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// In this example, our client wishes 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("newHeads")
// creates a subscription which fires block objects when new blocks arrive.
type Block struct {
Number *hexutil.Big
}
func ExampleClientSubscription() {
// Connect the client.
client, _ := rpc.Dial("ws://127.0.0.1:8545")
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
err = client.CallContext(ctx, &lastBlock, "eth_getBlockByNumber", "latest", false)
if 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())
}