1# Worker Thread Synchronously Calling Methods of the Host Thread
2
3If the Worker thread needs to call the method that has been implemented in the main thread, you can perform the following operations:
4
5The following uses an example in which the worker synchronously calls the host thread interface for description.
6
71. First, implement the method in the host thread, create a **Worker** object, and register the method on the **Worker** object.
8
9   ```ts
10   // IconItemSource.ets
11   export class IconItemSource {
12     image: string | Resource = '';
13     text: string | Resource = '';
14
15     constructor(image: string | Resource = '', text: string | Resource = '') {
16       this.image = image;
17       this.text = text;
18     }
19   }
20   ```
21
22   ```ts
23   // WorkerCallGlobalUsage.ets
24   import worker from '@ohos.worker';
25   import { IconItemSource } from './IconItemSource';
26
27   // Create a Worker object.
28   const workerInstance: worker.ThreadWorker = new worker.ThreadWorker("entry/ets/pages/workers/Worker.ts");
29
30   class PicData {
31     public iconItemSourceList: IconItemSource[] = [];
32
33     public setUp(): string {
34       for (let index = 0; index < 20; index++) {
35         const numStart: number = index * 6;
36         // Six images are used cyclically.
37         this.iconItemSourceList.push(new IconItemSource('$media:startIcon', `item${numStart + 1}`));
38         this.iconItemSourceList.push(new IconItemSource('$media:background', `item${numStart + 2}`));
39         this.iconItemSourceList.push(new IconItemSource('$media:foreground', `item${numStart + 3}`));
40         this.iconItemSourceList.push(new IconItemSource('$media:startIcon', `item${numStart + 4}`));
41         this.iconItemSourceList.push(new IconItemSource('$media:background', `item${numStart + 5}`));
42         this.iconItemSourceList.push(new IconItemSource('$media:foreground', `item${numStart + 6}`));
43
44       }
45       return "setUpIconItemSourceList success!";
46     }
47   }
48
49   let picData = new PicData();
50   // Register the method on the Worker object.
51   workerInstance.registerGlobalCallObject("picData", picData);
52   workerInstance.postMessage("run setUp in picData");
53   ```
54
552. Then, the setUp () method in the host thread can be called through the callGlobalCallObjectMethod interface in the worker.
56
57   ```ts
58   // Worker.ets
59   import { ErrorEvent, MessageEvents, ThreadWorkerGlobalScope, worker } from '@kit.ArkTS';
60   const workerPort: ThreadWorkerGlobalScope = worker.workerPort;
61   try {
62     // The method to call does not carry an input parameter.
63     let res: string = workerPort.callGlobalCallObjectMethod("picData", "setUp", 0) as string;
64     console.error("worker: ", res);
65   } catch (error) {
66     // Exception handling.
67     console.error("worker: error code is " + error.code + " error message is " + error.message);
68   }
69   ```
70