// Copyright 2016 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
package rpc_test
import (
"context"
"fmt"
"math/big"
"time"
"github.com/ethereum/go-ethereum/rpc"
)
// In this example, our client whishes to track the latest 'block number'
// known to the server. The server supports two methods:
//
// eth_getBlockByNumber("latest", {})
// returns the latest block object.
//
// eth_subscribe("newBlocks")
// creates a subscription which fires block objects when new blocks arrive.
type Block struct {
Number *big.Int
}
func ExampleClientSubscription() {
// Connect the client.
client, _ := rpc.Dial("ws://127.0.0.1:8485")
subch := make(chan Block)
// Ensure that subch receives the latest block.
go func() {
for i := 0; ; i++ {
if i > 0 {
time.Sleep(2 * time.Second)
}
subscribeBlocks(client, subch)
}
}()
// Print events from the subscription as they arrive.
for block := range subch {
fmt.Println("latest block:", block.Number)
}
}
// subscribeBlocks runs in its own goroutine and maintains
// a subscription for new blocks.
func subscribeBlocks(client *rpc.Client, subch chan Block) {
ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
defer cancel()
// Subscribe to new blocks.
sub, err := client.EthSubscribe(ctx, subch, "newHeads")
if err != nil {
fmt.Println("subscribe error:", err)
return
}
// The connection is established now.
// Update the channel with the current block.
var lastBlock Block
if err := client.CallContext(ctx, &lastBlock, "eth_getBlockByNumber", "latest"); err != nil {
fmt.Println("can't get latest block:", err)
return
}
subch <- lastBlock
// The subscription will deliver events to the channel. Wait for the
// subscription to end for any reason, then loop around to re-establish
// the connection.
fmt.Println("connection lost: ", <-sub.Err())
}
