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Posted on Jun 26

🗝️ Huawei Hongmeng Context In-depth Analysis: The Practical Guide to the Application of "Master Key"

As a developer who once caused memory leaks due to improper use of Context, today we will reveal the HarmonyOS Context!The first time I misuse Context, the APP crashed, and now I will make the correct usage of this "app master key" public~

1. Context: The "universal ID card" used

1. What is Context?

Imagine you entering the mall:

Context is your "Mall Pass", with it: ✅ Enter the store (start component) ✅ Get map (get resources) ✅ Use elevator (system adjustment service)

HarmonyOS的Context核心能力：

Resource access: Get strings, pictures and other resources
Component Start: Open a new page, adjust the background service
System interaction: Use sensors, store data, etc.

2. Two types: "different permissions" of passes

Type	Permission Range	Analog
AbilityContext	Permissions for a single Ability	Store-only pass
ApplicationContext	All application permissions	Shopping Mall Universal Pass

2. Obtain Context: Find the right "certificate issuing authority"

1. Get it in Ability (most commonly used)

import { UIAbility } from '@ohos.app.ability';

export default class MainAbility extends UIAbility {
  onStart() {
// Method 1: Get it directly
    const context1 = this.context;

// Method 2: By applying the context
    const context2 = this.getApplicationContext();

    console.log(`AbilityContext: ${context1}`);
    console.log(`ApplicationContext: ${context2}`);
  }
}

2. Get it in AbilitySlice

import { AbilitySlice } from '@ohos.app.ability';

export class MainSlice extends AbilitySlice {
  onPageShow() {
// Get Context from the Ability
    const context = this.ability.context;
console.log(Context obtained in `Slice: ${context}`);
  }
}

3. Get it in custom components

import { getContext } from '@ohos.app.ability';

// Get it from the custom component
const context = getContext(this) as typeof import('@ohos.app.ability').Context;

3. File operation: Use Context to open the "file drawer"

1. Common file path acquisition

Method	Purpose	Sample Code
getFilesDir()	Application Data Directory	const filesDir = context.getFilesDir();
getCacheDir()	CacheDir (will be cleaned)	const cacheDir = context.getCacheDir();
getEncryptedFilesDir()	Encrypted directory (storage sensitive data)	const encryptedDir = context.getEncryptedFilesDir();

2. Practical combat: Save user data to the encrypted directory

// Check whether the device is encrypted
if (context.isDeviceEncrypted()) {
// Get the encrypted directory
  const encryptedDir = context.getEncryptedFilesDir().path;

// Store sensitive data (such as payment information)
  const filePath = `${encryptedDir}/payment_info.json`;
  fs.writeFileSync(filePath, JSON.stringify(paymentData));
}

4. Cross-component communication: Context's "Walkjet" function

1. EventHub: Broadcasting between components

// Subscribe to events (in the target component)
context.eventHub.on('newMessage', (data) => {
console.log(`Message received: ${data}`);
  this.updateUI(data);
});

// Send event (at the sender)
context.eventHub.publish('newMessage', 'Hello from Context!');

2. AppStorage: All apps share data

// Store data (such as user information)
context.appStorage.put('userInfo', { name: 'HarmonyUser', level: 'VIP' });

// Get data (in other components)
const userInfo = context.appStorage.get('userInfo');
console.log(`user information: ${userInfo.name}`);

3. LocalStorage: component-specific data

// Store data (only available to the current component)
context.localStorage.put('componentState', { isLoading: false });

// Get data
const state = context.localStorage.get('componentState');
if (!state.isLoading) {
  this.loadData();
}

5. Practical pit avoidance: "Using specifications" of passes

1. Memory leak risk (the easiest pit)

// Error demonstration: Long life cycle object holds short life cycle Context
let badContext: Context;

function wrongWay(abilityContext: AbilityContext) {
badContext = abilityContext; // Dangerous!Context leaks after Ability is destroyed
}

// Correct way: Use ApplicationContext
function rightWay(context: Context) {
// Use application-level Context for longer life cycle
  const appContext = context.getApplicationContext();
}

2. Choose the right Context type in different scenarios

Scenario	Correct Context Type	Error Type	Consequence
Start a new Ability	AbilityContext	ApplicationContext	Some features are not available
Global Data Storage	ApplicationContext	AbilityContext	Data Lost after Component Destruction

6. Performance optimization: "Efficient use method" of the pass

1. Cache Context references

export class AppData {
  private static _context: Context;

// Save once and use it everywhere
  static init(context: Context) {
    this._context = context.getApplicationContext();
  }

  static get context() {
    return this._context;
  }
}

// Initialization
AppData.init(context);

// use
AppData.context.getFilesDir();

2. Avoid frequent acquisitions

// Bad writing method (take it every time)
for (let i = 0; i < 100; i++) {
  const dir = this.context.getFilesDir();
// operate...
}

// Optimized writing method (take it once first)
const dir = this.context.getFilesDir();
for (let i = 0; i < 100; i++) {
// Use dir variable
}

7. Interview FAQ: Context’s “soul-totoring”

1. What is the difference between AbilityContext and ApplicationContext?

Life Cycle: AbilityContext is destroyed with Ability, ApplicationContext is destroyed with application exit
Purpose: The former is used to start the component, the latter is used to global data storage

2. Why can't you start an Activity with Application's Context in Activity?

Report android.util.AndroidRuntimeException
Correct way: Use the Context of Activity or create the FLAG_ACTIVITY_NEW_TASK flag

The last thought

I remember the first time I used the Context of Activity in a custom view, which caused the View to hold the Context after the Activity was destroyed, and the memory leaked to the APP crashed~ Now I have developed the habit of using ApplicationContext to store global data, and I am no longer afraid of leaking~

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