I am drawing a tableview via BehaviorRelay.
Currently, I am using the code below as a way to add data.
viewModel.user.append(Person(name: "king", phoneNumber: "12341234"))
viewModel.personObservable.accept(viewModel.user)
I wonder if this code changes the user itself so that the whole tableView is redrawn.
If so, what method can I use to change only the data I added?
The code presented causes the personObservable (which is actually a BehaviorRelay apparently,) to emit a next event that contains an entire array of Person values, not just the latest Person added. Importantly, it's not emitting the viewModel.user object (at least not conceptually) but an entirely different object that happens to be equal to viewModel.user.
The default dataSource, the one that you get when you call items with anything other than a DataSource object, will call reloadData on the table view. This doesn't cause "the whole tableView" to be redrawn though, but it will cause the table view to query the data source for all of the visible cells, even if they haven't changed.
If you only want the table view to load the new cell, then the data source object needs to be smart enough to compare the new array with the array it's currently displaying so it can figure out which values are different and add/remove/move cells as appropriate, instead of just calling reloadData. As #Sweeper said in the comments, the RxDataSources library contains a set of data source classes that have that logic built in. If you wanted to reinvent the wheel, just write a class that conforms to both RxTableViewDataSourceType & UITableViewDataSource and implement the diffing yourself.
Related
I have three elements in my UICollectionViewCell:
Two labels with name and price, and a quantity button.
I would like to add the name and the quantity with "didSelectItemAtIndexPath" to a specific collection view in the previous view controller and the price in another view controller. I may achieve this by using a segue to pass data to any view controller of my choice.
Also, at the same time, I want to keep track of the selections I make adding them to a table view below the collection view.
My guess is to create empty arrays for each item.. one for name, price and quantity.
I may be wrong.. and I tried to append my current selection and I know I am missing something.
To pass information to a previous viewController use a delegate protocol. Suppose a is your first viewController, and b is your second viewController.
You can create a delegate protocol in b (here you specify a func you want to execute inside of a)
Set a as the delegate for b
write a function yourDelegateFunction in a , which your delegate protocol will call to update the viewController with your first collection.
in didSelectItemAtIndexPath call delegate.yourDelegateFunction
Here's a video with an easy guide for how this works:
https://www.youtube.com/watch?v=3BcBu30thIA
If you provide a sample of your code, and highlight where you experienced issues, I can provide a more precise solution.
I'm trying to set up a view-based table in Swift using bindings. All of the examples I've seen use a datasource/delegate setup.
I have an array of Flag objects which has two properties - flagName: String and flagImage: NSImage. I have an NSArrayController managing this array.
If I set up a cell-based table, and bind one column to arrangedObjects.flagImage and the other to arrangedObjects.flagName, I get a table displaying images and names, and I can use the array controller's add and remove methods, so there are no problems with my datasource or my array controller.
I have been following the instructions in Apple's TableView Programming Guide to bind my view-based table to my array controller:
tableView Content binding: FlagController.arrangedObjects
textField Value binding: TableCellView.objectValue.flagName
imageView Value binding: TableCellView.objectValue.flagImage
(IBs autocomplete is not happy with the paths for objectValue.flagName respectively flagImage; it doesn't feel that there should be any completion whatsoever and says it can't resolve the path, so it looks as if the problem is with the tableView's content.)
If I do this, my table has a number of rows that corresponds to the number of elements that my array controller is managing at that moment (I have two simple setups, one object vs. 50 objects, so it's clear that something is bound). What I don't get is a display; and selecting a table row does not seem to send back a message to my flagController.
What am I missing? Has anyone been able to make this work? I've had no problems with other bindings in Swift so far, but I'm starting to think that the sudden reappearance of datasource examples is not unrelated to this.
It sounds like you've failed to implement the delegate method -tableView:viewForTableColumn:row:. That's a common cause of blank tables.
You don't actually have to implement that if you make sure the identifier of the table column and the identifier of the table cell view are the same in IB.
Otherwise, the method can just do this:
- (NSView*) tableView:(NSTableView*)tableView viewForTableColumn:(NSTableColumn*)tableColumn row:(NSInteger)row
{
return [tableView makeViewWithIdentifier:#"TheIdentifierOfYourTableCellView" owner:self];
}
(It could also do more, if desired.)
I have a view controller that has a private NSArray variable. The variable is initialised in the viewDidLoad method. A few questions arise for the case when the didReceiveMemoryWarning is called:
Should I set the private variable to nil?
If I set it to nil in what method must it be recreated? Does the view controller call the viewDidLoad method to recreate it?
I'm asking because other methods of the view need this variable and won't work if it's nil.
Thank you!
Typically you unload a private property by assigning nil via the setter (e.g. self.propertyName = nil). Or you could set the ivar to nil after calling release, e.g. [_propertyName release]; _propertyName = nil;, but the former is preferable.
The didReceiveMemoryWarning method is called when there is a low memory situation. It is called on every view controller, including the one(s) responsible for the currently visible UI!
Therefore, you can't just unload data arbitrarily when you get a call to didReceiveMemoryWarning -- the view controller might need that data if it is currently visible on the display.
The general principle is that didReceiveMemoryWarning can get rid of any resources it can to assist freeing up memory, but only those that aren't immediately needed. For example, in an OpenGL game you wouldn't unload textures that are currently visible on the display. However, see my last paragraph.
Typically you reload the resources by checking they are loaded when you need them, and if not, loading them.
It's not worth niling/releasing tiny resources such as a single normally sized string. You should concentrate on items taking up significant amounts of memory.
