[Laszlo-dev] API change proposal: <node>/destroy behavior, when destroying child nodes recursively

[André Bargull]

On 1/27/2009 1:03 AM, P T Withington wrote:
> On 2009-01-26, at 18:24EST, André Bargull wrote:
> 
>> Oh wait, you just said nothing in the LFC depends on the current way 
>> things work. I really believed in you,
> 
> But wait, I said:
> 
> "where a child node _needs_ to access it's parent when the parent is in 
> the process of being  deleted"
> 
> so, there may be cases where it _does_ access the parent, but does it 
> _need_ to?  I don't think it does. So it is just a matter of fixing 
> those cases to be careful.

How do you plan to find those cases? Whenever (immediate)parent is 
referenced, you need to add extra if-condition to protect against 
nullpointer dereferencing. For example what is going to happen for 
constraints? The example [2] isn't a real-life example (it makes no 
sense to do anything like that), but through replication [1] and other 
means, you can end up destroying the parent-node and then it's really 
difficult to find the source of possible runtime errors.

> 
> Both of your examples remind me of the chainsaw user who cuts the branch 
> he is standing on and then complains that the chainsaw did not prevent 
> him from a horrible accident.  Are there really nodes that believe they 
> can delete their own parent (knowing that will result in their own 
> deletion)?

For example an one-way message dialog, if you close the dialog it should 
be destroyed, too. So you add a button which calls destroy on the 
parent-node on "onclick".

And I can also make the examples less obvious [1]:
- when replication is set to lazy, everything works
- but when replication is "normal", the app breaks and produces the same 
error as in my earlier example

[1]
<canvas debug="true" >
   <dataset name="ds" ><item /></dataset>
   <view width="100%" height="100%" layout="axis:y" >
     <view>
       <datapath xpath="ds:/item" replication="normal" />
       <view width="180" height="40" bgcolor="#eaeaea" clickable="true" 
focusable="true" >
         <text text="click and then press tab" />
         <handler name="onclick" >
           canvas.ds.appendChild(new lz.DataElement("item"));
         </handler>
       </view>
     </view>
   </view>
</canvas>

[2]
<canvas debug="true" >
   <view>
     <view width="100%" height="100%" bgcolor="#eaeaea" >
       <handler name="onwidth" >
         this.parent.destroy();
       </handler>
     </view>
   </view>
</canvas>