Android子线程与更新UI问题的深入讲解_android教程

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Android子线程与更新UI问题的深入讲解

发布时间：2019-08-08 发布网站：脚本宝典

脚本宝典收集整理的这篇文章主要介绍了Android子线程与更新UI问题的深入讲解，脚本宝典觉得挺不错的，现在分享给大家，也给大家做个参考。

前言

在AndROId项目中经常有碰到这样的问题，在子线程中完成耗时操作之后要更新UI，下面就自己经历的一些项目总结一下更新的方法。话不多说了，来一起看看详细的介绍吧

引子：

情形1

  @override  PRotected void onCreate(Bundle savedInstancestate) {  suPEr.onCreate(savedInstanceState);  setContentView(R.layout.activITy_main);   TextView textView = findViewById(R.id.home_tv);  ImageView imageView = findViewById(R.id.home_img);   new Thread(new Runnable() {   @Override   public void run() {   textView.setText("更新TextView");   imageView.setImageResource(R.drawable.img);   }  }).start();  }

运行结果：正常运行！！！

情形二

  @Override  protected void onCreate(Bundle savedInstanceState) {  super.onCreate(savedInstanceState);  setContentView(R.layout.activity_main);   TextView textView = findViewById(R.id.home_tv);  ImageView imageView = findViewById(R.id.home_img);   new Thread(new Runnable() {   @Override   public void run() {   try {    Thread.sleep(5000);   } catch (InterruptedException e) {    e.printStackTrace();   }   textView.setText("更新TextView");   imageView.setImageResource(R.drawable.img);   }  }).start();  }

运行结果：异常

    android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierArchy can touch its views.
        at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:6357)
        at android.view.ViewRootImpl.requestLayout(ViewRootImpl.java:874)
        at android.view.View.requestLayout(View.java:17476)
        at android.view.View.requestLayout(View.java:17476)
        at android.view.View.requestLayout(View.java:17476)
        at android.view.View.requestLayout(View.java:17476)
        at android.view.View.requestLayout(View.java:17476)
        at android.view.View.requestLayout(View.java:17476)
        at android.widget.RelativeLayout.requestLayout(RelativeLayout.java:360)
        at android.view.View.requestLayout(View.java:17476)
        at android.widget.TextView.checkForRelayout(TextView.java:6871)
        at android.widget.TextView.setText(TextView.java:4057)
        at android.widget.TextView.setText(TextView.java:3915)
        at android.widget.TextView.setText(TextView.java:3890)
        at com.dong.demo.MainActivity$1.run(MainActivity.java:44)
        at java.lang.Thread.run(Thread.java:818)

不是说，子线程不能更新UI吗，为什么情形一可以正常运行，情形二不能正常运行呢；

子线程修改UI出现异常，与什么方法有关

首先从出现异常的LOG日志入手，发现出现异常的方法调用顺序如下：

TextView.setText(TextView.java:4057)

TextView.checkForRelayout(TextView.java:6871)

View.requestLayout(View.java:17476)

RelativeLayout.requestLayout(RelativeLayout.java:360)

View.requestLayout(View.java:17476)

ViewRootImpl.requestLayout(ViewRootImpl.java:874)

ViewRootImpl.checkThread(ViewRootImpl.java:6357)

更改ImageView时，出现的异常类似；

首先看TextView.setText()方法的源码

  private void setText(CharSequence text, BufferType type,     boolean notifyBefore, int oldlen) {    //省略其他代码   if (MLayout != null) {   checkForRelayout();  }   sendOnTextChanged(text, 0, oldlen, textLength);  onTextChanged(text, 0, oldlen, textLength);   //省略其他代码

然后，查看以下checkForRelayout()方法的与源码。

  private void checkForRelayout() {  // If we have a fixed width, we can just swap in a new text layout  // if the text height stays the same or if the view height is fixed.   if ((mLayoutParams.width != LayoutParams.WRAP_CONTENT    //省略代码    // We lose: the height has changed and we have a dynamic height.   // Request a new view layout using our new text layout.   requestLayout();   invalidate();  } else {   // Dynamic width, so we have no choice but to request a new   // view layout with a new text layout.   nullLayouts();   requestLayout();   invalidate();  }  }

checkForReLayout方法，首先会调用需要改变的View的requestLayout方法，然后执行invalidate()重绘操作；

TextView没有重写requestLayout方法，requestLayout方法由View实现；

查看RequestLayout方法的源码：

  public void requestLayout() {  //省略其他代码  if (mParent != null && !mParent.isLayoutRequested()) {   mParent.requestLayout();  }  if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {   mAttachInfo.mViewRequestingLayout = null;  }  }

View获取到父View（类型是ViewParent，ViewPaerent是个接口，requestLayout由子类来具体实现），mParent，然后调用父View的requestLayout方法，比如示例中的父View就是XMl文件的根布局就是RelativeLayout。

  @Override  public void requestLayout() {  super.requestLayout();  mDirtyHierarchy = true;  }

继续跟踪super.requestLayout()方法，即ViewGroup没有重新，即调用的是View的requestLayout方法。

经过一系列的调用ViewParent的requestLayout方法，最终调用到ViewRootImp的requestLayout方法。ViewRootImP实现了ViewParent接口，继续查看ViewRootImp的requestLayout方法源码。

  @Override  public void requestLayout() {   if (!mHandlingLayoutInLayoutRequest) {    checkThread();    mLayoutRequested = true;    scheduleTraversals();   }  }

