I have read several articles about UITableView, including the official doc and some on SO. But my situation seems to be different.
I want to update the Table each time the view loaded. And I must fetch the data using HTTP request.
What I got now is:
When enter the table view, I should use a non-synchronous HTTP request to update the data. Because I don't want the main thread to wait. One place to do that, is in the tableView:cellForRowAtIndexPath: method. So I return 0 for no data exist at the beginning.
When I get the HTTP respond, I update rows on main thread using beginUpdates endUpdates insertRowsAtIndexPaths:withRowAnimation:
And I must update the "Data Source" at the same time, but how to do that?
Or should I make a daemon thread and update my data every once in a while? So that the data will be ready when TableView is loaded.
You would do it like this:
Have a boolean or some variable where you can reliably detect whether you have all the data.
In viewWillAppear, reset everything. Start loading your data.
If you don't have the data yet, you only display one section with one cell (a placeholder cell that reads "Loading..." and shows a spinner, for instance).
Once the data is completely loaded, you set the bool or whatever.
Call [self.tableView reloadData];
In all of your UITableViewDataSource methods you would need to check whether you've got the data already or not. If not, you return the placeholder data.
[yourtablename reloadData]; will help you relaod the data in the tableview, You can call this once you get the response from your server
I'm not sure there's a "best method" for what you're trying to accomplish here. I would suggest trying the method you have, and seeing if it provides an adequate user experience (whatever that means to you) and if it doesn't, try something else. I would definitely suggest having some sort of "loading" indicator while the table is empty and waiting for http response.
In terms of your question about the "data source", the data source of a UITableView is simply an object that implements the UITableViewDataSource protocol which you can read about here. Often times, you will have XCode set up a UITableViewController object which will act as both delegate and data source to your table view. How you actually store your data is up to you. The data source protocol simply provides the methods by which a table view will "ask" for the data it needs to load.
Related
I've been expanding my horizons recently and am trying to start utilizing KVO more in my programming.
I have a view controller in my iPhone application which acts as the datasource and delegate for a UITableView. I also have a singleton model controller which coordinates populating my model with data fetched from the web.
In my view controller, I request that the model's controller load new data from the web. Then I can observe the "dataset" property of the singleton and receive KVO notifications when items are added to or removed from the set.
Now, each cell in my table view has an indicator which specifies whether the content in that cell has been read or not (like the blue "unread" dot in mail). Like mail, when a row is selected, I'll display details about that row. In the viewDidLoad for the detail view, I set the object's "read" property to YES. I would like the original view controller to be able to observe this "read" property of each object in the dataset, so that [tableView reloadData] can automatically be called as necessary and redraw the cells without the blue dot.
In researching this, I found the following link: http://homepage.mac.com/mmalc/CocoaExamples/controllers.html#observingACollection
According to this, it looks like I will do the following:
1) Be an observer of the array
2) Whenever I get a notification of a change to the array, I add (or remove) myself as an observer for the individual properties I am interested in.
3) When I get a notification of a change to the property I'm interested in, I can call [tableView reloadData]
I'm currently in the process of attempting to implement this approach. Can anyone with experience doing this offer some advice on this approach? If this the best way to handle this type of situation?
If this is the correct approach, would anyone be willing to share their implementation of adding/removing the observers for objects in the collection when the collection changes?
Thanks!
I think you can accomplish this task by using Core Data and the Fetched Results Controller.
I'm sure this can save you a lot of work.
Here's a good guide: Ray Wenderlich Core Data Tutorial, getting started
I am processing several large RSS feeds and displaying results in a TableView. The processing starts after the user clicks on the relevant tab. All works well, but as it takes a couple of seconds to process, the NIB and Table don't load until the processing finishes and it looks like the iPhone has seized up. I have added an Activity indicator to the NIB, but because it doesn't load until the table is ready to display, it appears too late to be of any use.
Does anyone have any ideas how to display a message to a user while the table builds/loads? I have tried loading a UIView first and adding the Table as a subview but, again, both seem to load only after the table is ready.
Guidance appreciated.
It's kind of hard to guess what's going on from your description but it looks like your calls aren't asynchronous. Here's what you should be doing in your code:
Make all calls asynchronous. You said your phone is seizing up. Makes it sound like your requests and responses are happening on the main thread. There are many libraries you could use to handle asynchronous calls. ASIHTTPRequest for one example....
Don't wait for the data to come in before displaying the tableView. It's a design principle that you load as much of the UI as possible so that the user has something to look at while your data loads up in the background. What you should be doing is initializing an NSMutableArray to hold the data. Initially this array will contain no objects. This is the array that you use in your data source methods: Use array size for numberOfRowsInSection and use the array objects in cellForRowAtIndexPath. Once your RSS feed XML comes in and is parsed, store that in your arrays and call [tableView reloadData]. This way you don't leave your users looking at a blank screen. (Another good practice is when the array size is zero, show one cell in your tableview that says "data is loading" or something).
When you first initialize and load up your table and then fire off those RSS feed requests, that's where you show an activity indicator view on the tableView. Stop animating the indicator when the RSS data comes in and your tableView reloads.
These are the standard steps you should follow while showing non local data in a tableview. Makes for a smooth user experience.
Like I said before, it seems from your question that your calls are not asynchronous. If I'm wrong, correct me and let's take it from there...
In my UIView I've got a UITableView (UITV) which is controlled by an NSFetchedResultsController (NSFRC). The UIView is inside a UINavigationController.
