You can do the following:

1- add a local variable in the parent screen call it ChildGUID .

2- Add an input parameter in the block contain GUID.

3- Add an OnParametersChanged action on the block and assign it to the block events.

4- On the parent action, change the ChildGUID to trigger the OnParametersChanged action.

When you click the action on the parent screen the GUID will change and this will trigger the action on the block

Hi Lukasz,

Please check this article, it should asnwer your question: Pass Data Between Blocks - OutSystems 11 Documentation 

Way 1:

In Child block:

In_Flag: False

Local_Flag: False 

u use OnParametersChanged if In_Flag <> Local_Flag then call your action() and Local_Flag = In_Flag

In Parent block:

In_Flag of child block use Local_Flag in Parent

when u want trigger child action u can set Local_Flag = ! Local_Flag in parent blog

Way 2: 

use ParametersChanged in DevPatterns_Lib to trigger your action

In Parent block: implement same way 1

In_Flag of child block use Local_Flag in Parent

when u want trigger child action u can set Local_Flag = ! Local_Flag in parent blog

Hope this helps!

Hello,

It is the same as the relationship between a parent screen and a child block. For example, you add an input parameter to the block and place the block in the screen, then assign the input parameter with a value in the screen (the parent), keeping in mind to refresh the block in OnParametersChanged.

The exact same concept applies to a parent block and a child block. In this case, the parent block takes the role of the screen from the previous example.

@Łukasz Kaproń :  What you are asking is possible but using JavaScript. Documentation of the same is available at https://success.outsystems.com/documentation/how_to_guides/front_end/how_to_call_a_block_action_in_a_mobile_screen/ . Even though it says its for mobile it works on Web App too.

Basically you create a top level container in the block and on Onready you register your methods to be exposed.  Then in the consumer you get the blockId and call the method you registered earlier.  Please try it out.

You can create an input parameter in the child block, and I suggest making it a DateTime type. In the OnParametersChanged event, check whether this value is null or not. If it’s null, nothing happens. If it’s not null, you call the action you need. 

In the parent block/screen, create a local variable that always starts with NullDate(). Whenever you need to trigger the child block’s action, simply change this variable from NullDate() to CurrDateTime(). After executing the operation, the child block should trigger an event back to the parent so you can reset the local variable again to NullDate(). 

Check the OML in the attachment, I created an example where the parent screen triggers an action that performs form validation in a child block.

I'm using this in a scenario where I need to handle multiple types of changes from the parent block and want to trigger them in the child block.

Create a structured input parameter for the child block. 

InputContract

 └── ActionFlags (structure)

     ├── IsAChanged (Boolean)

     └── IsBChanged (Boolean)

LocalParam (to save the change at the end of process OnParametersChanged )

 └── ActionFlags (structure)

     ├── IsAChanged (Boolean)

     └── IsBChanged (Boolean)

OnParametersChanged:

If ActionFlags.IsAChanged  <> LocalParam.IsAChanged 

Then

   Run ActionA()

If ActionFlags.IsBChanged  <> LocalParam.IsBChanged 

Then

   Run ActionB()

LocalParam  = ActionFlags 



=> On Parent Block
=> when need to trigger action to child block
=> just change value = not current value
set value =>  ActionFlags.IsAChanged  = not ActionFlags.IsAChanged

1. Introduce a new Boolean variable in Parent Block. eg: Flag

2. In Parent block assign flag = If(Flag,False,True) (whenever you want to trigger child block action)

3. Add mandatory boolean input parameter in child block

4. pass Flag from parent block to child block input parameter 

5. use OnParameterChange to trigger a child block action 

The simplest way to trigger a child block action from a parent block using OnParameterChanged is this:

1. Expose an input parameter in the child block This parameter will control when to run the child’s action. Example: Input Parameter: TriggerAction (Boolean)

2. Inside the child block, use OnParameterChanged for this parameter When TriggerAction changes, the child block automatically detects the change.

3. In OnParameterChanged, write a condition If the parameter value meets your condition, call the child block action. Example: If TriggerAction = True → Run the internal logic

4. From the parent block/screen, update the TriggerAction parameter Whenever the parent updates the parameter value, the child block action will run.

Here is the clean logic flow:

Parent block
• Has an event or condition
• Sets ChildBlock.TriggerAction = True (or any value you want to check)
• This automatically fires OnParameterChanged in the child

Child block
• OnParameterChanged runs
• Inside it, check: If TriggerAction = True
• Call the child’s action
• Optionally reset TriggerAction back to False to avoid looping

This method works universally in OutSystems because:

• OnParameterChanged is invoked automatically each time the parent updates a value

• No manual event wiring is needed

• No dependencies or refresh issues

• Cleanest and fastest way to control child logic from the parent