IDL Specifications and User Guide (for System Applications Only)

IDL Overview

To ensure successful communications between the client and server, interfaces recognized by both parties must be defined. The OpenHarmony Interface Definition Language (IDL) is a tool for defining such interfaces. OpenHarmony IDL decomposes objects to be transferred into primitives that can be understood by the operating system and encapsulates cross-boundary objects based on developers’ requirements.

Figure 1 IDL interface description

IDL provides the following functions:

• Declares interfaces provided by system services for external systems, and based on the interface declaration, generates C, C++, JS, or TS code for inter-process communication (IPC) or remote procedure call (RPC) proxies and stubs during compilation.

• Declares interfaces provided by abilities for external systems, and based on the interface declaration, generates C, C++, JS, or TS code for IPC or RPC proxies and stubs during compilation.

Figure 2 IPC/RPC communication model

IDL has the following advantages:

• Services are defined in the form of interfaces in IDL. Therefore, you do not need to focus on implementation details.

• Interfaces defined by IDL can be used in IPC or RPC scenarios. The information or code generated based on the definitions in IDL simplifies IPC or RPC implementation.

IDL File Structure

Data Types

Primitive Type

The preceding table lists the primitive types supported by IDL and the mappings to the C++ and TS primitive types.

sequenceable Type

The sequenceable type is declared using the keyword sequenceable. This type can be passed during IPC or RPC through Parcel objects. The declaration mode of the sequenceable type in C++ is different from that in TS.

In C++, the declaration is placed in the file header in the format of sequenceable includedir..namespace.typename. It can be in any of the following forms:

sequenceable includedir..namespace.typename
sequenceable includedir...typename
sequenceable namespace.typename

In the preceding information, includedir indicates the directory where the header file of the type is located, and the dot (.) is used as the separator. namespace indicates the namespace where the type is located, and the dot (.) is used as the separator. typename indicates the data type, which can contain only English characters. includedir and namespace are separated by two dots (..). If the declaration statement does not contain two dots, all characters except the last typename will be parsed as a namespace. Example:

sequenceable a.b..C.D

The preceding statement is parsed into the following code in the C++ header file:

#include "a/b/d.h"
using C::D;

In TS, the declaration is placed in the file header in the format of sequenceable namespace.typename;. An example is provided below, where idl is the namespace and MySequenceable is the type name:

sequenceable idl.MySequenceable

In the preceding information, namespace indicates the namespace to which the data type belongs, typename indicates the data type name, and MySequenceable indicates that data can be passed during IPC using Parcel objects. The sequenceable type is not defined in the IDL file, but in the .ts file. Therefore, IDL adds the following statement to the generated .ts file based on the declaration:

import MySequenceable from "./my_sequenceable"

Note that IDL does not implement code for this type. It only imports the header file in the specified format or imports the specified module and uses the type. Therefore, you must ensure that the imported directory, namespace, and type are correct.

Interface Type

The interface type refers to interfaces defined in IDL files. The interfaces defined in an IDL file can be directly used as the parameter type or return value type of a method declared in the file. If an IDL file attempts to use interfaces defined in other IDL files, forward declaration must be contained in the header of that IDL file.

The declaration form in C++ is similar to that of the sequenceable type. The declaration form is as follows:

interface includedir..namespace.typename

In TS, the declaration form is as follows:

interface namespace.interfacename

In the preceding information, namespace indicates the namespace to which the interface belongs, and interfacename indicates the name of the interface. For example, interface OHOS.IIdlTestObserver; declares the IIdlTestObserver interface defined in another IDL file. This interface can be used as the parameter type or return value type of a method in the current file. IDL adds the following statement to the generated .ts file based on the statement:

import IIdlTestObserver from "./i_idl_test_observer"

Array Type

The array type is represented by T[], where T can be the primitive, sequenceable, interface, or array type. In C++, this type is generated as std::vector<T>. The table below lists the mappings between the IDL array type and TS and C++ data types.

Container Type

IDL supports two container types: List and Map. The List container is represented in the format of List<T>. The Map container is represented in the format of Map, where T, KT, and VT can be of the primitive, sequenceable, interface, array, or container type.

In C++, the List container type is generated as std::list, and the Map container type is generated as std::map.

