Android子线程与更新UI问题的深入讲解

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。

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

以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，如果有疑问大家可以留言交流，谢谢大家对呐喊教程的支持。

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