android非UI线程调用Toast

开发android游戏的时候，相拥Toast测试一下数据，报错了。

05-26 21:15:56.681: W/System.err(14558): java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
05-26 21:15:56.681: W/System.err(14558): 	at android.os.Handler.<init>(Handler.java:121)
05-26 21:15:56.691: W/System.err(14558): 	at android.widget.Toast$TN.<init>(Toast.java:335)
05-26 21:15:56.691: W/System.err(14558): 	at android.widget.Toast.<init>(Toast.java:101)
05-26 21:15:56.691: W/System.err(14558): 	at android.widget.Toast.makeText(Toast.java:248)

google了一下，找到了答案：（原理：Toast里面需要一个handler来处理内部的消息）

解决办法：

1）、使用Handler

// 声明一个Handler对象
private static Handler handler=new Handler();
 
// 新启动一个子线程
new Thread(new Runnable() {                    
    @Override
    public void run() {
        // 使用post方式加到主线程的消息队列中
        handler.post(new Runnable() {                    
            @Override
            public void run() {
                Toast.makeText(getApplicationContext(), 
                               "test",Toast.LENGTH_LONG).show();
                        }
        });
    }
}).start();

2）、使用Looper . prepare() 和 Looper . loop()

Looper.prepare()源码：（保证只有一个Looper）

private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

Looper.loop()源码：（loop()方法使用一个死循环不断支取MessageQueue中的消息，并取出消息分给该消息对应的Handler进行处理）

public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;

        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();

        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

            // This must be in a local variable, in case a UI event sets the logger
            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            msg.target.dispatchMessage(msg);

            if (logging != null) {
                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
            }

            // Make sure that during the course of dispatching the
            // identity of the thread wasn't corrupted.
            final long newIdent = Binder.clearCallingIdentity();
            if (ident != newIdent) {
                Log.wtf(TAG, "Thread identity changed from 0x"
                        + Long.toHexString(ident) + " to 0x"
                        + Long.toHexString(newIdent) + " while dispatching to "
                        + msg.target.getClass().getName() + " "
                        + msg.callback + " what=" + msg.what);
            }

            msg.recycle();
        }

其实，Handler指定MessageQueue发送消息，而MessageQueue有Looper负责管理的。有Looper的构造函数可以看出。

在线程中使用Handler的步骤如：

调用Looper的prepare()创建当前线程的Looper对象（only one），创建对象时会创建与之对应的MessageQueue。有了MessageQueue就可以给主线程传递message了，

然后调用Handler的handleMessage（），处理对应的事件，操作主线程的UI等。然后调用Looper.loop()启动Looper。