Question

I have an interface like this:

@interface AView : UIScrollView
{
    UIView* m_view1;
    UIView* m_view2;
    ...
}

-(void) method1;
-(void) method2;
...
@end

I need to access views from methods of the interface. I need to create, release, re-create them and also set properties.

The problem is, some methods of the interface are running in different threads. Since these methods access same views I have issues like one thread trying to re-create a view when another thread is trying to set some properties of a view being recreated.

How should I synchronize access to views?

Answer 1

First of all, do you know you can ONLY call the methods of the UIView class (and it’s subclasses) in the main thread? But, if you are just doing create and release job in second thread, it’s OK to do it.

Threading Considerations

Manipulations to your application’s user interface must occur on the
main thread. Thus, you should always call the methods of the UIView
class from code running in the main thread of your application. The
only time this may not be strictly necessary is when creating the view
object itself; but all other manipulations should occur on the main
thread.

In addition, you can use @synchronized() {object} to lock an object. But still, you can NOT call UIView’s methods in second thread (in Objective-C even set property is calling method) even you’ve locked it.

Objective-C supports multithreading in applications. Therefore, two
threads can try to modify the same object at the same time, a
situation that can cause serious problems in a program. To protect
sections of code from being executed by more than one thread at a
time, Objective-C provides the @synchronized() directive.

The @synchronized()directive locks a section of code for use by a
single thread. Other threads are blocked until the thread exits the
protected code—that is, when execution continues past the last
statement in the @synchronized() block.

The @synchronized() directive takes as its only argument any
Objective-C object, including self.