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- // 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, "newBlocks")
- 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())
- }
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