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Add (polling) Reactive Extensions for filters
nethereum
C#, JavaScript, Visual Basic, Solidity, HTML, Batchfile, TypeScript, CSS, Shell, ASP
# Filter changes
Keeping track of incoming changes of the blockchain state is at the moment rather cumbersome. One method of doing so, which we'll focus on in this issue, is by:
1. Registering a new filter using the filter service.
2. Initiate a polling mechanism that periodically looks for any state changes.
3. Parse one or more multiple state changes.
4. Deregister (dispose?) the registered filter when done.
The other method makes use of websockets and subscription services, which out of this issue's scope. It is likely however, that in the future the same proposed extensions can be re-implemented with these techniques.
In addition to this filter polling setup, you'll have to take care of the fact that:
1. Practically all operations are asynchronous.
2. You might want control over the polling logic, such that you can change the polling sample rate as you wish.
3. Parsing filter results often requires single usage of other services such as `BlockService`.
It is possible to encapsulate this complex logic by offering simple observable reactive extensions. Not only simplifies this the creation of filter observers, it also
1. Reduce filter bloat by simply reusing the same polling observable.
2. Simplify unregistering by simply disposing the subscriptions you no longer need.
3. Offers the power and flexibility of combining multiple event streams.
# Suggested extension methods.
My suggestion is to offer the following extensions regarding blocks, transactions and pending transactions:
```
namespace Nethereum.Spelunker.Extensions
{
public static class IEthApiServicePollingExtensions
{
// Creates a stream of the latest blocks with transaction hashes.
public static IObservable GetBlocksWithTransactionHashes(
this IEthApiService eth);
// Creates a stream of the latest blocks with transaction hashes, starting from the given block number.
public static IObservable GetBlocksWithTransactionHashes(
this IEthApiService eth, BigInteger from);
// Creates a stream of the latest blocks with transactions
public static IObservable GetBlocksWithTransactions(
this IEthApiService eth);
// Creates a stream of the latest blocks with transaction hashes, starting from the given block number.
public static IObservable GetBlocksWithTransactions(
this IEthApiService eth, BigInteger from);
// Creates a stream of the latest transactions.
public static IObservable GetTransactions(
this IEthApiService eth);
// Creates a stream of the latest transactions, starting from the given block number.
public static IObservable GetTransactions(
this IEthApiService eth, BigInteger from);
// Creates a stream of the latest pending transactions.
public static IObservable GetPendingTransactions(
this IEthApiService eth);
// Creates a stream of the latest pending transactions, starting from the given block number..
public static IObservable GetPendingTransactions(
this IEthApiService eth, BigInteger from);
}
}
```
# Polling methods
When users can't be bothering configuring a custom polling method, we may simply define a default polling method:
```
public static IObservable DefaultPoller = Observable
.Timer(TimeSpan.FromMilliseconds(100))
.Select(_ => Unit.Default);
```
The extension methods will look for filter changes each time the observable emits a `Unit.Default`.
If the users desires to use their own custom polling logic, we may offer an overloaded extension method that accepts an additional parameter `poller`, for example:
```
public static IObservable GetTransactions(
this IEthApiService eth,
IObservable poller);
```
# Example
```
// Creates a stream that returns viewmodels for the 10 latest mined blocks.
// An alternative method could be achieved by making use of the reactive .Scan() method.
var last10blocksSubscription = Observable
.FromAsync(() => eth.Blocks.GetBlockNumber.SendRequestAsync())
.SelectMany(blockNumber =>
eth.GetBlocksWithTransactionHashes(from: blockNumber.Value - 10))
.ObserveOnDispatcher()
.Select(BlockViewModel.FromBlock)
.Subscribe(block =>
{
Blocks.Add(block);
Blocks = Blocks
.OrderByDescending(b => b.BlockNumber)
.Take(10)
.ToList();
})
// when you're done, you can simply dispose the subscription.
last10blocksSubscription.Dispose();
```
# Last remark
The three suggested streams for blocks, transactions and pending transactions should serve us a solid base for further reactive extensions, such as personal balance changes.
For more inspiration we may look at https://docs.web3j.io/filters.html.
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