Toast show的时候崩溃的问题

今天同事提交了一份代码，里面有一个简单的Toast.makeText(`````).show()，没想到真正运行的时候跑到这里挂掉了：

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

从崩溃上看这是looper没有启动导致的，但是为什么？ 查询源码后找到了原因（以6.0源码为例）：

先看Toast的show方法：

    /**
     * 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.getOpPackageName();
        TN tn = mTN;
        tn.mNextView = mNextView;

        try {
            service.enqueueToast(pkg, tn, mDuration);
        } catch (RemoteException e) {
            // Empty
        }
    }

这里直接交给了INotificationManager来进行显示的操作，按照android的命名规则，这个AIDL的实现位于NotificationManagerService中：

	@Override
        public void enqueueToast(String pkg, ITransientNotification callback, int duration)
        {
            ......

            synchronized (mToastQueue) {
                int callingPid = Binder.getCallingPid();
                long callingId = Binder.clearCallingIdentity();
                try {
                    ToastRecord record;
                    int index = indexOfToastLocked(pkg, callback);
                    // If it's already in the queue, we update it in place, we don't
                    // move it to the end of the queue.
                    if (index >= 0) {
                        record = mToastQueue.get(index);
                        record.update(duration);
                    } else {
                        // Limit the number of toasts that any given package except the android
                        // package can enqueue.  Prevents DOS attacks and deals with leaks.
                        

			......

                        record = new ToastRecord(callingPid, pkg, callback, duration);
                        mToastQueue.add(record);
                        index = mToastQueue.size() - 1;
                        keepProcessAliveLocked(callingPid);
                    }
                    // If it's at index 0, it's the current toast.  It doesn't matter if it's
                    // new or just been updated.  Call back and tell it to show itself.
                    // If the callback fails, this will remove it from the list, so don't
                    // assume that it's valid after this.
                    if (index == 0) {
                        showNextToastLocked();
                    }
                } finally {
                    Binder.restoreCallingIdentity(callingId);
                }
            }
        }

一系列的判断后，交给了showNextToastLocked方法：

     void showNextToastLocked() {
        ToastRecord record = mToastQueue.get(0);
        while (record != null) {
            if (DBG) Slog.d(TAG, "Show pkg=" + record.pkg + " callback=" + record.callback);
            try {
                record.callback.show();
                scheduleTimeoutLocked(record);
                return;
            } catch (RemoteException e) {
                Slog.w(TAG, "Object died trying to show notification " + record.callback
                        + " in package " + record.pkg);
                // remove it from the list and let the process die
                int index = mToastQueue.indexOf(record);
                if (index >= 0) {
                    mToastQueue.remove(index);
                }
                keepProcessAliveLocked(record.pid);
                if (mToastQueue.size() > 0) {
                    record = mToastQueue.get(0);
                } else {
                    record = null;
                }
            }
        }
    }

最终的展示是在这句：record.callback.show()，这个record.callback其实是由Toast通过AIDL传递过来的，回头再看Toast的代码，其实是调用了一个内部类TN.show()：

        /**
         * schedule handleShow into the right thread
         */
        @Override
        public void show() {
            if (localLOGV) Log.v(TAG, "SHOW: " + this);
            mHandler.post(mShow);
        }

而这个mHandler也就是我们报错时所提示的looper所在，它的创建就是一句简单的new而已：

    private static class TN extends ITransientNotification.Stub {
        

	......
        

	private final WindowManager.LayoutParams mParams = new WindowManager.LayoutParams();
        final Handler mHandler = new Handler();

    }

至此也就搞清楚了这个崩溃的根本原因了，运行Toast.show()的线程，自身的looper并没有开始工作，所以导致了这个崩溃。