Obsidean_VM/04-InLavoro/9..... MASTER Transport/Standard Transport/FB495 - LIFO HMI Page.md

```pascal

IF #CurrentPage <> #LastPage THEN
    #LastPage := #CurrentPage;
    
    #found := False;
    #index := -1;
    
    // Search if the current page is already in the history
    FOR #i := 1 TO #Length DO
        IF #PageHistory[#i] = #CurrentPage THEN
            #found := True;
            #index := #i; // Exit the loop as we found the page
            EXIT;
        END_IF;
    END_FOR;
    IF #found THEN
        // If the page is found, move the history to remove loops
        FOR #i := #index TO #Length DO
            #PageHistory[#i-#index] := #PageHistory[#i];
        END_FOR;
        #Length := #Length - #index;
    ELSE
        // If the page is new, add it to the history
        IF #Length < #MaxHistory THEN
            #Length := #Length + 1;
            FOR #i := #Length TO 1 BY -1 DO
                #PageHistory[#i] := #PageHistory[#i - 1];
            END_FOR;
            #PageHistory[0] := #CurrentPage;
        END_IF;
    END_IF;
    // Return the previous page
    #ReturnPage := #PageHistory[1];
ELSE
    // Special initial case if history is empty
    IF #Length=0 THEN
        #PageHistory[0] := #CurrentPage;
        #Length := 1;
        #ReturnPage := 1;
    END_IF;
END_IF;

IF #ReturnPage = 0 THEN
    #ReturnPage := 1;
END_IF;

```

### Then on the FC10:

![[Pasted image 20240919154404.png]]

we save the last page on the DBW66


### Description:
***
This script manages the navigation history of Human-Machine Interface (HMI) pages using a Last-In-First-Out (LIFO) approach. It ensures efficient page tracking and allows users to navigate back to previous pages without encountering loops or redundant entries.

**Key Functionalities:**
***
1. **Page Change Detection:**
    - The script checks if the current page (`#CurrentPage`) is different from the last visited page (`#LastPage`).
    - If there is a change, it updates the navigation history accordingly.
2. **Updating the Last Visited Page:**
    - Assigns the value of `#CurrentPage` to `#LastPage` to keep track of the latest page.
3. **Searching the Current Page in History:**
    - Initializes a flag `#found` to `False` and an index `#index` to `-1`.
    - Iterates through the `#PageHistory` array to check if the current page already exists in the history.
    - If found, sets `#found` to `True`, records the index, and exits the loop.
4. **Removing Loops in History:**
    - If the current page is found in the history (`#found` is `True`):
        - Shifts the pages after the found index to the beginning of the `#PageHistory` array.
        - Updates the `#Length` of the history to reflect the removed pages, effectively eliminating loops.
5. **Adding a New Page to History:**
    - If the current page is not found in the history:
        - Checks if the history length is less than the maximum allowed (`#MaxHistory`).
        - Increments the `#Length` of the history.
        - Shifts existing pages one position backward to make space at the top of the history stack.
        - Adds the current page to the top of the history (`#PageHistory[0]`).
6. **Setting the Return Page:**
    - Assigns the value of `#PageHistory[1]` to `#ReturnPage`, which represents the previous page for navigation.
7. **Handling Initial Case:**
    - If there is no page change and the history is empty (`#Length = 0`):
        - Initializes the history with the current page.
        - Sets the `#ReturnPage` to `1` as a default.
8. **Default Return Page:**
    - Ensures that `#ReturnPage` is not zero; if it is, sets it to `1` to avoid invalid page references.

**Usage Guidelines:**
***
- **Navigation History Management:**
    - Utilize this script to keep an accurate and loop-free history of HMI page navigation.
    - It is particularly useful for implementing "Back" functionality in HMI applications.
- **Customizing History Size:**
    - Adjust the `#MaxHistory` variable to change the maximum number of pages stored in the history.
- **Understanding the History Stack:**
    - The `#PageHistory` array operates as a stack where `#PageHistory[0]` is the most recently visited page.
    - Pages are pushed onto the stack when new pages are visited and popped off when navigating back.
- **Avoiding Navigation Errors:**
    - The script ensures that there is always a valid page to return to by setting a default `#ReturnPage` if necessary.

**Tips:**
***
- **Preventing Loops:**
    - The script removes any pages between the current position and a previously visited page to prevent navigation loops.

**Conclusion:**
***
This script effectively manages HMI page navigation by maintaining a clean history stack, allowing users to navigate backward efficiently. By preventing loops and redundant entries, it enhances the user experience in HMI applications.