My application builds its user interface and a core data model through code (no .xib .nib or .xcdatamodel files).
I'm having trouble finding documentation on how to bind core data entity attributes (in an NSManagedObject) with the object properties of a UIView or UIViewControler such that the two are kept synchronized with each other.
I could simply write code to move data between Core Data and the UI, then trap all events for field changes, but there are hints everywhere that this can be handled automatically using Key-Value mapping somehow.
Can anyone point me in the right direction or show me some sample code on how this binding is achieved through code?
Much appreciated.
Chris.
Bindings are not available on iOS. Instead there is the NSFetchedResultController.
Related
I am learning Objective-C Cocoa programming for OS X, and object-based programming in general, so I am a big novice here, so my question is a bit general and my guess is the answer to this is simply "experience"; however, I am curious if there is some route of knowledge to understanding what methods in various classes are best or perhaps required for getting tasks done.
For example, in a programming guide I am instructed to create a document-based program, and the document class contains an array to store data, with the following method bound to a button to create a new entry in the array:
- (IBAction)insertItem:(id)sender {
if (!theItems) {
theItems = [NSMutableArray array];
}
[theItems addObject:#"Double-click to edit."];
[theTableView reloadData];
[self updateChangeCount:NSChangeDone];
}
The array is "theItems" and its data is being presented in a TableView object. I understand that the steps here add a new string to the array and then refresh the table to display it, followed by setting the document to be set to an unsaved state.
What I am not getting is how one would know these specific steps and methods are required. Intuitively it seems one would just add items to the array, and that would be all that's required to have the new values simply show up in the table view for which the array is the data source, so how would one know that the tableView would need to be refreshed with the "reloadData" call? I can see someone (myself) figuring it out by trial and error, but is there some quick resource or guide (ie, some quick flow-chart) either in XCode or elsewhere that indicates for a table view that this would have to be a required action to display the new entry?
If I look at Apple's NSTableView class reference, it claims in the overview that you "modify the values in the data source and allow the changes to be reflected in the table view" which suggest the view is updated automatically, so the requirement to call "reloadData" on the view seems a little obscure.
Look for the guides. In the online class reference for NSTableView, there's a section at the top called "Companion Guides". For NSTableView, it lists the Table View Programming Guide for Mac. (In the prerelease 10.10 docs, the guides are listed under Related Documentation in the left-hand sidebar.)
I could have sworn this same information was available in Xcode's Documentation window, albeit somewhat hidden behind a "More related items" pseudo-link, but when I check right now there's no link to the guide anywhere in the NSTableView class reference. Which is a terrible oversight.
You can also browse or search the Guides section of the developer library.
Familiarity, studying the documentation and possibly reading some good books is the answer. For example, in the docs you quoted (emphasis mine)
you should modify the values in the data source and allow the changes to be reflected in the table view
You should do both these things. If you want it to happen "automatically", look into bindings, which uses several other Cocoa features you won't understand at this point either to do the table data source stuff for you. I'd recommend understanding what is happening manually before handing over control to bindings, so you have some chance of understanding when things go wrong.
As well as looking at the table view documentation, you also need to study the cell, delegate and datasource references. All of those objects work together to give you the functioning table view.
I have to check a frame window with RFT which is written with a .net framework. My problem is, after adding the frame as Testobject via drag and drop to the script it works fine. But after restarting RFT, it isnt able to recognize that frame any more, neither with find method or the highlight function for objects.
I read that there is a way to add objects to the proxys. But this frame is declared at proxy .Net.FormProxy and this proxy is existing in the file rational_ft.rftcust as
<Obj L=".Proxy">
<ClassName>Rational.Test.Ft.Domain.Net.FormProxy</ClassName>
<Replaces/>
<UsedBy>[System.Windows.Forms]System.Windows.Forms.Form</UsedBy>
</Obj>
I dont get what is the problem. Especially, why is it working some times but not always.
Thx for help..
The problem you have mentioned may happen for the following reasons.
