Android Question [B4X] [XUI] B4XCanvas - how to remove a line

Alexander Stolte

Expert
Licensed User
Hello,

i draw a line with this code:
B4X:
If Action = xpnl_base.TOUCH_ACTION_DOWN Then
        g_y = y
        g_x = x
        xpath_main.Initialize(X,Y) 
    Else if Action = xpnl_base.TOUCH_ACTION_MOVE Then
        xcv_main.DrawLine(X, Y, g_x, g_y, xui.Color_Red,5)
        xpath_main.LineTo(x,y)
        g_x = X
        g_y = Y       
        xcv_main.Invalidate
        End If
And save the line in a B4XPath.
how can i remove this line with the help of the B4XPath?
 

JordiCP

Well-Known Member
Licensed User
A simple solution would be to use a list to add/remove the coords, and only create the path based on it, when/if needed. This will allow you for an undo-depth greater than one.

To remove the line once it is drawn, you could either just clear the canvas and redraw everything using a path created with the new list (once the last element is removed) , or redraw the last line with the background color (if it is solid)
 
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JordiCP

Well-Known Member
Licensed User
A cleaner approach, if you only want a 1-undo-depth, would be to draw the latest line always on a different canvas (on top of the current one).
So,
  • When a new line is drawn, the previous one is removed from the temporal canvas and drawn on the final one (you keep the latest couple of coords)
  • When a line is removed, just clear the temporal canvas.
 
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stevel05

Expert
Licensed User
Unless you are creating fast moving graphics (in which case you should probably be looking to use a game library approach), redrawing the canvas is quick enough to redraw all of the objects. Save the paths in a list, then manage the list to delete the ones you no longer want and clear and redraw them all.

It'll save problems where lines may cross or overlay at corners etc..
 
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