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，谢谢大家对的支持。