Purpose

Fix MAGN-9568 Crash in preview control

There are two issues, both related to outdated mirror data that used in preview control to query the node value from the DesignScript virtual machine.

Updating a graph will update mirror data in all executed UI nodes. Ideally, the following preview operation will get the latest mirror data to display value. But because updating graph (UpdateGraphAsyncTask), querying mirror data (QueryMirrorDataAsyncTask) and displaying preview value (wrapped in DelegateBasedAsyncTask) are all asynchronous operations, and their creation is not necessarily in scheduler thread, their execution order is not guaranteed and the outdated mirror data will be used and cause crash.

The first issue is DelegateBasedAsyncTask is scheduled before QueryMirrorDataAsyncTask.

In this case, UpdateGraphAsyncTask has been executed and node's value has been updated. Now a DelegateBasedAsyncTask is created and put in scheduler thread (because of previewing operation on the UI thread). It will be executed immediately because QueryMirrorDataAsyncTaskhas not been scheduled yet:

Scheduler task queue
--------------------

         +----------if complete, schedule query task -----+
         |                                                |
         |                                                v
+----------------------+------------------------+--------------------------+
| UpdateGraphAsyncTask | DelegateBasedAsyncTask | QueryMirrorDataAsyncTask |
+----------------------+------------------------+--------------------------+
                                ^
                                |
                                +----- Scheduled from UI thread

NodeModel.CachedValue has not been updated yet, but the value of node has been changed, so NodeModel.CachedValue becomes invalid.

There are three ways to fix this case:

1. Set NodeModel.CachedValue to some invalid state. In DelegateBasedAsyncTask we will check if mirror data is invalid or not, and display "..." in the preview control accordingly. Mirror data will finally be updated in QueryMirrorDataAsyncTask. But introducing more states will make the code be harder to maintain.
2. As MirrorData is a snapshot of value, we could add the other kind of mirror data which always reflects current value. But MirrorData is widely used now, making this change will incur too many other changes. Besides, always querying value from the engine would be a potential performance issue.
3. Or, immediately updating mirror data after UpdateGraphAysncTask is executed. Firstly, it is a trivial change; secondly, if we can ensure all these operations happen in scheduler thread, DelegateBasedAsyncTask would never get a change to kick in and get executed. This PR uses this option.

Related change: NodeModel.RequestValueUpdateAsync() is updated to synchronous NodeModel.RequestValueUpdate().

The second issue is, PreviewControl holds a cached of mirror data (PreviewControl.mirrordata), this mirrordata will only be updated after UpdateGraphAsyncTask is scheduled, but before that, DelegateBasedAsyncTask may be scheduled already:

Scheduler task queue
--------------------

         +----------if complete, update preview control's mirror data .....
         |                                                                +
         |                                                                |
+----------------------+------------------------+                         |
| UpdateGraphAsyncTask | DelegateBasedAsyncTask |                         |
+----------------------+------------------------+                         |
                                ^                                         |
                                |                                         |
                        Scheduled from UI thread                           |
                                                                          v
                                                     Update preview control's mirror data

Again, invalid mirror data is used.

As PreviewControl knows about NodeModel, it can always get the latest mirror data from NodeModel. So this PR removes PreviewControl.mirrorData.

Declarations

Check these if you believe they are true

• The code base is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.

Reviewers

@benglin : @riteshchandawar has helped to do some play testing.

@aosyatnik please help to review preview control part in case I miss something, and sorry for stealing the task from you... :-)