When the view is about to be loaded/displayed I start some background activities which fetch data from a remote server (JSON) and parse into Core Data.
The NSFRC is being called when the parsing is done and the threaded NSManagedObjectContext have been merged into the main context.
The problem is that sometimes many rows are being inserted to Core Data at once, a lot of table cells are being added and there is quite a delay from that the actual fetching and parsing is done, until the rows are being displayed.
Now I wonder if anyone knows of any solution to, for example:
hook up a spinner to some "fetched results controller inserted all its rows for this time" (or something) notification/delegate call to at least tell the user that "something is going to show up soon"?
Or might the best solution simply be to not initialize the NSFRC until the background fetching and processing is completed?
Thanks!
If I understand your question correctly, you may want to look into the NSFetchedResultsControllerDelegate methods, with documentation available here: http://developer.apple.com/library/ios/#documentation/CoreData/Reference/NSFetchedResultsControllerDelegate_Protocol/Reference/Reference.html
There are delegate methods available for pre changes with controllerWillChangeContent:, post changes with controllerDidChangeContent and during changes with didChangeSection: and didChangeObject.
I hope it helps!
Rog
I am trying to load a table view from a cache very quickly and have the cached data in the table view appear. Then I want download new data, and then reload the table. Right now I am downloading the new data on viewDidAppear, but the view still refreshes before it displays. Any idea how I can do this?
viewDidAppear is not a good place to download data; it is really intended for clean up after presenting data, so I can understand why you used it. You should request your data reload as early as possible, such as in viewDidLoad or viewWillAppear (depending on your reuse or otherwise of view controllers).
If you are doing asynchronous downloads, which you should be, put the reloadData call in your delegate callback function for when the data is completed.
Simply calling [tableView reloadData] after the download might do the trick. This will trigger a refresh of your table cells.
To download the new data, you may consider using Cocoa Streams, particular an asynchronous Socket Stream. In the stream delegate, call reloadData when the download is completed.
I ended up implementing the delegate class to do this asynchronously. This example was extremely helpful, and I implemented much of its code:
http://developer.apple.com/iphone/library/samplecode/URLCache/Introduction/Intro.html#//apple_ref/doc/uid/DTS40008061-Intro-DontLinkElementID_2
I have been solving the Stanford free iPhone course project called Presence 3 (found on the stanford site: www.stanford.edu/class/cs193p/cgi-bin/downloads.php, which pulls data from twitter for users, which are stored in a plist. A UIActivityIndicator (spinner) is visible while the data is loading. Once the data has been loaded, a TableView displays the users in a list with their photos, and user statuses show up when a user's cell is clicked. I can successfully display the TableView with the photos and bring up another view controller with statuses when clicked. But when I add in the spinner, my program crashes. I set up my program almost identically to the ThreadedFlickrTableView example project, which can also be found at the same link above (sorry, I'm a new user and can only post one link), which works. I put breakpoints in my code to see where the problem was, and I found that the program crashes when it is loading a cell in the cellForRowAtIndexPath method, specifically when it is retrieving the photo from the appropriate array (followeesPhotoURLs). This is because the array is empty - the photos were never downloaded since the main thread decides to execute the cell-loading method before the thread dedicated to downloading from the internet finishes executing (it does start executing).
I looked on the auditors discussion group page for the course and found that someone else had the same problem, but the thread never resolved the issue, and I emailed to no avail:
http://groups.google.com/group/iphone-appdev-auditors/browse_thread/thread/ccfc6ae99b4cf45d/ef1b8935e749c7c2?hl=en&lnk=gst&q=presence3#ef1b8935e749c7c2
My first rule of UITableView is never report sections or rows that aren't ready (with something, even if only a placeholder) because it will crash every time.
The spinner is spinning while the resource is loading. So you are waiting for a resource that may or may not be ready because you don't know the exact state of your background process. How about setting a value in your main thread indicating that things are not ready. Then when your secondary thread finishes loading things you can do performSelectorOnMainThread to cause some main thread function to set the value to indicate you can proceed. Until the value says proceed, your main thread does not try to access those values the secondary thread might be touching. Maybe your cells will display "loading" or similar until the data is ready, or you will just add cells as they become ready.
One more thing - ONLY the main thread can touch the UI. Nothing in UIKit is thread safe unless explicitly stated. The changes to the progress indicator must be handled by the main thread, it should start the indicator and stop it (probably when your secondary thread notifies "done", as above).
I just finished working through this today and here's the sequence of events that I observed:
Your table view will load up with nothing because your arrays contain nothing
Your thread will go and retrieve Twitter data
Your table view will be refreshed with the Twitter data [self.tableView reloadData]
If you're like me, you try to set the user name using something similar to
cell.textLabel.text = [[userInfo objectAtIndex:indexPath.row] valueForKey:#"name"];
I think that because it is very specific, the application really tries to look for it and doesn't return null if it doesn't find anything, unlike the code in the example which is
cell.text = [photoNames objectAtIndex:indexPath.row];
...so your application errors out the first time it tries to load data with empty arrays.
The way I worked around this is to create an array that loads the user names from the property list at the very beginning so I know how many entries there should be in my array of content. The key part is to create a condition before setting up your cell so that you know you have all the information that you need such as...
// Set up the cell...
if ([userInfo count] == [userList count]) {
userInfo is my array of dictionaries with the data that comes from Twitter.
userList is my array of values from the property list.