Iphone – Cocoa – Return information from NSOperation

cocoacocoa-touchiphonemultithreadingnsoperation

I have an IPhone app that uses webservices to get data from a server. I'm putting each call to the webservice in a NSOperation subclass so that it can be threaded. My question is, what is the recommended way to pass back information from a completed NSOperation subclass. I'm currently sending a NSNotification at the end of my main method and any code that is waiting for the NSOperation to complete, subscribes to the notification. And then I will use the object part of NSNotificationWithName:Object: to pass information back from the operation.

I wasn't sure if there was a better way to do this. I have heard of Key Value Coding and then I could use Key Value Observing to detect when the the isFinished property for the operation is changed, but I'm not sure what the best practice is.

I'm also trying to ensure that my application is thread safe, which I assume wouldn't be an issue as long as a my notification was sent out at the end of the main method thus the object in the notification would no longer be used in the thread since it will end.

Finally, I am calling the notification like so:

[[NSNotificationCenter defaultCenter] performSelectorOnMainThread:@selector(postNotification:) withObject:[NSNotification notificationWithName:@"notificationName" object:dataObject] waitUntilDone:NO];

Best Answer

I've personally done this in two ways. Both worked quite well.

First Way

You can have a function that returns the "output", and you call that function after the operation has finished. You can observe the key isFinished on the NSOperation object, or use -[NSOperationQueue waitUntilAllOperationsAreFinished] to determine when the operation is finished.

Second Way

You can use a delegate or target/action from the NSOperation. Just make sure that when you call the delegate function, or trigger the target/action, you do so on the main thread by using -[NSObject performSelectorOnMainThread:withObject:waitUntilDone:]

Related Solutions

C# – How to update the GUI from another thread

The simplest way is an anonymous method passed into Label.Invoke:

// Running on the worker thread
string newText = "abc";
form.Label.Invoke((MethodInvoker)delegate {
    // Running on the UI thread
    form.Label.Text = newText;
});
// Back on the worker thread

Notice that Invoke blocks execution until it completes--this is synchronous code. The question doesn't ask about asynchronous code, but there is lots of content on Stack Overflow about writing asynchronous code when you want to learn about it.

Cocoa: What’s the difference between the frame and the bounds

The bounds of an UIView is the rectangle, expressed as a location (x,y) and size (width,height) relative to its own coordinate system (0,0).

The frame of an UIView is the rectangle, expressed as a location (x,y) and size (width,height) relative to the superview it is contained within.

So, imagine a view that has a size of 100x100 (width x height) positioned at 25,25 (x,y) of its superview. The following code prints out this view's bounds and frame:

// This method is in the view controller of the superview
- (void)viewDidLoad {
    [super viewDidLoad];

    NSLog(@"bounds.origin.x: %f", label.bounds.origin.x);
    NSLog(@"bounds.origin.y: %f", label.bounds.origin.y);
    NSLog(@"bounds.size.width: %f", label.bounds.size.width);
    NSLog(@"bounds.size.height: %f", label.bounds.size.height);

    NSLog(@"frame.origin.x: %f", label.frame.origin.x);
    NSLog(@"frame.origin.y: %f", label.frame.origin.y);
    NSLog(@"frame.size.width: %f", label.frame.size.width);
    NSLog(@"frame.size.height: %f", label.frame.size.height);
}

And the output of this code is:

bounds.origin.x: 0
bounds.origin.y: 0
bounds.size.width: 100
bounds.size.height: 100

frame.origin.x: 25
frame.origin.y: 25
frame.size.width: 100
frame.size.height: 100

So, we can see that in both cases, the width and the height of the view is the same regardless of whether we are looking at the bounds or frame. What is different is the x,y positioning of the view. In the case of the bounds, the x and y coordinates are at 0,0 as these coordinates are relative to the view itself. However, the frame x and y coordinates are relative to the position of the view within the parent view (which earlier we said was at 25,25).

There is also a great presentation that covers UIViews. See slides 1-20 which not only explain the difference between frames and bounds but also show visual examples.

Best Answer

Related Solutions

C# – How to update the GUI from another thread

Cocoa: What’s the difference between the frame and the bounds

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