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In my .qml file, when a Button is pressed, it emits a signal, and some action is taken. This action takes a long time (it is encrypting data) and I want the button to "unpush" itself before this action starts, so I can show a progress bar on top.

I know Qt has event loop, and somehow, I need this signal to be emitted after this event loop , or when the GUI completes the Paint() of all objects, so the button is shown unpressed.

How is this done?

This is the code that is being executed on the signal, the heavy process is aewm_obj.save_account(key,account) , it takes 2 seconds on desktop PC and 10 seconds on Android:

function save_account() 
 if (fld_password1.length===0) 
 if (account.type==WalletAccount.WACCT_TYPE_FULL) 
 return
 
 

 if (fld_password1.text!==fld_password2.text) 
 console.log("passwords do not match")
 else 
 btn_save.enabled=true;
 if (account.type==WalletAccount.WACCT_TYPE_FULL) 
 key.password=fld_password1.text
 account.password=fld_password1.text
 
 btn_save.enabled=false
 encrypting_message.open()
 if (aewm_obj.save_account(key,account)) 
 encrypting_message.close()
 accounts_root.back_to_accounts()
 else 
 encrypting_message.close()
 
 

The problem is, the dialog with the progress bar is shown after the heavy process finishes, however it was called before. This happens because the event loop to show and hide components is processed too late.

Use a single shot timer with a timeout of 0 to queue up an event that will be triggered after returning to the event loop. In C++ this typically looks like:

QTimer::singleShot(0, /* do something */ );

The QML equivalent is:

Timer 
 interval: 0; running: true; repeat: false
 onTriggered: /* do something */

After reading this article: https://doc.qt.io/archives/qq/qq27-responsive-guis.html

I have added code to my C++ function to process all the events before it starts the heavy process. This way, I empty event queue and the GUI updates automatically showing the progress bar dialog. (a sort of repaint() of all QML objects)

for (int i=0; i<10000; i++) 
 QCoreApplication::sendPostedEvents();
 QCoreApplication::processEvents();

The delay caused by this loop is unnoticeable compared to the 2 seconds of processing time. The 10,000 iterations are needed on Android, on desktop 10 iterations is enough. Using small amount of iterations, will show the dialog not at full opacity, due to fade effects.