handlemessage:全面解析Android进阶面试常客之Handler

 2021-07-01 8:53    77  

前言:又到了一年一度的跳槽季,准备跳槽的你在关于Android面试方面的知识都完全掌握了吗handlemessage?Android面试中经常被问到的知识——Android消息机制即Handler有关的问题你都能解释的清楚吗?如果你对Android消息机制比较模糊或者能够回答与Handler有关的问题但是不清楚其中的原理,那么你将会在本文得到你想要的答案。

handlemessage:全面解析Android进阶面试常客之Handler

阅读本文后的收货

handlemessage:全面解析Android进阶面试常客之Handler

阅读本文后你将会有以下收获handlemessage:

handlemessage:全面解析Android进阶面试常客之Handler

清楚的理解Handler的工作原理理清Handler、Message、MessageQueue以及Looper之间的关系知道Looper是怎么和当前线程进行绑定的是否能在子线程中创建Handler获得分析Handler源码的思路要想有以上的收获,就需要研究Handler的源码,从源码中来得到答案handlemessage。

handlemessage:全面解析Android进阶面试常客之Handler

开始探索之路

Handler的使用

先从Handler的使用开始。我们都知道Android的主线程不能处理耗时的任务,否者会导致ANR的出现,但是界面的更新又必须要在主线程中进行,这样,我们就必须在子线程中处理耗时的任务,然后在主线程中更新UI。

但是,我们怎么知道子线程中的任务何时完成,又应该什么时候更新UI,又更新什么内容呢?为了解决这个问题,Android为我们提供了一个消息机制即Handler。下面就看下Handler的常见使用方式,代码如下

public class MainActivity extends AppCompatActivity implements View.OnClickListener { private Button mStartTask; @SuppressLint("HandlerLeak") private Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { super.handleMessage(msg); if (msg.what == 1) { Toast.makeText(MainActivity.this, "刷新UI、", Toast.LENGTH_SHORT).show(); } } }; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); initView(); } private void initView() { mStartTask = findViewById(R.id.btn_start_task); mStartTask.setOnClickListener(this); } @Override public void onClick(View v) { switch (v.getId()) { case R.id.btn_start_task: new Thread(new Runnable() { @Override public void run() { try { Thread.sleep(1000); mHandler.sendEmptyMessage(1); } catch (InterruptedException e) { e.printStackTrace(); } } }).start(); break; } }}可以看到在子线程中,让线程睡了一秒,来模仿耗时的任务,当耗时任务处理完之后,Handler会发送一个消息,然后我们可以在Handler的handleMessage方法中得到这个消息,得到消息之后就能够在handleMessage方法中更新UI了,因为handleMessage是在主线程中嘛。到这里就会有以下疑问了:

Handler明明是在子线程中发的消息怎么会跑到主线程中了呢?Handler的发送消息handleMessage又是怎么接收到的呢?带着这两个疑问,开始分析Handler的源码。

Handler的源码分析

先看下在我们实例化Handler的时候,Handler的构造方法中都做了那些事情,看代码

final Looper mLooper; final MessageQueue mQueue; final Callback mCallback; final boolean mAsynchronous;/** * Default constructor associates this handler with the {@link Looper} for the * current thread. * * If this thread does not have a looper, this handler won't be able to receive messages * so an exception is thrown. */ public Handler() { this(null, false); }/** * Use the {@link Looper} for the current thread with the specified callback interface * and set whether the handler should be asynchronous. * * Handlers are synchronous by default unless this constructor is used to make * one that is strictly asynchronous. * * Asynchronous messages represent interrupts or events that do not require global ordering * with respect to synchronous messages. Asynchronous messages are not subject to * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}. * * @param callback The callback interface in which to handle messages, or null. * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for * each {@link Message} that is sent to it or {@link Runnable} that is posted to it. * * @hide */ public Handler(Callback callback, boolean async) { if (FIND_POTENTIAL_LEAKS) { final Class<? extends Handler> klass = getClass(); if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) && (klass.getModifiers() & Modifier.STATIC) == 0) { Log.w(TAG, "The following Handler class should be static or leaks might occur: " + klass.getCanonicalName()); } } mLooper = Looper.myLooper(); if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; mCallback = callback; mAsynchronous = async; }通过源码可以看到Handler的无参构造函数调用了两个参数的构造函数,而在两个参数的构造函数中就是将一些变量进行赋值。

