examples: add a perfect oracle example

examples/README.md

@@ -25,6 +25,7 @@ See the table below for the detailed examples description.
 | [engine](engine) | This contract demonstrates how to use `runtime` interop package which implements an API for `System.Runtime.*` NEO system calls. Please, refer to the `runtime` [package documentation](../pkg/interop/doc.go) for details. |
 | [events](events) | The contract shows how execution notifications with the different arguments types can be sent with the help of `runtime.Notify` function of the `runtime` interop package. Please, refer to the `runtime.Notify` [function documentation](../pkg/interop/runtime/runtime.go) for details. |
 | [iterator](iterator) | This example describes a way to work with NEO iterators. Please, refer to the `iterator` [package documentation](../pkg/interop/iterator/iterator.go) for details. |
+| [oracle](oracle) | Oracle demo contract exposing two methods that you can use to process URLs. It uses oracle native contract, see [interop package documentation](../pkg/interop/native/oracle/oracle.go) also. |
 | [runtime](runtime) | This contract demonstrates how to use special `_initialize` and `_deploy` methods. See the [compiler documentation](../docs/compiler.md#vm-api-interop-layer ) for methods details. It also shows the pattern for checking owner witness inside the contract with the help of `runtime.CheckWitness` interop [function](../pkg/interop/runtime/runtime.go). |
 | [storage](storage) | The contract implements API for basic operations with a contract storage. It shows hos to use `storage` interop package. See the `storage` [package documentation](../pkg/interop/storage/storage.go). |
 | [timer](timer) | The idea of the contract is to count `tick` method invocations and destroy itself after the third invocation. It shows how to use `contract.Call` interop function to call, update (migrate) and destroy the contract. Please, refer to the `contract.Call` [function documentation](../pkg/interop/contract/contract.go) |

examples/oracle/oracle.go

@@ -0,0 +1,36 @@
+package oraclecontract
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/interop/native/oracle"
+	"github.com/nspcc-dev/neo-go/pkg/interop/native/std"
+	"github.com/nspcc-dev/neo-go/pkg/interop/runtime"
+)
+
+// Request does an oracle request for the URL specified. It adds minimum
+// response fee which should suffice for small requests. The data from this
+// URL is subsequently processed by OracleCallback function. This request
+// has no JSONPath filters or user data.
+func Request(url string) {
+	oracle.Request(url, nil, "oracleCallback", nil, oracle.MinimumResponseGas)
+}
+
+// FilteredRequest is similar to Request but allows you to specify JSONPath filter
+// to run against data got from the url specified.
+func FilteredRequest(url string, filter []byte) {
+	oracle.Request(url, filter, "oracleCallback", nil, oracle.MinimumResponseGas)
+}
+
+// OracleCallback is called by Oracle native contract when request is finished.
+// It either throws an error (if the result is not successful) or logs the data
+// got as a result.
+func OracleCallback(url string, data interface{}, code int, res []byte) {
+	// This function shouldn't be called directly, we only expect oracle native
+	// contract to be calling it.
+	if string(runtime.GetCallingScriptHash()) != oracle.Hash {
+		panic("not called from oracle contract")
+	}
+	if code != oracle.Success {
+		panic("request failed for " + url + " with code " + std.Itoa(code, 10))
+	}
+	runtime.Log("result for " + url + ": " + string(res))
+}

examples/oracle/oracle.yml

@@ -0,0 +1,3 @@
+name: "Oracle example"
+supportedstandards: []
+events: