of a listview or something) and then just display it? All I want to happen is for the program to simply refresh the data without taking up time in the main thread.

I recommend loading the data into memory and using a virtual mode ListView. That way, you only create the ListViewItem objects as they are needed.

If you have to load really huge amount of data into UI it will require time and it will block our app. The option is smart scrolling or pagination. You load all data but you put it piece by piece upon user request.

Since most of the above are good advice, but don't really solve your immediate problem, here is another approach:
This will update your GUI and keep it responsive. Assuming you are in WinForm App?
Application.DoEvents();
this.Refresh();
Nevertheless, this does not mean that maybe you should not listen to the ideas from above :-)

In addition to virtualization, I would recommend breaking the items into batches of, say, 100 and adding each batch in its own message. That way, the UI has a change to process other messages whilst the batches are being added to the ListView.
In other words, all the RunWorkerCompleted handler does is queue the first batch for adding in a separate message. The adding method will then add the items and then queue the next batch. This will continue until there are not more items left to add. At that point, you would re-enable the relevant portion of your UI (the ListView).

Winforms: How to display a "loading" form?

I have a grid and when a row is double clicked a form is loaded. However a lot of data must be loaded, so I'd like to display a simple form with the text 'loading, please wait..'. And when all loading is finished, the form must disappear.
This is what I have right now, but it doesn't work:
Code that invokes the form with lots of data:
FormWithLotData form = new FormWithLotData();
form.ShowDialog(this);
Constructor of FormWithLotData:
// Show load form
FormIsLoading frm = new FormIsLoading();
_CloseLoadForm closeForm = new _CloseLoadForm(frm.Close);
System.Threading.Thread thread = new System.Threading.Thread(frm.Show);
thread.Start();
InitializeComponent();
this.Visible = false;
LoadAllData();
this.Visible = true;
// Close load form
Invoke(closeForm);
Hope you can help me out.
EDIT:
I'd like to show an animated gif on the loading form.
SOLUTION:
I've created a background worker. The DoWork event handles all the loading and by using the invoke() method I add nodes to the treeview. Now, the GUI doesn't hang and the user don't have the idea that the application is hanging.

You need to reverse your code.
The constructor of FormWithLotData is running in the UI thread. This is the thread that must show your FormIsLoading form. So instead of trying to display this form using the new Thread, do your data loading with it.
The DoEvents way others have suggested is the easiest to implement and (possibly? never done it myself) may work well.
The better pattern to use is to do your data loading on a worker thread. Before you show your FormWithLotData, Begin loading data on a background thread and show your Loading dialog. The method that loads the data should have a callback method into the Loading dialog to signal when it should Close(). Once it closes you can then construct a new FWLD, pass it the already loaded data, and Show it.
Trying to load your data after the form has already been invoked mixes your UI with your data operations, forcing your form to not only be in charge of the UI but also be in charge of data retrieval. Bad for KISS and Single Responsibility, imho.
After your update, it seems like DoEvents is going to be the only real answer to your question, but with some caveats.
You will not be able to show another form MODALLY while you construct your tree. You will still have to do your heavy work within your form's constructor. You will still have to hide your main form and Show() (not ShowDialog) your loading form. You will also have to call DoEvents at every single possible moment while constructing your tree. Its not exactly an elegant solution, but it will probably be your best bet at this point.

how about ...
FormIsLoading frm = new FormIsLoading();
frm.Show();
Application.DoEvents();
// ... load data ...
frm.Close();

In the Form_Load event, add the following:
this.Visible = true;
Application.DoEvents();
before any other processing occurs. The Application.DoEvents caused the UI to show the form at the current state, where normally the UI thread is locked while you other processing is taking place.

Don't do your LoadAllData() directly in a UI thread, instead start up a background thread to do it. The in your Form_Loaded event handler, use an AutoResetEvent and wait till it becomes signalled by the background data retrieving thread. Once it is signalled you can then continue to do whatever you need to do with the data, like bind it into the UI.
This method is still a little clunky for various reasons, but it will get you started.
Edit: i was being lazy with my answer above... a better option is to pass a delegate (callback) to the background thread, when the delegate is invoked (upon completion of the data retrieval) it marshals itself back on to the UI thread, and starts doing the required work with the data.

Inside your form, you can use a Timer control to simulate loading
using ProgressBar when it reaches 100 it unloads the form or any
kind of animation.
for progress-bar code just add the control from the toolbox and then
write the following code:
ProgressBar1.Value = ProgressBar1.Value+1;
if(ProgressBar1.Value == 100)
{
timer1.Enabled = false; this.Hide();
MessageBox.Show("Complete Loading...");
}

Delay loading of combobox when form loads

I've got a Windows Forms (C#) project with multiple comboboxes/listboxes etc that are populated when the form loads.
The problem is that the loading of the comboboxes/listboxes is slow, and since the loading is done when the form is trying to display the entire form isn't shown until all the controls have been populated. This can in some circumstances be 20+ seconds.
Had there been a Form_finished_loaded type of event I could have put my code in there, but I can't find an event that is fired after the form is done drawing the basic controls.
I have one requirement though - the loading has to be done in the main thread (since I get the items from a non-threading friendly COM-application).
I have found one potential solution, but perhaps there is a better way?
I can create a System.Timer.Timer when creating the form, and have the first Tick be called about 1 second later, and then populate the lists from that tick. That gives the form enough time to be displayed before it starts filling the lists.
Does anyone have any other tips on how to delay the loading of the controls?

There is the Shown event that "occurs whenever the form is first displayed.". Also you may want to use the BeginUpdate and EndUpdate functions to make the populating of your combobox faster.

It has that certain smell of workaround, but this approach should fulfil your needs:
private bool _hasInitialized = false;
private void Form1_Shown(object sender, EventArgs e)
{
if (!_hasInitialized)
{
ThreadPool.QueueUserWorkItem(state =>
{
Thread.Sleep(200); // brief sleep to allow the main thread
// to paint the form nicely
this.Invoke((Action)delegate { LoadData(); });
});
}
}
private void LoadData()
{
// do the data loading
_hasInitialized = true;
}
What it does is that it reacts when the form is shown, checks if it has already been initialized before, and if not it spawns a thread that will wait for a brief moment before calling the LoadData method on the main thread. This will allow for the form to get painted properly. The samething could perhaps be achieve by simply calling this.Refresh() but I like the idea of letting the system decide how to do the work.
I would still try to push the data loading onto a worker thread, invoking back on the main thread for populating the UI (if it is at all possible with the COM component).

Can you get your data from a web service that calls the COM component?
That way, you can display empty controls on a Locked form at the start, make Asynchronous calls to get the data, and on return populate the respective combos, and once all of them are loaded, you can unlock the form for the user to use.

You could listen for the VisibleChanged event and the first time it's value is true you put your initialization code.

Isn't FormShown the event you're looking for?

When you say that you cannot use a background thread because of COM what do you mean? I am using many COM components within my apps and running them on background threads.
If you create a new thread as an STAThread you can probably load the ComboBox/ListBox on a Non-UI thread. IIRC the ThreadPool allocates worker threads as MTAThread so you'll need to actually create a thread manually instead of using ThreadPool.QueueUserWorkItem.