看下下面的代码

mLooper = Looper.myLooper(); if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); }这里是通过Looper中的myLooper方法来获得Looper实例的,如果Looper为null的话就会抛异常,抛出的异常内容翻译过来就是

无法在未调用Looper.prepare()的线程内创建handler

从这句话中,我们可以知道,在调用Looper.myLooper()之前必须要先调用Looper.prepare()方法,现在来看下prepare方法中的内容,如下

/** Initialize the current thread as a looper. * This gives you a chance to create handlers that then reference * this looper, before actually starting the loop. Be sure to call * {@link #loop()} after calling this method, and end it by calling * {@link #quit()}. */ public static void prepare() { prepare(true); } private static void prepare(boolean quitAllowed) { if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper(quitAllowed)); }从上面代码中可以看到,prepare()方法调用了prepare(boolean quitAllowed)方法,prepare(boolean quitAllowed) 方法中则是实例化了一个Looper,然后将Looper设置进sThreadLocal中,到了这里就有必要了解一下ThreadLocalle。

什么是ThreadLocal

ThreadLocal 为解决多线程程序的并发问题提供了一种新的思路。使用这个工具类可以很简洁地编写出优美的多线程程序。

当使用ThreadLocal 维护变量时,ThreadLocal 为每个使用该变量的线程提供独立的变量副本,所以每一个线程都可以独立地改变自己的副本,而不会影响其它线程所对应的副本。

如果看完上面这段话还是搞不明白ThreadLocal有什么用,那么可以看下下面代码运行的结果,相信看下结果你就会明白ThreadLocal有什么作用了。public class MainActivity extends AppCompatActivity { private static final String TAG = "MainActivity"; private ThreadLocal<Integer> mThreadLocal = new ThreadLocal<>(); @SuppressLint("HandlerLeak") private Handler mHandler = new Handler(){ @Override public void handleMessage(Message msg) { super.handleMessage(msg); if (msg.what == 1) { Log.d(TAG, "onCreate: "+mThreadLocal.get()); } } }; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mThreadLocal.set(5); Thread1 thread1 = new Thread1(); thread1.start(); Thread2 thread2 = new Thread2(); thread2.start(); Thread3 thread3 = new Thread3(); thread3.start(); new Thread(new Runnable() { @Override public void run() { try { Thread.sleep(2000); mHandler.sendEmptyMessage(1); } catch (InterruptedException e) { e.printStackTrace(); } } }).start(); } class Thread1 extends Thread { @Override public void run() { super.run(); mThreadLocal.set(1); Log.d(TAG, "mThreadLocal1: "+ mThreadLocal.get()); } } class Thread2 extends Thread { @Override public void run() { super.run(); mThreadLocal.set(2); Log.d(TAG, "mThreadLocal2: "+ mThreadLocal.get()); } } class Thread3 extends Thread { @Override public void run() { super.run(); mThreadLocal.set(3); Log.d(TAG, "mThreadLocal3: "+ mThreadLocal.get()); } }}看下这段代码运行之后打印的log

可以看到虽然在不同的线程中对同一个mThreadLocal中的值进行了更改,但最后仍可以正确拿到当前线程中mThreadLocal中的值。由此我们可以得出结论ThreadLocal.set方法设置的值是与当前线程进行绑定了的。

知道了ThreadLocal.set方法的作用,则Looper.prepare方法就是将Looper与当前线程进行绑定(当前线程就是调用Looper.prepare方法的线程)。

文章到了这里我们可以知道以下几点信息了

在对Handler进行实例化的时候,会对一些变量进行赋值。对Looper进行赋值是通过Looper.myLooper方法,但在调用这句代码之前必须已经调用了Looper.prepare方法。Looper.prepare方法的作用就是将实例化的Looper与当前的线程进行绑定。这里就又出现了一个问题:在调用Looper.myLooper方法之前必须必须已经调用了Looper.prepare方法,即在实例化Handler之前就要调用Looper.prepare方法,但是我们平常在主线程中使用Handler的时候并没有调用Looper.prepare方法呀!这是怎么回事呢?

