Undo Selection with UISegmentedControl - ios

When a user taps on a UISegmentedControl and changes which segment is selected, I want to perform some validation. If the validation fails, I want to 'undo' their selection, and re-select the item that they previously had selected.
What's the easiest way to do this? I think I need something like
- (BOOL) segment:(UISegmentedControl *)seg shouldSelectSegmentAtIndex:(NSInteger)newIndex`
but I was looking at the Apple Docs, but I didn't see anything like that - really I didn't see anything with will in it at all. Will I have to create a category or subclass to do this, or am I just missing it?
I suppose I could add an ivar that holds the selected segment index after it has been validated, and then call [mySegmentedControl setSelectedSegmentIndex:myIvar]; if validation fails, or myIvar = [mySegmentedControl selectedSegmentIndex]; when it passes, but that feels a little messier. It's also a bit of a pain if I have a lot of segmented controls on the same screen. I believe if I do create my own subclass, I'll have to do something like this though.
EDIT:
Other programming languages I've used have natively supported an 'undo' feature, without the programmer having to implement the undo on his own. I'm looking to see if anyone is aware of something like that for Objective-C. If it does not exist, does anyone know of a better way to implement it other than the ivar method I described above?

I don't believe there really is an elegant way to do that without subclassing. But since there are no subclassing notes on UISegmentedControl, that could be an option. Override setSelectedSegmentIndex, where you check through delegation whether that index should be selected. If it should, call super's implementation. If not, just do nothing.
This does assume that there are no internal methods that mess things up (and which you can't override).

Im not exactly sure what you are asking, but if I understand correctly I do believe this would do the trick:
[mySegmentedControl addTarget:self action:#selector(segmentChanged:) forControlEvents:UIControlEventChanged];
- (void)segmentChanged:(id)sender
{
if (myIvar != validated){
[mySegmentedControl setSelectedSegmentIndex:defaultSegment];
}
}

Related

How to use method swizzle or something on subclasses

I has many UITableViewController subclasses in my app.
Now i just needed to modify them all to add +1 row in all cases, and one simple equal row in all.
I do not want to modify all of them by hand, better way seem's to replace UITableViewDataSource method to modify values in way like:
+(void)load {
[[self class] jr_swizzleMethod:#selector(tableView:numberOfRowsInSection:) withMethod:#selector(swizzledTableView:numberOfRowsInSection:) error:nil];
}
- (NSInteger)swizzledTableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
return [self swizzledTableView:tableView numberOfRowsInSection:section] + 1;
}
But it replaces superclass function, that does not called in subclasses, so this is not working. Is there method to do that what i want, without modifying all subclasses?
You'd need to swizzle every subclass specifically. You can find them by introspecting the class hierarchy at runtime with objc_getClassList, but I can't begin to describe how dangerous and fragile this approach is. You're trying to apply this to every tableview in the system, which you hope is just the tableviews you mean it to be (i.e. your tableviews). But what about tableviews that might be used by the system or from third-party libraries? You're modifying them, too. And when you try to understand the crash this causes, the stack trace will be unintelligible because of the swizzle.
In order for this to work, tableView:cellForRowAtIndexPath: also needs to correctly handle this extra row, so it's hard to see how every table view controller in the system is going to be implemented correctly without knowing about this +1.
Either subclass your table view controller (and have them call super), or use a separate object that all of them call to add the extra row if it's needed. This other object (or superclass) is also where you should handle the cell for this extra row.
I have little experience in swizzling. But I have two possible solutions to your problem.
First:
Create a subclass: YouBaseTableView: UITableView, and add a row in YouBaseTableView. And inherit all your table view classes from YouBaseTableView.
Second:
Create an extension for UITableView, and write your row in this extension.
I'm probably late for the train...
But for future reference, a solution for the problem would be to swizzle setDataSource of UITableView and replace it with an NSProxy instance.
Usually nobody overrides the setDelegate / setDataSource methods, and that would allow you to swizzle those and intercept all calls to these delegates and exchange the implementation.
Check this out for more info: https://developer.apple.com/documentation/foundation/nsproxy
You are going into two different areas that need full understanding to be used correctly and are highly dangerous: Performing code in the +load method, and using method swizzling. I would never dare doing anything in +load. +initialize is ok if you know what you are doing, but +load is something you mustn't even think of touching if you ask questions here.
Now ask yourself first: What is "self" in a class method, and what is "[self class]"? Do you think this has even a chance of working?
I'd also recommend that you google for "swizzle" and pick up some other code for method swizzling. It looks quite dubious to me. And writing it as a category instead of a plain C function feels just horrible.

Should UIAlertView be subclassed?

