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  • Android子线程与更新UI问题的深入讲解

    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

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    @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

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    @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

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    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。

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    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：

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    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。

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    @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。

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    @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

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    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：

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    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

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

    继续追踪doTraversal()方法

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    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

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    @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：

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    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

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    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

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    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：

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    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：

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      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。
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  • 原文地址：https://www.cnblogs.com/liumin-txgt/p/13130018.html
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