其实,在主线程中Android系统已经帮我们调用了Looper.prepare方法,可以看下ActivityThread类中的main方法,代码如下

public static void main(String[] args) { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain"); // CloseGuard defaults to true and can be quite spammy. We // disable it here, but selectively enable it later (via // StrictMode) on debug builds, but using DropBox, not logs. CloseGuard.setEnabled(false); Environment.initForCurrentUser(); // Set the reporter for event logging in libcore EventLogger.setReporter(new EventLoggingReporter()); // Make sure TrustedCertificateStore looks in the right place for CA certificates final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId()); TrustedCertificateStore.setDefaultUserDirectory(configDir); Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } // End of event ActivityThreadMain. Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }上面的代码中有一句

Looper.prepareMainLooper();这句话的实质就是调用了Looper的prepare方法,代码如下

public static void prepareMainLooper() { prepare(false);//这里调用了prepare方法 synchronized (Looper.class) { if (sMainLooper != null) { throw new IllegalStateException("The main Looper has already been prepared."); } sMainLooper = myLooper(); } }到这里就解决了,为什么我们在主线程中使用Handler之前没有调用Looper.prepare方法的问题了。

让我们再回到Handler的构造方法中,看下

mLooper = Looper.myLooper();myLooper()方法中代码如下

/** * Return the Looper object associated with the current thread. Returns * null if the calling thread is not associated with a Looper. */ public static @Nullable Looper myLooper() { return sThreadLocal.get(); }其实就是从当前线程中的ThreadLocal中取出Looper实例。

再看下Handler的构造方法中的

mQueue = mLooper.mQueue;这句代码。这句代码就是拿到Looper中的mQueue这个成员变量,然后再赋值给Handler中的mQueue,下面看下Looper中的代码

final MessageQueue mQueue; private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }同过上面的代码,我们可以知道mQueue就是MessageQueue,在我们调用Looper.prepare方法时就将mQueue实例化了。

Handler的sendMessage方法都做了什么

还记得文章开始时的两个问题吗?

Handler明明是在子线程中发的消息怎么会跑到主线程中了呢?Handler的发送消息handleMessage又是怎么接收到的呢?下面就分析一下Handler的sendMessage方法都做了什么,看代码

public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); }public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); }/** * Enqueue a message into the message queue after all pending messages * before the absolute time (in milliseconds) <var>uptimeMillis</var>. * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b> * Time spent in deep sleep will add an additional delay to execution. * You will receive it in {@link #handleMessage}, in the thread attached * to this handler. * * @param uptimeMillis The absolute time at which the message should be * delivered, using the * {@link android.os.SystemClock#uptimeMillis} time-base. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. Note that a * result of true does not mean the message will be processed -- if * the looper is quit before the delivery time of the message * occurs then the message will be dropped. */ public boolean sendMessageAtTime(Message msg, long uptimeMillis) { MessageQueue queue = mQueue; if (queue == null) { RuntimeException e = new RuntimeException( this + " sendMessageAtTime() called with no mQueue"); Log.w("Looper", e.getMessage(), e); return false; } return enqueueMessage(queue, msg, uptimeMillis); }由上面的代码可以看出,Handler的sendMessage方法最后调用了sendMessageAtTime这个方法,其实,无论时sendMessage、sendEmptyMessage等方法最终都是调用sendMessageAtTime。可以看到sendMessageAtTime这个方法最后返回的是enqueueMessage(queue, msg, uptimeMillis);下面看下这个方法,代码如下

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }这里有一句代码非常重要。

msg.target = this;这句代码就是将当前的Handler赋值给了Message中的target变量。这样,就将每个调用sendMessage方法的Handler与Message进行了绑定。

enqueueMessage方法最后返回的是queue.enqueueMessage(msg, uptimeMillis);也就是调用了MessageQueue中的enqueueMessage方法,下面看下MessageQueue中的enqueueMessage方法,代码如下