The object recognition is indeed changing . And it is not necessarily the object that you are having problem with has object recognition issue , but an object in the parent heirarchy of this object (unless this object is the top level object).
The second reason could be the application is not getting enabled during playback time and you can try getRootTestObject().enableForTesting(/) API to force enable the application.
On a recorded Form object in the Object Map try to use the "update recognition properties" to see if there is change in recognition properties in actual vs recorded object.
You can also just test a different simple Form Application to see if the issue is related to your application or if it is general issue in your environment( I suspect it to be related to the application).
I am implementing a view model that is shared by applications on multiple platforms. I am using MvvmCross v3 that has its own MvxEventToCommand class, but I believe the challenge is the same for other frameworks like MVVM Light. As long as the event is used without parameters, the implementation is straightworward, and this is the case for simple interactions like tapping the control.
But when the command needs to handle event arguments things become more complicated. For example, the view model needs to act on certain scroll bar changes (and load more items in the associated list view). Here is the example of XAML:
<cmd:EventToCommand
Command="{Binding ScrollChanged}"
CommandParameter="{Binding EventArgs}" />
(MvvmCross uses MvxEventToCommand, but the principle is the same).
Then in my model I can have the following command handler:
public ICommand ScrollChanged
{
get
{
return new RelayCommand<ScrollChangedEventArgs>(e =>
{
MessageBox.Show("Change!");
});
}
}
(MvxCommand in MvvmCross).
The problem is that ScrollChangedEventArgs is platform specific and this code simply will not compile in a portable class library. This is a general problem with any command that needs not only a push when an event was fired but requires more specific event details. Moving this code in platform-specific part is silly because it more or less kills the concept of portable view models and code-behind-free views. I tried to search for projects that share view models between different platforms, but they all use simple events like "Tap" with no attached event details.
UPDATE 1 I agree with Stuart's remark that view models should only deal with higher level abstractions, so I will rephrase the original concern: how to map results of low-level interactions to a platform-neutral event that triggers a business logic command? Consider the example above: the business logic command is "load more items in a list", i.e. we deal with a list virtualization where a limited number of items from a large collection are loaded initially, and scrolling down to a bottom of a list should cause additional items to be loaded.
WinRT can take care of list virtualization by using observable collections that support ISupportIncrementalLoading interface. The runtime detects this capability and automatically requests extra items from a respective service when the user scrolls down the list. On other platforms this feature should be implemented manually and I can't find any other way than reacting on ScrollViewer ScrollChanged event. I can see then two further options:
Place OnScrollChanged handler in a code-behind file and call the portable view model higher level event (such as "OnItemsRequested");
Avoid code-behind stuff and struggle to wire the ScrollChanged event directly to a view model, then we will need to remap the platform-specific event first.
As long as there is no support for second option, putting event handler in code-behind file is OK as long as it is done for the sole purpose of event mapping. But I would like to investigate what can be done using the second option. MvvmCross has MapCommandParameter class which seems to be able to help, so I wonder if I should exploit that one.
UPDATE 2 I tried MapCommandParameter approach, and it worked allowing me to insert a platform-specific adapter that would map low-level events to view model-specific commands. So the second option worked without any struggle. Stuart also suggested listview-subclassing so there is no need to care about scrolling events. I plan to play with it later.
I agree that viewmodel commands should normally be expressed in terms of viewmodel concepts - so it would be 'strange' to send the viewmodel a command about the scrollbar value changing, but it might be ok to send the viewmodel a command about the user selecting certain list elements to be visible (which she does via scrolling)
One example where I've done this type of thing previously is in list selection.
I originally did this across multiple platforms using a cross-platform eventargs object -
https://github.com/slodge/MvvmCross/blob/vnext/Cirrious/Cirrious.MvvmCross/Commands/MvxSimpleSelectionChangedEventArgs.cs
this was then used on WindowsPhone (for example) via an EventToCommand class like https://github.com/slodge/MvvmCross/blob/vnext/Cirrious/Cirrious.MvvmCross.WindowsPhone/Commands/MvxSelectionChangedEventToCommand.cs
However... I have to admit that this code hasn't been used much... For list selection we have instead mainly used selecteditem binding, and there simply haven't been any apps that have needed more complex parameterized commands (so far) - you might even need to go back to very old v1 mvvmcross code to find any samples that use it.