ViewRootImp的requestLayout方法中有两个方法：

一、checkThread，检查线程，源码如下

  void checkThread() {   if (mThread != Thread.currentThread()) {    throw new CalleDFromWrongThreadException(      "Only the original thread that created a view hierarchy can touch its views.");   }  }

判断当前线程，是否是创建ViewRootImp的线程，而创建ViewRootImp的线程就是主线程，当前线程不是主线程的时候，就抛出异常。

二、scheduleTraversals(),查看源码：

  void scheduleTraversals() {   if (!mTraversalScheduled) {    mTraversalScheduled = true;    mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();    mChoreographer.postCallback(      Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);    if (!mUnbufferedInputDispatch) {     scheduleConsumeBatchedInput();    }    notifyRendererOfFramePending();    pokeDrawLockIfNeeded();   }  }

查看mTraversalRunnable中run()方法的具体操作

  final class TraversalRunnable implements Runnable {   @Override   public void run() {    doTraversal();   }  }

继续追踪doTraversal()方法

  void doTraversal() {   if (mTraversalScheduled) {    mTraversalScheduled = false;    mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);     if (mProfile) {     Debug.startMethodTracing("ViewAncestor");    }     performTraversals();     if (mProfile) {     Debug.stopMethodTracing();     mProfile = false;    }   }  }

查看到performTraversals()方法，熟悉了吧，这是View绘制的起点。

Android子线程与更新UI问题的深入讲解

总结一下：

1.Android更新UI会调用View的requestLayout()方法，在requestLayout方法中，获取ViewParent，然后调用ViewParent的requestLayout()方法，一直调用下去，直到调用到ViewRootImp的requestLayout方法；

2.ViewRootImp的requetLayout方法，主要有两部操作一个是checkThread()方法，检测线程，一个是scheduleTraversals，执行绘制相关工作；

情形3

  @Override  protected void onCreate(Bundle savedInstanceState) {   Log.i("Dong", "Activity: onCreate");   super.onCreate(savedInstanceState);   setContentView(R.layout.activity_main);    new Thread(new Runnable() {    @Override    public void run() {      Looper.prepare();      try {      Thread.sleep(5000);     } catch (InterruptedException e) {      e.printStackTrace();     }      Toast.makeText(MainActivity.this, "显示Toast", Toast.LENGTH_LONG).show();      Looper.loop();    }   }).start();  }

运行结果：正常

分析

下面从Toast源码进行分析：

  public static Toast makeText(Context context, CharSequence text, @Duration int duration) {   return makeText(context, null, text, duration);  }

makeText方法调用了他的重载方法，继续追踪

  public static Toast makeText(@NonNull Context context, @Nullable Looper looper,    @NonNull CharSequence text, @Duration int duration) {   Toast result = new Toast(context, looper);    LayoutInflater inflate = (LayoutInflater)     context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);   View v = inflate.inflate(com.android.internal.R.layout.transient_notification, null);   TextView tv = (TextView)v.findViewById(com.android.internal.R.id.message);   tv.setText(text);    result.mNextView = v;   result.mDuration = duration;    return result;  }

新建了一个Toast对象，然后对显示的布局、内容、时长进行了设置，并返回Toast对象。

继续查看new Toast()的源码

  public Toast(@NonNull Context context, @Nullable Looper looper) {   mContext = context;   mTN = new TN(context.getPackageName(), looper);   mTN.mY = context.getResources().getDimensionPixelSize(     com.android.internal.R.dimen.toast_y_offset);   mTN.mGravity = context.getResources().getInteger(     com.android.internal.R.integer.config_toastDefaultGravity);  }

继续查看核心代码 mTN = new TN(context.getPackageName(), looper);

TN初始化的源码为：

   TN(String packageName, @Nullable Looper looper) {    //省略部分不相关代码    if (looper == null) {     // 没有传入Looper对象的话，使用当前线程对应的Looper对象     looper = Looper.myLooper();     if (looper == null) {      throw new RuntimeException(        "Can't toast on a thread that has not called Looper.prepare()");     }    }    //初始化了Handler对象    mHandler = new Handler(looper, null) {     @Override     public void handleMessage(Message msg) {      switch (msg.what) {       case SHOW: {        IBinder token = (IBinder) msg.obj;        handleShow(token);        break;       }       case HIDE: {        handleHide();        // Don't do this in handleHide() because it is also invoked by        // handleShow()        mNextView = null;        break;       }       case CANCEL: {        handleHide();        // Don't do this in handleHide() because it is also invoked by        // handleShow()        mNextView = null;        try {         getService().cancelToast(mPackageName, TN.this);        } catch (RemoteException e) {        }        break;       }      }     }    };   }

继续追踪handleShow(token)方法：

   public void handleShow(IBinder windowToken) {    //省略部分代码    if (mView != mNextView) {     // remove the old view if necessary     handleHide();     mView = mNextView;     Context context = mView.getContext().getApplicationContext();     String packageName = mView.getContext().getOpPackageName();     if (context == null) {      context = mView.getContext();     }     mWM = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);     /*     ・*省略设置显示属性的代码     ・*/     if (mView.getParent() != null) {      if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);      mWM.removeView(mView);     } =    try {      mWM.addView(mView, mParams);      trySendAccessibilityEvent();     } catch (WindowManager.BadTokenException e) {      /* ignore */     }    }   }

通过源码可以看出，Toast显示内容是通过mWM（WindowManager类型）的直接添加的，更正：mWm.addView 时，对应的ViewRootImp初始化发生在子线程，checkThread方法中的mThread != Thread.currentThread()判断为true，所以不会抛出只能在主线程更新UI的异常。

总结

@L_406_1@
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