boolean enqueueMessage(Message msg, long when) { if (msg.target == null) { throw new IllegalArgumentException("Message must have a target."); } if (msg.isInUse()) { throw new IllegalStateException(msg + " This message is already in use."); } synchronized (this) { if (mQuitting) { IllegalStateException e = new IllegalStateException( msg.target + " sending message to a Handler on a dead thread"); Log.w(TAG, e.getMessage(), e); msg.recycle(); return false; } msg.markInUse(); msg.when = when; Message p = mMessages; boolean needWake; if (p == null || when == 0 || when < p.when) { // New head, wake up the event queue if blocked. msg.next = p; mMessages = msg; needWake = mBlocked; } else { // Inserted within the middle of the queue. Usually we don't have to wake // up the event queue unless there is a barrier at the head of the queue // and the message is the earliest asynchronous message in the queue. needWake = mBlocked && p.target == null && msg.isAsynchronous(); Message prev; for (;;) { prev = p; p = p.next; if (p == null || when < p.when) { break; } if (needWake && p.isAsynchronous()) { needWake = false; } } msg.next = p; // invariant: p == prev.next prev.next = msg; } // We can assume mPtr != 0 because mQuitting is false. if (needWake) { nativeWake(mPtr); } } return true; }上面的代码就是将消息放进消息队列中,如果消息已成功放入消息队列,则返回true。失败时返回false,而失败的原因通常是因为处理消息队列正在退出。代码分析到这里可以得出以下两点结论了

Handler在sendMessage时会将自己设置给Message的target变量即将自己与发送的消息绑定。Handler的sendMessage是将Message放入MessageQueue中。到了这里已经知道Handler的sendMessage是将消息放进MessageQueue中,那么又是怎样从MessageQueue中拿到消息的呢?想要知道答案请继续阅读。

怎样从MessageQueue中获取Message

在文章的前面,贴出了ActivityThread类中的main方法的代码,不知道细心的你有没有注意到,在main方法的结尾处调用了一句代码

Looper.loop();好了,现在可以看看Looper.loop();这句代码到底做了什么了loop方法中的代码如下

/** * Run the message queue in this thread. Be sure to call * {@link #quit()} to end the loop. */ public static void loop() { final Looper me = myLooper();//通过myLooper方法拿到与主线程绑定的Looper if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } final MessageQueue queue = me.mQueue;//从Looper中得到MessageQueue // Make sure the identity of this thread is that of the local process, // and keep track of what that identity token actually is. Binder.clearCallingIdentity(); final long ident = Binder.clearCallingIdentity(); //开始死循环 for (;;) { //从消息队列中不断取出消息 Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return; } // This must be in a local variable, in case a UI event sets the logger final Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs; final long traceTag = me.mTraceTag; if (traceTag != 0 && Trace.isTagEnabled(traceTag)) { Trace.traceBegin(traceTag, msg.target.getTraceName(msg)); } final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis(); final long end; try { //这句代码是重点 msg.target.dispatchMessage(msg); end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis(); } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } if (slowDispatchThresholdMs > 0) { final long time = end - start; if (time > slowDispatchThresholdMs) { Slog.w(TAG, "Dispatch took " + time + "ms on " + Thread.currentThread().getName() + ", h=" + msg.target + " cb=" + msg.callback + " msg=" + msg.what); } } if (logging != null) { logging.println("<<<<< Finished to " + msg.target + " " + msg.callback); } // Make sure that during the course of dispatching the // identity of the thread wasn't corrupted. final long newIdent = Binder.clearCallingIdentity(); if (ident != newIdent) { Log.wtf(TAG, "Thread identity changed from 0x" + Long.toHexString(ident) + " to 0x" + Long.toHexString(newIdent) + " while dispatching to " + msg.target.getClass().getName() + " " + msg.callback + " what=" + msg.what); } msg.recycleUnchecked(); } }上面的代码,我已经进行了部分注释,这里有一句代码非常重要

msg.target.dispatchMessage(msg);执行到这句代码,说明已经从消息队列中拿到了消息,还记得msg.target吗?就是Message中的target变量呀!也就是发送消息的那个Handler,所以这句代码的本质就是调用了Handler中的dispatchMessage(msg)方法,代码分析到这里是不是有点小激动了呢!稳住!下面看下dispatchMessage(msg)这个方法,代码如下

/** * Handle system messages here. */ public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }现在来一句句的来分析上面的代码,先看下这句

if (msg.callback != null) { handleCallback(msg); } msg.callback就是Runnable对象,当msg.callback不为null时会调用 handleCallback(msg)方法,先来看下 handleCallback(msg)方法,代码如下

private static void handleCallback(Message message) { message.callback.run(); }上面的代码就是调用了Runnable的run方法。那什么情况下if (msg.callback != null)这个条件成立呢!还记得使用Handler的另一种方法吗?就是调用Handler的post方法呀!这里说明一下,使用Handler其实是有两种方法的

