Toast，AlertDialog的误解

在一般的软件开发中，子线程中是不能更改UI主线程中创建的UI控件的。之前的理解是Toast也不能在子线程中创建。事实上并不是这样子的。

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    new Thread(new Runnable(){
        @Override
        public void run() {
            // TODO Auto-generated method stub
            Toast.makeText(MainActivity.this, "test", Toast.LENGTH_LONG).show();
        }}).start();
}

在Activity的onCreate中写入以上代码运行。LogCat中会抱错

java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

不看这个错误信息第一反应肯定是UI控件不能在子线程调用
事实上并不是这样的

我们可以查看Toast的源码

public Toast(Context context) {
        mContext = context;
        mTN = new TN();
        mTN.mY = context.getResources().getDimensionPixelSize(
                com.android.internal.R.dimen.toast_y_offset);
    }

    /**
     * Show the view for the specified duration.
     */
public void show() {
    if (mNextView == null) {
        throw new RuntimeException("setView must have been called");
    }

    INotificationManager service = getService();
    String pkg = mContext.getPackageName();
    TN tn = mTN;
    tn.mNextView = mNextView;
    try {
        service.enqueueToast(pkg, tn, mDuration);
    } catch (RemoteException e) {
            // Empty
    }
}

看这个mTN = new TN();构造函数

final Handler mHandler = new Handler();    

Handler的构造函数

public Handler(Callback callback, boolean async) {
       ...
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
       ...
    }

而Looper.myLooper()

 /**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static Looper myLooper() {
        return sThreadLocal.get();
    }

android中的子线程默认ThreadLocal中未设置Looper，所有会抛出这个异常，至于Looper是啥，可以参考我另一篇文章：http://www.cnblogs.com/cqcmdwym/archive/2013/05/12/3074138.html

解决方法

new Thread(new Runnable(){
    @Override
    public void run() {
        // TODO Auto-generated method stub
        Looper.prepare();
        Toast.makeText(MainActivity.this, "test", Toast.LENGTH_LONG).show();
        Looper.loop();
}}).start();

或者在主线程中声明一个Handler，然后在run方法中hanlder.post(new Runnable(){})一下。

同样AlertDialog也是如此，他包含一个AlertController字段，其内部也需要创建一个Handler.

参考网址：http://www.oschina.net/question/163910_31439