In TS, the List container type is not supported, and the Map container type is generated as Map.

The table below lists the mappings between the IDL container type and TS and C++ data types.

Specifications for Compiling IDL Files

Only one interface type can be defined in an IDL file, and the interface name must be the same as the file name. The interface definition of the IDL file is described in Backus-Naur form (BNF). The basic definition format is as follows:

[<*interface_attr_declaration*>]interface<*interface_name_with_namespace*>{<*method_declaration*>}

In the preceding information, <interface_attr_declaration> declares interface attributes. Currently, only the oneway attribute is supported, indicating that all methods in the interface are unidirectional. Such a method returns value without waiting for the execution to complete. This attribute is optional. If this attribute is not set, synchronous call is used. The interface name must contain the complete interface header file directory, namespace, and method declaration. Empty interfaces are not allowed. The method declaration format in the interface is as follows:

[<*method_attr_declaration*>]<*result_type*><*method_declaration*>

In the preceding information, <method_attr_declaration> describes the interface attributes. Currently, only the oneway attribute is supported, indicating that the method is unidirectional. Such a method returns value without waiting for the execution to complete. This attribute is optional. If this attribute is not set, synchronous call is used. <result_type> indicates the type of the return value, and <method_declaration> indicates the method name and parameter declaration. The parameter declaration format is as follows:

[<*formal_param_attr*>]<*type*><*identifier*>

The value of <formal_param_attr> can be in, out, or inout, indicating that the parameter is an input parameter, an output parameter, or both an input and an output parameter, respectively. A oneway method does not allow output or inout parameters or return values.

How to Develop

Obtaining IDL

On DevEco Studio, choose Tools > SDK Manager to view the local installation path of the OpenHarmony SDK. The following figure uses DevEco Studio 3.0.0.993 as an example.

Go to the local installation path, choose toolchains > 3.x.x.x (the folder named after the version number), and check whether the executable file of IDL exists.

NOTE

Use the SDK of the latest version. The use of an earlier version may cause errors in some statements.

If the executable file does not exist, download the SDK package from the mirror as instructed in the Release Notes. The following uses 3.2 Beta3 as an example.

For details about how to replace the SDK package, see Full SDK Compilation Guide.

After obtaining the executable file, perform subsequent development steps based on your scenario.

Development Using TS

Creating an IDL File

You can use TS to create IDL files.

For example, create a file named IIdlTestService.idl with the following content:

  interface OHOS.IIdlTestService {
      int TestIntTransaction([in] int data);
      void TestStringTransaction([in] String data);
      void TestMapTransaction([in] Map<int, int> data);
      int TestArrayTransaction([in] String[] data);
  }

Run the idl -gen-ts -d dir -c dir/IIdlTestService.idl command in the folder where the executable file is located.

-dir next to d is the target output folder. For example, if the target output folder is IIdlTestServiceTs, run the idl -gen-ts -d IIdlTestServiceTs -c IIdlTestServiceTs/IIdlTestService.idl command in the folder where the executable file is located. The interface file, stub file, and proxy file are generated in the dir directory (IIdlTestServiceTs directory in this example) in the execution environment.

NOTE

The generated interface class file name must be the same as that of the .idl file. Otherwise, an error occurs during code generation.

For example, for an .idl file named IIdlTestService.idl and target output directory named IIdlTestServiceTs, the directory structure is similar to the following:

├── IIdlTestServiceTs  # IDL code output folder
│   ├── i_idl_test_service.ts  # File generated
│   ├── idl_test_service_proxy.ts  # File generated
│   ├── idl_test_service_stub.ts  # File generated
│   └── IIdlTestService.idl  # Constructed .idl file
└── idl.exe  # Executable file of IDL

Exposing Interfaces on the Server

The stub class generated by IDL is an abstract implementation of the interface class and declares all methods in the IDL file.

