Object Not Locked Message When Calling RecyclerView.NotifyAll()

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Object Not Locked Message When Calling RecyclerView.NotifyAll()

Nathan Sokalski 4,111

I have a RecyclerView & several Slider(s). I want to update the RecyclerView items when the user moves the Slider. I have created the OnValueChange for the Slider(s). This contains a call to the NotifyAll() method of the RecyclerView, but when I call this I receive the following Exception:

Java.Lang.RuntimeException
object not locked by thread before notify()

This happens at the beginning of my app because I call the OnValueChange method/handler explicitly to set the Slider(s) to their desired initial values. I have never worked with locking stuff before. What should I do?

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Answer 1

Yonglun Liu (Shanghai Wicresoft Co,.Ltd.) 50,126 Microsoft External Staff

Hello,

This issue is caused by your code not meeting the execution conditions of the notify() or notifyAll() methods.

You could find the following conditions via referring to the official documentation: notify.

This method should only be called by a thread that is the owner of this object's monitor. A thread becomes the owner of the object's monitor in one of three ways:
By executing a synchronized instance method of that object.
By executing the body of a synchronized statement that synchronizes on the object.
For objects of type Class, by executing a synchronized static method of that class.

Therefore, you need to lock the object it before calling NotifyAll() in C#.

Please refer to lock statement (C# reference) to get more details.

Update:

This should be due to an error thrown by the compiler.

When you use the RecycleView.Adapter, if the adapter gets an address different from the actual address when initializing, the adapter mistakenly thinks that there are multiple different threads operating it, which will cause this error to cause no matter what object we lock.

In fact, the use case of notifyall is not for data updates, but for waking up threads, see:

The notifyAll() method of thread class is used to wake up all threads. This method gives the notification to all waiting threads of a particular object.

Please use the notifyDataSetChanged method to update the dataset, it is the most recommended way to do it.

Best Regards,

Alec Liu.

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Nathan Sokalski 4,111 Reputation points

2022-10-24T18:14:08.767+00:00
I have never used the lock statement before, so I am completely new to it. My original statement (the line of code that the debugger points to when the Exception occurs) is:

this.rvPointCards.NotifyAll();

Because the Exception does not tell me what object is not locked, I am not completely sure what to do or exactly what needs locked (or how to lock it). The OnBindViewHolder method in the Adapter for rvPointCards is as follows:

public override void OnBindViewHolder(ViewHolder holder, int position) { (holder as PointCardViewHolder).Card.CardType = psapp.CurrentCards[position]; }

I am assuming I will need to declare an object in MainActivity (which is what contains the method from which I call NotifyAll()) similar to the following:

private readonly object currentcardslock = new object();

I tried modifying my original statement to the following:

lock (currentcardslock) { this.rvPointCards.NotifyAll(); }

But it did not seem to help. I am very confused as to how use the lock statement (probably because it is my first time using it), but I am not sure what to do next. What should I do?
Yonglun Liu (Shanghai Wicresoft Co,.Ltd.) 50,126 Reputation points Microsoft External Staff

2022-10-25T06:39:52.353+00:00
You need to lock the CurrentThread as the following code:

void Update() { lock (Thread.CurrentThread) { try { rvPointCards.NotifyAll(); } catch (Exception ex) { Console.WriteLine(ex.StackTrace); } } }

In addition, it is more recommended to use BaseAdapter.NotifyDataSetChanged Method to update the items of Adapter on Android.
Nathan Sokalski 4,111 Reputation points

2022-10-25T16:44:13.7+00:00
I added the following:

lock (Thread.CurrentThread) { try { this.rvPointCards.NotifyAll(); } catch (Exception ex) { System.Diagnostics.Debug.WriteLine(ex.Message); } }

But I still get the following message:

object not locked by thread before notify()

Do I need something else to go with this?
Yonglun Liu (Shanghai Wicresoft Co,.Ltd.) 50,126 Reputation points Microsoft External Staff

2022-10-26T02:41:01.913+00:00

This should be due to an error thrown by the compiler.

When you use the RecycleView.Adapter, if the adapter gets an address different from the actual address when initializing, the adapter mistakenly thinks that there are multiple different threads operating it, which will cause this error to cause no matter what object we lock.

In fact, the use case of notifyall is not for data updates, but for waking up threads, see:

The notifyAll() method of thread class is used to wake up all threads. This method gives the notification to all waiting threads of a particular object.

Please use the notifyDataSetChanged method to update the dataset, it is the most recommended way to do it.
Nathan Sokalski 4,111 Reputation points

2022-10-26T03:12:16.77+00:00

Thanks! Once I started using NotifyDataSetChanged() instead of NotifyAll() it works great!
Yonglun Liu (Shanghai Wicresoft Co,.Ltd.) 50,126 Reputation points Microsoft External Staff

2022-10-26T04:08:00.38+00:00

Thanks for your feedback, I have updated my answer. Could you please accept it? It would be helpful for others who meet the same issue.

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Object Not Locked Message When Calling RecyclerView.NotifyAll()

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