I see every now and again that Apple needs to make it easier to allow developers to save the state of their application so that they open up exactly as they were when they were shut down.
Obviously I’m all for Apple making my life easy, but that’s not going to happen for a while yet so I thought I’d share how I implemented it in Yummy.
The key is this simple protocol:
@protocol SaveState
- (NSData*) saveState;
- (id) initWithSaveState:(NSData*)data;
@end
This is what I have in my applicationWillTerminate: method:
NSMutableArray* vcList = [NSMutableArray arrayWithCapacity:3];
for (UIViewController* vc in self.navigationController.viewControllers) {
// Classes that don't implement the SaveState protocol will be ignored
if ([vc conformsToProtocol:@protocol(SaveState)]) {
NSArray* state = [NSArray arrayWithObjects:NSStringFromClass([vc class]),
[(UIViewController*)vc saveState],
nil];
[vcList addObject:state];
}
}
This is in the applicationDidFinishLaunching::
for (NSArray* screen in screenList) {
UIViewController* next =
[[NSClassFromString([screen objectAtIndex:0]) alloc]
initWithSaveState:([screen count] == 2) ?
[screen objectAtIndex:1] : nil];
if (next != nil) {
[[self navigationController] pushViewController:next animated:NO];
[next release];
}
}
So a simple example, for a view controller that needed to be remembered but didn’t need to store any extra state, would be:
- (NSData*) saveState {
return nil;
}
- (id) initWithSaveState:(NSData*)data {
return [self init];
}
But you can initialise each view controller with anything that can be converted into an NSData. (I picked NSData rather than, say, id because an NSData can always be serialised. Made sense to make that assumption.)
One weakness is that it doesn’t cope with the situation where a modal view is on top but that should be pretty easy to implement if it’s important (it generally isn’t in Yummy).