Recent advances in behind the scenes memory management mean you're less likely to need to actually unload data these days - the operating system can unload and reload uncompressed image data and the like behind the scenes.
As Hot Licks mentions, the simulator has an option for simulating a memory warning. It's worth triggering this memory warning at various points in your app to see how it behaves.
Create custom getter that load data lazily. Something such as this snippet is good for non-mutithreading eviroment:
- (NSArray*) dataArray {
if(_dataArray) return _dataArray;
_dataArray = [self lordata];
return _dataArray;
}
In this way data are always reloaded if you "release" them in memory warnings
The ViewDidLoad method is called only once, when your ViewController is initialized. If you must reload some data to your NSArray, you should call your own methods to do that when needed.
If this array is used by various parts of the code, maybe you should think about redesigning your code structure to avoid a huge concentration of data inside only one object.
Edit: As pointed by #occulus in the comments below, it is not called when the View is initialized, but when the View is loaded by the ViewController.. my mistake
As an example, I had an app that downloaded data to a very long table view (potentially 1000s of records). To support this I implemented a "sparse" array, allowing empty elements that would be "faulted in" over the net when referenced (with a "downloading" indicator in the table cell while downloading).
This was rigged so that when didReceiveMemoryWarning occurred the array would be purged, using a least-recently-used algorithm to delete the oldest N% of the array. Recovery would be automatic -- the emptied cells would reload when they were referenced.
Not that I recommend this specific scheme, but note the general features of having a lot of data, having a way to "prioritize" what should be deleted, and having a "soft" way to reload the data (ideally only reloading the parts that are needed in the near future).
Its better to set your variable to nil. I mean release the memory it is holding in didReceiveMemoryWarning and set a dirty flag.
You can always check the dirty flag in array's getter(you can write your own) and repopulate it. It is probably not the best way. It entirely depends on the array's usage.
How does look the practical usage of IBOutletCollection? Unfortunately Apple documentation mentions it briefly without giving an wider idea of usage. OK, it maintains one-to-many relation with IB, but how to access and use particular objects efficiently? With TagName? How to ensure the order of objects?
I've recently used this to easily initialize a grid of labels. I have a n by n matrix of labels on a view, and reference each one individually (via an IBOutlet) in order to display relevant data. However when the view first loads I wanted to control the default text that displayed in all the labels. Initially i wanted a dash to display, but since this is for a client I wanted it to be easy to change. The view contents have and continue to change over time, per client requests.
Instead of writing N line of code, I created an IBOutletCollection and accomplished the same results in 4 (#property, #synthesize, and for loop). YMMV but I found it very useful in this situation.
Read again this section in the Interface Builder User Guide.
IBOutletCollections are actually just NSArrays you can connect to more than one object in IB. All the objects you connected end up in this array and can be accessed from code like any other object in an array.
I used it to minimize code. I have a range of UIViews that should react on "touch up inside" events of some UIButtons (custom mode).
I gave all UIButtons a tag (lets say 1005 to 1010) and all UIViews the same tag as the UIButton they shall respond to.
Then I connected the UIViews with the collection in Interface Builder. All UIButton touch up events go to the same function in my controller. This function gets the tag of the sender object, iterates through the NSArray list (of "IBOutletCollection(UIView)") and compares the tag. Everytime it hits, the appropriate action is done.
It is a pity that NSArrays seem not to hold the order...
I have a UITableView with style "Grouped" which I use to set some options in my App. I'd like for one of the cells of this UITableView to only show up depending on whether another of this UITableView's cells is activated or not. If it's not, the first cell should show up (preferably with a smooth animation), if it is, the first cell should hide.
I tried returning nil in the appropriate -tableView:cellForRowAtIndexPath: to hide the cell, but that doesn't work and instead throws an exception.
I'm currently stuck and out of ideas how to solve this, so I hope some of you can point me in the right direction.
You should remove the data behind the hidden cells from the table view's data source.
For example, if you are using an array, when an action occurs that causes a cell to be hidden, you would remove the object for that row from the array. Then, as the table view's data source, the array will return one less total count and only return valid cells for every row in that count (no nil).
This approach may require maintaining a second array with all of the objects (including hidden).
To update the view, check out reloadRowsAtIndexPaths:withRowAnimation:.
Here's a handy post in which the author provides some source code for performing animations on the currently selected cell:
http://iphonedevelopment.blogspot.com/2010/01/navigation-based-core-data-application.html
He's using this in a NSFetchedResultsController context, but you can see how he's using various calls to add/remove cells & sections.
Now, in your case, you'll need to modify whatever array you're using to host the data used to generate the rows in your tableView when you "activate" your cell, then selectively use:
tableView:insertRowsAtIndexPaths:withRowAnimation:
tableView:deleteRowsAtIndexPaths:withRowAnimation:
tableView:insertSections:withRowAnimation:
tableView:deleteSections:withRowAnimation:
to adjust things accordingly (you can start with tableView:reloadData:, but it's inefficient).
I realize that the API can be a bit daunting, but take the time to read through it and understand what the various calls do. Understanding how the UITableView uses its datasource and delegate, as well as the chain of events that occur when cells are selected/deleted/etc., is important if you want to get things just right (and crash-free).
[tableView beginUpdates];
[tableView deleteRowsAtIndexPaths:withRowAnimation:]; // or insertRowsAtIndexPaths:withAnimation:
[tableView endUpdates];
Before cellForRowAtIndexPath is called, numberOfRowsInSection is called. You should return the appropriate value of cells in the section there, so if you only want to show 1 cell, return one. The logic what cells are shown has to be implemented partially in both methods