import {testIntTransactionCallback} from "./i_idl_test_service";
import {testStringTransactionCallback} from "./i_idl_test_service";
import {testMapTransactionCallback} from "./i_idl_test_service";
import {testArrayTransactionCallback} from "./i_idl_test_service";
import IIdlTestService from "./i_idl_test_service";
import rpc from "@ohos.rpc";

export default class IdlTestServiceStub extends rpc.RemoteObject implements IIdlTestService {
    constructor(des: string) {
        super(des);
    }
    
    async onRemoteMessageRequest(code: number, data: rpc.MessageSequence, reply: rpc.MessageSequence,
        option: rpc.MessageOption): Promise<boolean> {
        console.log("onRemoteMessageRequest called, code = " + code);
        if (code == IdlTestServiceStub.COMMAND_TEST_INT_TRANSACTION) {
            let _data = data.readInt();
            this.testIntTransaction(_data, (errCode: number, returnValue: number) => {
                reply.writeInt(errCode);
                if (errCode == 0) {
                    reply.writeInt(returnValue);
                }
            });
            return true;
        } else if (code == IdlTestServiceStub.COMMAND_TEST_STRING_TRANSACTION) {
            let _data = data.readString();
            this.testStringTransaction(_data, (errCode: number) => {
                reply.writeInt(errCode);
            });
            return true;
        } else if (code == IdlTestServiceStub.COMMAND_TEST_MAP_TRANSACTION) {
            let _data: Map<number, number> = new Map();
            let _dataSize = data.readInt();
            for (let i = 0; i < _dataSize; ++i) {
                let key = data.readInt();
                let value = data.readInt();
                _data.set(key, value);
            }
            this.testMapTransaction(_data, (errCode: number) => {
                reply.writeInt(errCode);
            });
            return true;
        } else if (code == IdlTestServiceStub.COMMAND_TEST_ARRAY_TRANSACTION) {
            let _data = data.readStringArray();
            this.testArrayTransaction(_data, (errCode: number, returnValue: number) => {
                reply.writeInt(errCode);
                if (errCode == 0) {
                    reply.writeInt(returnValue);
                }
            });
            return true;
        } else {
            console.log("invalid request code" + code);
        }
        return false;
    }
    
    testIntTransaction(data: number, callback: testIntTransactionCallback): void{}
    testStringTransaction(data: string, callback: testStringTransactionCallback): void{}
    testMapTransaction(data: Map<number, number>, callback: testMapTransactionCallback): void{}
    testArrayTransaction(data: string[], callback: testArrayTransactionCallback): void{}

    static readonly COMMAND_TEST_INT_TRANSACTION = 1;
    static readonly COMMAND_TEST_STRING_TRANSACTION = 2;
    static readonly COMMAND_TEST_MAP_TRANSACTION = 3;
    static readonly COMMAND_TEST_ARRAY_TRANSACTION = 4;
}

You need to inherit the interface class defined in the IDL file and implement the methods in the class. The following code snippet shows how to inherit the IdlTestServiceStub interface class and implement the testIntTransaction, testStringTransaction, testMapTransaction, and testArrayTransaction methods.

import {testIntTransactionCallback} from "./i_idl_test_service"
import {testStringTransactionCallback} from "./i_idl_test_service"
import {testMapTransactionCallback} from "./i_idl_test_service";
import {testArrayTransactionCallback} from "./i_idl_test_service";
import IdlTestServiceStub from "./idl_test_service_stub"


class IdlTestImp extends IdlTestServiceStub {

    testIntTransaction(data: number, callback: testIntTransactionCallback): void
    {
        callback(0, data + 1);
    }
    testStringTransaction(data: string, callback: testStringTransactionCallback): void
    {
        callback(0);
    }
    testMapTransaction(data: Map<number, number>, callback: testMapTransactionCallback): void
    {
        callback(0);
    }
    testArrayTransaction(data: string[], callback: testArrayTransactionCallback): void
    {
        callback(0, 1);
    }
}

After the service implements the interface, the interface needs to be exposed to the client for connection. If your service needs to expose this interface, extend Ability and implement onConnect() to return IRemoteObject so that the client can interact with the service process. The following code snippet shows how to expose the IRemoteAbility interface to the client:

import Want from '@ohos.app.ability.Want';
import rpc from "@ohos.rpc";

class ServiceAbility {
  onStart() {
    console.info('ServiceAbility onStart');
  }
  onStop() {
    console.info('ServiceAbility onStop');
  }
  onCommand(want: Want, startId: number) {
    console.info('ServiceAbility onCommand');
  }
  onConnect(want: Want) {
    console.info('ServiceAbility onConnect');
    try {
      console.log('ServiceAbility want:' + typeof(want));
      console.log('ServiceAbility want:' + JSON.stringify(want));
      console.log('ServiceAbility want name:' + want.bundleName)
    } catch(err) {
      console.log('ServiceAbility error:' + err)
    }
    console.info('ServiceAbility onConnect end');
    return new IdlTestImp('connect') as rpc.RemoteObject;
  }
  onDisconnect(want: Want) {
    console.info('ServiceAbility onDisconnect');
    console.info('ServiceAbility want:' + JSON.stringify(want));
  }
}

export default new ServiceAbility()