I am sure the answer to this is "no" as the documentation is very clear. But I am a little confused. A standard UIAlertView is pretty dull and I want to improve the look and it seems that other apps do it (see the example below).
Another possibility is that they are not subclassed UIAlertViews. In which case, how is this achieved?
The page UIAlertViews states
Appearance of Alert Views
You cannot customize the appearance of alert views.
So how do we get the something like the example shown here?
No, do not subclass it. From the docs:
Subclassing Notes
The UIAlertView class is intended to be used as-is
and does not support subclassing. The view hierarchy for this class is
private and must not be modified.
What you can do though is create a UIView and have it act similar to a UIAlertView. It's isn't very difficult and seems to be what they are doing in your op.
Apple's docs say that you should not subclass it. That means that there are probably internal reasons that would make it difficult to make it work right.
You might or might not be able to make a subclass of UIAlertView work, but you do so at your own risk, and future iOS releases might break you without warning. If you tried to complain Apple would laugh and tell you "I told you so".
Better to create a view that looks and acts like an alert but is your own custom view/view controller. Beware that even this is dangerous, because Apple has been making sweeping changes to the look and feel of it's UI elements recently. If you implement a view controller that looks and acts like a variant of the current alert view, Apple could change that look and/or behavior in the future and your UI app would end up looking odd and outdated. We've been bitten by this sort of thing before.
Rethink your strategy. Why do you need to use an Alert View? Besides having a modal view displayed top-most on your view stack, there's not much else that it does. Instead, subclass UIView or UIViewController to define your own interface, using images and ui elements to give it the style and input functionality as needed.
I usually subclass UIView, and attach it to the app's window's view so that I'm certain that it will be displayed on top of anything else. And you can use blocks to provide hooks into the various input elements of your new view (did user press OK, or did user enter text?)
For example:
// Instantiate your custom alert
UIView *myCustomAlert = [[UIMyCustomUIViewAlert alloc] initWithFrame:CGRectMake(...)];
// Suppose the new custom alert has a completion block for when user clicks on some button
// Or performs some action...
myCustomAlert.someEventHandler = ^{
// This block should be invoked internally by the custom alert view
// in response to some given user action.
};
// Display your custom alert view
UIWindow *window = [[UIApplication sharedApplication] keyWindow];
[window addSubview: myCustomAlert];
// Make sure that your custom alert view is top-most
[window bringSubviewToFront: myCustomAlert];
Using this method, however, will not pause the thread's execution like UIAlertView does. Using this method, everything will continue running as usual. So if you need to pause execution while your custom alert is showing, then it gets much trickier.
But otherwise, creating your own custom alerts is quite straightforward, just as you would customize any other view. You could even use Interface Builder.
Hope this helps.
No. You absolutely should not subclass a UIAlertView for any reason. Apple explicitly states this in their documentation (see "Subclassing Notes"). They even tell you that it relies on private methods - and we all know that meddling in private methods in an AppStore app is immediate grounds for rejection.
HOWEVER, there isn't a need to subclass UIAlertView on iOS 7. Apple introduced a new Custom ViewController Transitions feature in iOS 7.0 that lets you present completely custom ViewControllers with completely custom transitions. In other words, you could very easily make your own UIAlertView or even something better. There's a nice tutorial on the new feature here:
In fact, there are lots of good tutorials on this - a quick Google search on the topic turns up a huge wealth of information.

Does willMoveToSuperview will also deallocate the UIView on which its got called?

I was wondering if I can call willMoveToSuperview on UIView and after that retain that view to reuse later for one ? something like following
if (!CGRectIntersectsRect(cell.frame, visibleRegion)) {
[cell willMoveToSuperview:nil];
[self.resuableCells addObject:cell];
}
I am not sure about your intent here...
But WillMoveToSuperview - According to doc:
The default implementation of this method does nothing. Subclasses can override it to perform additional actions whenever the superview changes.
So your code,
[cell willMoveToSuperview:nil];
Has no effect unless you override this method in a cell subclass and implement your own logic there.
Coming to your question -
Does willMoveToSuperview will also deallocate the UIView on which its got called?
Answer is obvious - NO.
willMoveToSuperview is an observer method that the system calls as a courtesy to you in order to give you a chance to handle special cases before it completes some other hidden tasks.
It's default behavior is to do nothing, but you might want to tidy up something in your code prior to a move by overriding this method.
A proper use case might be if you had a view playing a video clip or an animation, and something else in your code is about to rip the view out of it's current hierarchy and place it in some other un-related view hierarchy. You might want the chance to pause the clip or suspend the animation before the move took place.
I doubt it's the right method to handle what you are attempting, and I definitely know you should not be calling it directly.
Feel free to post some more code to show us what you're trying to accomplish and where it's going wrong.

Singleton-Like UIView Access?

I have a UIView I need to access the properties of from all around my app. I know you can't create a Singleton around a UIView object, so what might be a good way of doing similar?
E.g. The view has a label. From any view controller in my app I want to be able to change the text of this view (sitting in a parent view controller).
Thanks.
EDIT:
Success! Using KVO to track changes in my Singleton object worked a charm, and a very simple solution.
I think what you’re trying to do violates the separation of concerns of the MVC pattern: The only thing that should interact with a view is its controller. In your case, you should probably be creating a model that is watched by your view controller (maybe through key–value observing), and then the controller can propagate the necessary changes to your view.
If you know (read: you really know for now and forever!) that there will be at most one instance of that view alive at one point in time, you can just use a global variable to store that. Or use a class property on that view - which is as close as being a singleton as possible.
Or you might just fix your design, which has proven to be the better choice in every case I can remember. :) Just add some forward and backward references in your classes (and stick to MVC principle). It takes much less time to implement that worrying about those awkward workaround, and it will pay of rather sooner than later.

Select a range in UISlider

i asked me whether it is possible if i can create a uislider who has 2 handles to select a range. Just like here:
The problem i am facing is that i dont want to use a custom UIControl Subclass. I need a UISlider subclass or a other solution for this problem, because a lot of the code is based on UISlider specific propertys etc. So is there any possibility to achieve this ?
Look at the following example:
http://www.cocoacontrols.com/platforms/ios/controls/rangeslider
You can subclass UISlider, but it will be very difficult. Your class should offer quite some new properties, and the old ones won't make much sense at all.
Not sure how your code can be based much on UISlider specific things - as everything would change the meaning (i.e. ranges instead of one value).
If you really need a common base class, you could encapsulate ("has-a" relationship) the control in a custom class and let this handle the different types.
I implemented a similar control using a custom view, and it happened to be quite straight forward.
UISlider doesn't provide the functionality you're after, and subclassing UISlider probably won't work out. What would the value of such a control be? The value of a slider is a number, but you want it to be a range. Consider a custom control that duplicates the UISlider properties you need.

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