How to safely pass a context object in an UIAlertView delegate? - iphone

Question

In my application I use a UIAlertView to display to the user a message and some options. Depending on the button pressed, I want the application to perform something on an object.
The sample code I use is...
-(void) showAlert: (id) ctx {
UIAlertView *baseAlert = [[UIAlertView alloc]
initWithTitle: title
message: msg
delegate:self
cancelButtonTitle: cancelButtonTitle
otherButtonTitles: buttonTitle1, buttonTitle2, nil];
//baseAlert.context = ctx;
[baseAlert show];
[baseAlert release];
}
- (void)alertView:(UIAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex {
if (buttonIndex == 1) {
id context = ...;//alertView.context;
[self performSelectorOnMainThread:#selector(xxx:) withObject: context waitUntilDone: NO];
}
}
Is there any way to pass an object into the delegate as a context object? or maybe some other way?
I could add the property on the delegate but the same delegate object is being used by many different alert views. For this reason I would prefer a solution where the context object is attached to the UIAlertView instance and carried across to the delegate as part of the UIAlertView object.

Answer 1

I still think storing it locally is the best solution. Create a class local NSMutableDictionary variable to hold a map of context objects, store the context with UIAlertView as the key and the context as the value.
Then when the alert method is called just look into the dictionary to see which context object is related. If you don't want to use the whole Alert object as a key, you could use just the address of the UIAlertView object:
NSString *alertKey = [NSString stringWithFormat:#"%x", baseAlert];
The address should be constant on the phone. Or you could tag each alert as the other poster suggested and use the tag to look up a context in the map.
Don't forget to clear out the context object when you are done!

Answer 2

A complete implementation that allows you to pass context:
#interface TDAlertView : UIAlertView
#property (nonatomic, strong) id context;
#end
#implementation TDAlertView
#end
And a usage example, note how we pre-cast the pointer:
#implementation SomeAlertViewDelegate
- (void)alertView:(TDAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex {
NSLog(#"%#", [alertView context]);
}
#end

Answer 3

you can also use the tag property (since it's a UIView subclass). This is just an int, but may be enough for you.

Answer 4

Instead of debating the meaning of "does not support subclassing", I'll provide a better answer. I created a generic contextInfo category for my job a couple months ago. I just put it on github: JLTContextInfo.
#import "objc/runtime.h"
#interface NSObject (JLTContextInfo)
- (NSMutableDictionary *)JLT_contextInfo;
#end
#implementation NSObject (JLTContextInfo)
- (NSMutableDictionary *)JLT_contextInfo
{
static char key;
NSMutableDictionary *contextInfo = objc_getAssociatedObject(self, &key);
if (!contextInfo) {
contextInfo = [NSMutableDictionary dictionary];
objc_setAssociatedObject(self, &key, contextInfo, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
return contextInfo;
}
#end
This creates a place to easily store extra data for any object derived from NSObject. Now the answer is nearly identical to the original question.
-(void) showAlert: (id) ctx {
UIAlertView *baseAlert = [[UIAlertView alloc]
initWithTitle: title
message: msg
delegate:self
cancelButtonTitle: cancelButtonTitle
otherButtonTitles: buttonTitle1, buttonTitle2, nil];
[[baseAlert JLT_contextInfo] setObject:ctx forKey:#"ctx"];
[baseAlert show];
[baseAlert release];
}
- (void)alertView:(UIAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex {
if (buttonIndex == 1) {
id context = [[alertView JLT_contextInfo] objectForKey:#"ctx"];
[self performSelectorOnMainThread:#selector(xxx:) withObject: context waitUntilDone: NO];
}
}

Answer 5

Subclassing UIAlertView is not a good idea.
http://developer.apple.com/library/ios/#DOCUMENTATION/UIKit/Reference/UIAlertView_Class/UIAlertView/UIAlertView.html
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.
UIAlertView with a user supplied context and [self autorelease] answers this question in a different way.

Answer 6

I did a mix between Kendall's answer and the uses of blocks in one of my base view controller classes. Now I can use AlertView and ActionSheets with blocks which improves greatly readability. Here is how I did it :
In the .h of my ViewController I declare a block type (optional but recommanded)
typedef void (^AlertViewBlock)(UIAlertView*,NSInteger);
Also I declare a mutable dictionnary that will store the blocks for each alertview :
NSMutableDictionary* m_AlertViewContext;
In the implementation file I add a method to create the AlertView and save the block :
-(void)displayAlertViewWithTitle:(NSString *)title message:(NSString *)message cancelButtonTitle:(NSString *)cancelButtonTitle withBlock:(AlertViewBlock)execBlock otherButtonTitles:(NSArray *)otherButtonTitles
{
UIAlertView* alert = [[UIAlertView alloc] initWithTitle:title
message:message
delegate:self cancelButtonTitle:cancelButtonTitle otherButtonTitles: nil];
for (NSString* otherButtonTitle in otherButtonTitles) {
[alert addButtonWithTitle:otherButtonTitle];
}
AlertViewBlock blockCopy = Block_copy(execBlock);
[m_AlertViewContext setObject:blockCopy forKey:[NSString stringWithFormat:#"%p",alert]];
Block_release(blockCopy);
[alert show];
[alert release];
}
Note that I receive the same attributes as the constructor of the UIAlertView but the delegate (which will be self). Also I receive a AlertViewBlock object that I save in the m_AlertViewContext mutable dictionnary.
Then I show the alert as I would usually do.
In the delegate callbacks, I call the block and give it the parameters :
#pragma mark -
#pragma mark UIAlertViewDelegate
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
NSString* blockKey = [NSString stringWithFormat:#"%p",alertView];
AlertViewBlock block = [m_AlertViewContext objectForKey:blockKey];
block(alertView,buttonIndex);
[m_AlertViewContext removeObjectForKey:blockKey];
}
- (void)alertViewCancel:(UIAlertView *)alertView {
NSString* blockKey = [NSString stringWithFormat:#"%p",alertView];
[m_AlertViewContext removeObjectForKey:blockKey];
}
Now, whenever I need to use an AlertView I can call it like this :
[self displayAlertViewWithTitle:#"Title"
message:#"msg"
cancelButtonTitle:#"Cancel"
withBlock:^(UIAlertView *alertView, NSInteger buttonIndex) {
if ([[alertView buttonTitleAtIndex:buttonIndex] isEqualToString:#"DO ACTION"]){
[self doWhatYouHaveToDo];
}
} otherButtonTitles:[NSArray arrayWithObject:#"DO ACTION"]];
I did the same for the ActionSheet and now it's really easy to use those.
Hope it helps.

Answer 7

From my other answer, here's a quick and clean solution that takes advantage of associated objects. I mention in my other answer that you could even replace UIAlertView with NSObject and effectively add a context property to any object:
#import <objc/runtime.h>
#interface UIAlertView (Private)
#property (nonatomic, strong) id context;
#end
#implementation UIAlertView (Private)
#dynamic context;
-(void)setContext:(id)context {
objc_setAssociatedObject(self, #selector(context), context, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
-(id)context {
return objc_getAssociatedObject(self, #selector(context));
}
#end
And then you'll be able to do something like:
NSObject *myObject = [NSObject new];
UIAlertView *alertView = ...
alertView.context = myObject;
IMPORTANT:
And don't forget to nil the context in dealloc!!

Answer 8

You could subclass UIAlertView and add the property there.