In .NET I just do something like DataForm.Source = Object and then magic happens. Platform routes data changes from ui fileds to object properties, does validation and so on. Can I do something similar with Cocoa Touch and CoreData objects?
The closest thing in Cocoa is 'Key-Value Observing'. In the desktop Cocoa framework you can use bindings to hook user interface elements up to underlying objects so that changes in the objects or UI elements are reflected in the other.
Whilst Cocoa on iOS doesn't have this sort of UI bindings, you can still use 'Key-Value Observing' to synchronise changes in the data model with UI elements as described here:
http://developer.apple.com/library/iOS/#documentation/General/Conceptual/Devpedia-CocoaApp/KVO.html
I wrote a little open-source library that provides some simple data-binding functionality. It's basically just a wrapper around key-value observing (KVO).
http://github.com/kristopherjohnson/KJSimpleBinding
There are a few other similar libraries on GitHub:
http://github.com/dewind/KeyPathBindings
http://github.com/jonsterling/Observe
http://github.com/mruegenberg/objc-simple-bindings
http://github.com/zeasy/EasyBinding
Probably should also mention Github's Reactive Cocoa, a framework for composing and transforming sequences of values, an objective-C version of .NET's Reactive Extensions (Rx).
Binding mechanics can be done really simple (from the sample):
// RACObserve(self, username) creates a new RACSignal that sends a new value
// whenever the username changes. -subscribeNext: will execute the block
// whenever the signal sends a value.
[RACObserve(self, username) subscribeNext:^(NSString *newName) {
NSLog(#"%#", newName);
}];
Don't forget NSFetchedResultsController.
Not a full blown data bound controller, but makes table views a lot easier to use with Core Data.
If you're using Swift, check out Bond framework: https://github.com/ReactiveKit/Bond
Binding is as simple as:
textField.reactive.text.bind(to: label.reactive.text)
It plays well with functional:
textField.reactive.text
.map { "Hi " + $0 }
.bind(to: label.reactive.text)
And provides simple observations:
textField.reactive.text
.observeNext { text in
print(text)
}
STV (http://sensiblecocoa.com) is a framework that can do that within tableviews
I use CoreDataTableViewController from the Stanford University for my TableViewControllers. It hides a lot of details that you would normally implement in your TableViewController.
Googling for CoreDataTableViewController.h and .m will help you on the road. There are versions from several courses available, the latest does ARC, the old ones don't.
For syncing labels and edit fields with an NSManagedObject, I am still looking for a good solution.
Yes, there is a data binding framework that integrates well into Interface Builder and requires only minimal code overhead (if at all).
Take a look at https://github.com/mutech/aka-ios-beacon
EDIT:
You can for example bind a table view to a fetched results controller simply by setting the data source binding property of the table view in interface builder to:
[ yourResultsController ] { defaultCellMapping: "YourCellId" }
And the only thing you have to do is to define a property yourResultsController in your view controller.
The wiki provides a rather complete documentation and a lot of example use cases.
I just started playing with Core Data.
I created an entity called Task with a property called Description. I opened Interface Builder and I added Core Data Entity view.
Picked my entity, property and tried to build the application. After clicking on "Add" button it crashed with EXC_BAD_ACCESS.
After I've renamed this attribute to 'desc' it works fine.
Can anyone explain me why is this happening? Is 'description' some kind of reserved word in Core Data or something?
description is ann Objective-C property used for debugging and goes all the way down to Core Foundation, which has a corresponding CFDescription function. You should just name that property something else.
It's a method with a particular purpose in Cocoa, and Core Data dislikes it being overridden. More here.