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3.3 A Brief Analysis of Handler Message Passing Mechanism

Category Android Basic Tutorial

Introduction to This Section

In the previous two sections, we studied two event handling mechanisms in Android. Regarding the response to events, there are just these two types; this section will explain to you the information transmission of the Handler in the UI components of the Activity. I believe many friends already know that for thread safety, Android does not allow us to operate the UI outside the UI thread; many times when we refresh the interface, we need to notify the UI components to update through a Handler! In addition to using Handler to complete the interface update, you can also use runOnUiThread() to update, or even more advanced transaction buses, of course, here we only explain the Handler, what is a Handler, the execution process, related methods, and the differences in using Handler in the child thread and the main thread, etc.!

1. Learning Roadmap:

2. Introduction to the Handler Class:

3. Execution Flowchart of Handler:

Flowchart Analysis: Related Terms

>

In simpler terms:

>

When our child thread wants to modify the UI components in the Activity, we can create a new Handler object and send messages to the main thread through this object; the messages we send will first wait in the main thread's MessageQueue, and then the Looper will take them out in the order of first in first out, and distribute them to the corresponding Handler for processing according to the what attribute of the message object!

4. Related Methods of Handler:

>

5. Example of Using Handler:

1) Handler Written in the Main Thread

>

In the main thread, since the system has already initialized a Looper object, we can directly create a Handler object to send and process information!

Code Example: A simple program that switches pictures on a timer, using a Timer to periodically modify the content displayed by the ImageView, thus forming a frame animation.

Running Effect Picture:

Implementation Code:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"  
    xmlns:tools="http://schemas.android.com/tools"  
    android:id="@+id/RelativeLayout1"  
    android:layout_width="match_parent"  
    android:layout_height="match_parent"  
    android:gravity="center"  
    tools:context="com.jay.example.handlerdemo1.MainActivity" >  

    <ImageView  
        android:id="@+id/imgchange"  
        android:layout_width="wrap_content"  
        android:layout_height="wrap_content"  
        android:layout_alignParentLeft="true"  
        android:layout_alignParentTop="true" />  

</RelativeLayout>

MainActivity.java:

``` public class MainActivity extends Activity { 

// Define the array id for switching pictures  
int imgids[] = new int[]{  
    R.drawable.s_1, R.drawable.s_2,R.drawable.s_3,  
    R.drawable.s_4,R.drawable.s_5,R.drawable.s_6,  
    R.drawable.s_7,R.drawable.s_8

{ // Use j to iterate through all numbers starting from 2 up to the square root of i for (int j = 2; j <= Math.sqrt(i); j++) { // If divisible, it indicates that the number is not a prime if (i != 2 && i % j == 0) { continue outer; } } nums.add(i); } // Use Toast to display all the prime numbers counted Toast.makeText(CalPrime.this, nums.toString(), Toast.LENGTH_LONG).show(); } }; Looper.loop(); }

@Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); etNum = (EditText) findViewById(R.id.etNum); calThread = new CalThread(); // Start a new thread calThread.start(); }

// Provide an event handling function for the button's click event public void cal(View source) { // Create a message Message msg = new Message(); msg.what = 0x123; Bundle bundle = new Bundle(); bundle.putInt(UPPER_NUM, Integer.parseInt(etNum.getText().toString())); msg.setData(bundle); // Send a message to the Handler in the new thread calThread.mHandler.sendMessage(msg); } }

PS: This example is from "Android Craziness Lecture Notes"~

Summary of This Section

This section provides a simple analysis of the Handler event passing in Android, clarifying the concepts of Handler, Message, MessageQueue, and Loop, as well as the differences between writing Handlers in the main thread and in a child thread!

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