我什么时候应该使用未来建设者？

FutureBuilder removes boilerplate code.

Let's say you want to fetch some data from the backend on page launch and show a loader until data comes.

Tasks for ListBuilder:

• Have two state variables, dataFromBackend and isLoadingFlag
• On launch, set isLoadingFlag = true, and based on this, show loader.
• Once data arrives, set data with what you get from backend and set isLoadingFlag = false (inside setState obviously)
• We need to have a if-else in widget creation. If isLoadingFlag is true, show the loader else show the data. On failure, show error message.

Tasks for FutureBuilder:

• Give the async task in future of Future Builder
• Based on connectionState, show message (loading, active(streams), done)
• Based on data(snapshot.hasError), show view

Pros of FutureBuilder

• Does not use the two state variables and setState
• Reactive programming (FutureBuilder will take care of updating the view on data arrival)

Example:

FutureBuilder<String>(
future: _fetchNetworkCall, // async work
builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
switch (snapshot.connectionState) {
case ConnectionState.waiting: return Text('Loading....');
default:
if (snapshot.hasError)
return Text('Error: ${snapshot.error}');
else
return Text('Result: ${snapshot.data}');
}
},
)

Performance impact:

I just looked into the FutureBuilder code to understand the performance impact of using this.

• FutureBuilder is just a StatefulWidget whose state variable is _snapshot
• Initial state is _snapshot = AsyncSnapshot<T>.withData(ConnectionState.none, widget.initialData);
• It is subscribing to future which we send via the constructor and update the state based on that.

Example:

widget.future.then<void>((T data) {
if (_activeCallbackIdentity == callbackIdentity) {
setState(() {
_snapshot = AsyncSnapshot<T>.withData(ConnectionState.done, data);
});
}
}, onError: (Object error) {
if (_activeCallbackIdentity == callbackIdentity) {
setState(() {
_snapshot = AsyncSnapshot<T>.withError(ConnectionState.done, error);
});
}
});

So the FutureBuilder is a wrapper/boilerplate of what we do typically, hence there should not be any performance impact.

FutureBuilder Example

• When you want to rander widget after async call then use FutureBuilder()

  class _DemoState extends State<Demo> {
  
  
  @override
  Widget build(BuildContext context) {
  return FutureBuilder<String>(
  future: downloadData(), // function where you call your api
  builder: (BuildContext context, AsyncSnapshot<String> snapshot) {  // AsyncSnapshot<Your object type>
  if( snapshot.connectionState == ConnectionState.waiting){
  return  Center(child: Text('Please wait its loading...'));
  }else{
  if (snapshot.hasError)
  return Center(child: Text('Error: ${snapshot.error}'));
  else
  return Center(child: new Text('${snapshot.data}'));  // snapshot.data  :- get your object which is pass from your downloadData() function
  }
  },
  );
  }
  Future<String> downloadData()async{
  //   var response =  await http.get('https://getProjectList');
  return Future.value("Data download successfully"); // return your response
  }
  }
  

In future builder, it calls the future function to wait for the result, and as soon as it produces the result it calls the builder function where we build the widget.

AsyncSnapshot has 3 state:

1. connectionState.none = In this state future is null
   
   
2. connectionState.waiting = [future] is not null, but has not yet completed
   
   
3. connectionState.done = [future] is not null, and has completed. If the future completed successfully, the [AsyncSnapshot.data] will be set to the value to which the future completed. If it completed with an error, [AsyncSnapshot.hasError] will be true

FutureBuilder is a Widget that will help you to execute some asynchronous function and based on that function’s result your UI will update.

I listed some use cases, why you will use FutureBuilder?

1. If you want to render widget after async task then use it.

2. We can handle loading process by simply using ConnectionState.waiting

3. Don't need any custom error controller. Can handle error simply dataSnapshot.error != null

4. As we can handle async task within the builder we do not need any setState(() { _isLoading = false; });

When we use the FutureBuilder widget we need to check for future state i.e future is resolved or not and so on. There are various State as follows:

1. ConnectionState.none: It means that the future is null and initialData is used as defaultValue.

2. ConnectionState.active: It means the future is not null but it is not resolved yet.

3. ConnectionState.waiting: It means the future is being resolved, and we will get the result soon enough.

4. ConnectionState.done: It means that the future has been resolved.

A simple implementation

Here OrdersProvider is a provider class and fetchAndSetOrders() is the method of that provider class.

body: FutureBuilder(
future: Provider.of<OrdersProvider>(context, listen: false)
.fetchAndSetOrders(),
builder: (context, dataSnapshot) {
if (dataSnapshot.connectionState == ConnectionState.waiting) {
return Center(
child: CircularProgressIndicator(),
);
} else {
if (dataSnapshot.error != null) {
return Center(
child: Text('An error occured'),
);
} else {
return Consumer<OrdersProvider>(
builder: (context, orderData, child) => ListView.builder(
itemCount: orderData.orders.length,
itemBuilder: (context, i) => OrderItem(orderData.orders[i]),
),
);
}
}
},
),