Handler message delivery mechanism

Handler Introduction

Android Specifies that only the UI thread is allowed to modify the UI components in a ctivity . When the program starts for the first time, it launches a main thread, often referred to as the UI thread. If the newly-started thread wants to change the value of the attribute in the interface component, use the Handler message-passing mechanism.

Program Examples:

The layout file contains only one Imageview component.

1  Public classMainactivityextendsappcompatactivity {2 3  4 5  6 7     int[] Imageids =New int[]{8 9 R.drawable.java,Ten  One r.drawable.ee, A  - R.drawable.ajax, -  the R.drawable.xml, -  - R.drawable.classic -  +     }; -  +     intCurrentimageid = 0; A  at   -  - @Override -  -     protected voidonCreate (Bundle savedinstancestate) { -  in         Super. OnCreate (savedinstancestate); -  to Setcontentview (r.layout.activity_main); +  -   the  *         FinalImageView show =(ImageView) Findviewbyid (r.id.show); $ Panax Notoginseng       final Handler MyHandler = new Handler () {@Override40//import Android.os.handler;4                     2 public void Handlemessage (Message msg) {msg.what = = 0x1233) {46 47 Show.setimageresource (imageids[currentimageid++% imageids.length]); 48 49}50 51}52 5 3}; Wu  -   About  $         NewTimer (). Schedule (NewTimerTask () { -  - @Override -  A              Public voidrun () { +  theMyhandler.sendemptymessage (0x1233); -  $             } the  the}, 0, 1200); the  the     } -  in}

The essence of the TimerTask object is to start a new thread and therefore cannot make changes to the UI component.

The program passes a timer method timed to pass an empty message (0x1233) to MyHandler, and then the main thread handlemessage to handle the elimination ( 0x1233)

Handler Working principle

• message:  handler The message objects that are accepted and processed.
• looper:  Each thread has only one looper< Span lang= "ZH-CN", which is responsible for managing messagequeuemessagequeue< Span lang= "ZH-CN" to remove the message and divide the message into the corresponding handler processing.
• messagequeue:   looper responsible for management. It uses FIFO to manage message
• handler:  He could send the message to looper managed messagequeuelooper

steps to use Handler in a thread:

1. Calling Looper 's preper () method creates a Looper object for the current thread , and when the Looper object is created, its constructor creates a companion MessageQueue .
2. With Looper , create an instance of the Handler subclass, overriding the Handlrmessage () method, which is responsible for processing messages from other threads.
3. Call Looper 's Loop () method to start Looper.

Instance:

The layout file contains a key and a text box

where the key contains the property OnClick = "cal" Cal is the method declared in the activity.

1  2 3  Public classMainactivityextendsappcompatactivity {4 5  6 7     Static FinalString upper_num = "UPPER";8 9 EditText Etnum;Ten  One Calthread Calthread; A  -   -  the @Override -  -     protected voidonCreate (Bundle savedinstancestate) { -  +         Super. OnCreate (savedinstancestate); -  + Setcontentview (r.layout.activity_main); A  at   -  -Etnum =(EditText) Findviewbyid (r.id.etnum); -  -Calthread =NewCalthread (); -  in Calthread.start (); -  to     } +  -   the  *      Public voidcal (View source) { $ Panax NotoginsengMessage msg =NewMessage (); -  theMsg.what = 0x1233; +  ABundle bundle =NewBundle (); the  + Bundle.putint (Upper_num, Integer.parseint (Etnum.gettext (). toString ())); -  $ Msg.setdata (bundle); $  - calThread.mHandler.sendMessage (msg); -  the     } - Wuyi   the  -     Private classCalthreadextendsThread { Wu  -   About  $          PublicHandler Mhandler; -  - @Override -  A          Public voidrun () { +  the Looper.prepare (); -  $            Mhandler = new Handler (){ the  the @Override the  the                Public void handlemessage (Message msg) { -  in                     if(Msg.what = = 0x1233){ the  the                         intUpper =msg.getdata (). GetInt (upper_num); About  theList<integer> nums =NewArraylist<integer>(); the  the outer: +  -                              for(inti = 2; I <= Upper; i++){ the Bayi                                  for(intj = 2; J <= Math.sqrt (i); J + +){ the  the                                     if(I! = 2 && i% j = 0){ -  -                                         Continueouter; the  the                                     } the  the                                 } -  the Nums.add (i); the  the                             }94  theToast.maketext (mainactivity. This, Nums.tostring (), the  the  　　　　　　　　　　　　　　　　　　 Toast.length_long). Show ();98  About                     } - 101                 }102 103             };104  the Looper.loop ();106 107         }108 109     } the 111}

Handler message delivery mechanism