Calling Methods from the Client for IPC

When the client calls connectServiceExtensionAbility() to connect to a Service ability, the onConnect callback in onAbilityConnectDone of the client receives the IRemoteObject instance returned by the onConnect() method of the Service ability. The client and Service ability are in different applications. Therefore, the directory of the client application must contain a copy of the .idl file (the SDK automatically generates the proxy class). The onConnect callback then uses the IRemoteObject instance to create the testProxy instance of the IdlTestServiceProxy class and calls the related IPC method. The sample code is as follows:

import common from '@ohos.app.ability.common';
import Want from '@ohos.app.ability.Want';
import IdlTestServiceProxy from './idl_test_service_proxy'

function callbackTestIntTransaction(result: number, ret: number): void {
  if (result == 0 && ret == 124) {
    console.log('case 1 success');
  }
}

function callbackTestStringTransaction(result: number): void {
  if (result == 0) {
    console.log('case 2 success');
  }
}

function callbackTestMapTransaction(result: number): void {
  if (result == 0) {
    console.log('case 3 success');
  }
}

function callbackTestArrayTransaction(result: number, ret: number): void {
  if (result == 0 && ret == 124) {
    console.log('case 4 success');
  }
}

let onAbilityConnectDone: common.ConnectOptions = {
  onConnect: (elementName, proxy) => {
    let testProxy: IdlTestServiceProxy = new IdlTestServiceProxy(proxy);
    let testMap: Map<number, number> = new Map();
    testMap.set(1, 1);
    testMap.set(1, 2);
    testProxy.testIntTransaction(123, callbackTestIntTransaction);
    testProxy.testStringTransaction('hello', callbackTestStringTransaction);
    testProxy.testMapTransaction(testMap, callbackTestMapTransaction);
    testProxy.testArrayTransaction(['1','2'], callbackTestMapTransaction);
  },
  onDisconnect: (elementName) => {
    console.log('onDisconnectService onDisconnect');
  },
  onFailed: (code) => {
    console.log('onDisconnectService onFailed');
  }
};

let context: common.UIAbilityContext = this.context;

function connectAbility(): void {
    let want: Want = {
        bundleName: 'com.example.myapplicationidl',
        abilityName: 'com.example.myapplicationidl.ServiceAbility'
    };
    let connectionId = -1;
    connectionId = context.connectServiceExtensionAbility(want, onAbilityConnectDone);
}

Transferring a sequenceable Object During IPC

You can send a class from one process to another through IPC interfaces. However, you must ensure that the peer can use the code of this class and this class supports the marshalling and unmarshalling methods. OpenHarmony uses marshalling and unmarshalling to serialize and deserialize objects into objects that can be identified by each process.

To create a class that supports the sequenceable type, perform the following operations:

1. Implement the marshalling method, which obtains the current state of the object and serializes the object into a Parcel object.
2. Implement the unmarshalling method, which deserializes the object from a Parcel object.

The following is an example of the MySequenceable class code:

import rpc from '@ohos.rpc';
export default class MySequenceable implements rpc.Sequenceable {
    constructor(num: number, str: string) {
        this.num = num;
        this.str = str;
    }
    getNum() : number {
        return this.num;
    }
    getString() : string {
        return this.str;
    }
    marshalling(messageParcel: rpc.MessageParcel) {
        messageParcel.writeInt(this.num);
        messageParcel.writeString(this.str);
        return true;
    }
    unmarshalling(messageParcel: rpc.MessageParcel) {
        this.num = messageParcel.readInt();
        this.str = messageParcel.readString();
        return true;
    }
    private num: number;
    private str: string;
}

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