使用Handler的sendMessage方法,最后在handleMessage(Message msg)方法中来处理消息。使用Handler的post方法,最后在Runnable的run方法中来处理,代码如下public class MainActivity extends AppCompatActivity implements View.OnClickListener { private Button mTimeCycle,mStopCycle; private Runnable mRunnable; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); initView(); } private void initView() { mTimeCycle = findViewById(R.id.btn_time_cycle); mTimeCycle.setOnClickListener(this); mStopCycle = findViewById(R.id.btn_stop_cycle); mStopCycle.setOnClickListener(this); mRunnable = new Runnable() { @Override public void run() { Toast.makeText(MainActivity.this, "正在循环!!!", Toast.LENGTH_SHORT).show(); mHandler.postDelayed(mRunnable, 1000); } }; } @Override public void onClick(View v) { switch (v.getId()) { case R.id.btn_time_cycle: mHandler.post(mRunnable); break; case R.id.btn_stop_cycle: mHandler.removeCallbacks(mRunnable); break; } }}第一种方法,我们已经分析了,下面来分析一下第二种使用方式的原理,先看下Handler的post的方法做了什么,代码如下

/** * Causes the Runnable r to be added to the message queue. * The runnable will be run on the thread to which this handler is * attached. * * @param r The Runnable that will be executed. * * @return Returns true if the Runnable was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */ public final boolean post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0); } private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; return m; }由上面的代码不难看出,post方法最终也是将Runnable封装成消息,然后将消息放进MessageQueue中。下面继续分析dispatchMessage方法中的代码

else { //if中的代码其实是和if (msg.callback != null) {handleCallback(msg);} //原理差不多的,只不过mCallback是Handler中的成员变量。 if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } //当上面的条件都不成立时,就会调用这句代码 handleMessage(msg); }上面的代码就不分析了,我已经在代码中进行了注释,下面再看下handleMessage(msg)这个方法,代码如下

/** * Subclasses must implement this to receive messages. */ public void handleMessage(Message msg) { }其实,他就是一个空方法,具体的代码让我们自己重写这个方法进行处理。代码分析到这里,已经可以给出下面问题的答案了。

Handler明明是在子线程中发的消息怎么会跑到主线程中了呢?Handler的发送消息handleMessage又是怎么接收到的呢?在子线程中Handler在发送消息的时候已经把自己与当前的message进行了绑定,在通过Looper.loop()开启轮询message的时候,当获得message的时候会调用 与之绑定的Handler的handleMessage(Message msg)方法,由于Handler是在主线程创建的,所以自然就由子线程切换到了主线程。

总结

上面已经嗯将Handler的源码分析了一遍,现在来进行一些总结:

1、Handler的工作原理

在使用Handler之前必须要调用Looper.prepare()这句代码,这句代码的作用是将Looper与当前的线程进行绑定,在实例化Handler的时候,通过Looper.myLooper()获取Looper,然后再获得Looper中的MessageQueue。

在子线程中调用Handler的sendMessage方法就是将Message放入MessageQueue中,然后调用Looper.loop()方法来从MessageQueue中取出Message,在取到Message的时候,执行 msg.target.dispatchMessage(msg);这句代码,这句代码就是从当前的Message中取出Handler然后执行Handler的handleMessage方法。

2、Handler、Message、MessageQueue以及Looper之间的关系

在介绍它们之间的关系之前,先说一下它们各自的作用。

Handler:负责发送和处理消息。Message:用来携带需要的数据。MessageQueue:消息队列,队列里面的内容就是Message。Looper:消息轮巡器,负责不停的从MessageQueue中取Message。它们的关系如下图(图片来源于网上)

3、在子线程中使用Handler

在子线程中使用Handler的方式如下class LooperThread extends Thread { public Handler mHandler; public void run() { Looper.prepare(); mHandler = new Handler() { public void handleMessage(Message msg) { // process incoming messages here } }; Looper.loop(); }}结束语

本文将Handler的机制详细讲解了一遍,包括在面试中有关Handler的一些问题,在文章中也能找到答案。

顺便说下阅读代码应该注意的地方,在分析源码之前应该知道你分析代码的目的,就是你为了得到什么答案而分析代码;在分析代码时切记要避轻就重,不要想着要搞懂每句代码做了什么,要找准大方向。文中的代码已上传到GitHub,可以在这里获取,与Handler有关的源码在我上传的源码的handler包中。

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