Bug 7759 - Calling InvokeOnMainThread causes the app to hang
Summary: Calling InvokeOnMainThread causes the app to hang
Status: RESOLVED FIXED
Alias: None
Product: iOS
Classification: Xamarin
Component: General ()
Version: 6.0.x
Hardware: Other Other
: --- normal
Target Milestone: Untriaged
Assignee: Bugzilla
URL:
Depends on:
Blocks:
 
Reported: 2012-10-09 19:21 UTC by dj_technohead
Modified: 2012-10-09 22:31 UTC (History)
1 user (show)

Tags:
Is this bug a regression?: ---
Last known good build:

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Status:
RESOLVED FIXED

Description dj_technohead 2012-10-09 19:21:36 UTC
I have the following code:

public void HideProgressIndicator()
        {
            if (this.progressHUD != null)
            {
                if (!IsMainThread())
                    this.InvokeOnMainThread(()=>{
                        this.progressHUD.Hide(true);
                    });
                else
                    this.progressHUD.Hide(true);
            }
        }

        protected static bool IsMainThread() {
            return Messaging.bool_objc_msgSend(GetClassHandle("NSThread"),
new Selector("isMainThread").Handle);
        }

        protected static IntPtr GetClassHandle (string clsName)
        {
            return (new Class(clsName)).Handle;
        } 


It is hanging 100% of the time on the line where InvokeOnMainThread is being called, it is never making it to the line this.progressHUD.Hide(true). 

This seems like a problem with nested async event handling. To give a bit more context to the problem, the app that I am working on is doing the following:

Scan barcode, on scan event, show UIActivityIndicator, add http request to a queue, pull http request from queue in a thread and send http request for barcode lookup asynchronously, on completion of async http request hide UIActivityIndicator. Trying to track the code is pretty hard due to all events/asynchronous handlers, so sometimes not surprised that MT appears to be having issues.

This seems like a pretty bad problem as I don't know of a workaround and it is causing the app to completely hang, only recourse is to terminate the app.
Comment 1 dj_technohead 2012-10-09 22:31:52 UTC
I believe I found the problem. Within the code, some threads were coded to wait until a result is returned, to make an async method behave like a sync method. Removing the waiting for a result appears to have resolved this issue.