Flutter stream builders concept

Question

I have a map in which i store key and value pair. I have put this map in a stream, which we will call dataStream for ease.

now i have a stream builder, this stream-builder uses dataStream. I want this stream builder to build only if a specific key's value is altered.

Currently my stream builder responds to any change in the map. I want to instruct it to change only when a certain key's value in the map changes. Is this possible in stream builder and how?

return StreamBuilder(
    stream: dataBloc.dataStream,
    builder: (context, snapshot) {
     if(snapshot.hasData){
     Map<String, String> dataMap = snapshot.data;
     // some logic
     }
     return widget;
     }

I want to see if a specific key's value in this map changes, only then rebuild my existing widget else, leave it as it is. Do not build it again

Solution

You should be able to filter for specific events using where:

stream: dataBloc.dataStream.where((item) => /* check criteria */ item['key'] == 'foo')

Comments

[GoPro]

Do i access the map which is inside my dataStream using snapshot.data ? I am not sure how to access it in the where clause;

[Günter Zöchbauer]

I see. Didn't recognize this is about Firebase. item is the same what snapshot.data returns in builder: ...

[GoPro]

I have one more question, what happens if my where clause is false; Will it rebuild? Ideally i want it to rebuild if there is a change in the output of the where clause!

[GoPro]

for example, if the condition is satisfied build, if it is not satisfied build but differently.

[Günter Zöchbauer]

If the where clause is false, the event will be filtered and not passed to the listener (StreamBuilder), so no, it's as if the event were never sent.

[Günter Zöchbauer]

"if there is a change in the output of the where clause!" sorry, but I don't understand what that means.

[GoPro]

Let us continue this discussion in chat.