*********************子线程向主线程发消息**************************
描述：新建线程访问网络，将得到的信息显示在主线程的Listview中
1.网络线程

 new Thread() {
                    @Override
                    public void run() {
                        super.run();
                        String str = get("http://www.imooc.com/api/teacher?type=2&cid=1");
                        try {
                            JSONObject json = new JSONObject(str);
                            JSONArray arr = json.getJSONArray("data");
                            data = new ArrayList<>();
                            for (int i = 0; i < arr.length(); i++) {
                                Map<String,Object> map=new HashMap<>();
                                JSONObject obj=arr.getJSONObject(i);
                                map.put("id",obj.getInt("id"));
                                map.put("name",obj.getString("name"));
                                data.add(map);
                            }

                            String[] from={"id","name"};
                            int[] to={R.id.item_id,R.id.item_name};
                            SimpleAdapter adapter=new SimpleAdapter(MainActivity.this,data,R.layout.item,from,to);
                            Message msg=new Message();
//what 标识 ,arg 数字，obj对象
                            msg.what=1;
                            msg.obj=adapter;
                            handler.sendMessage(msg);

                        } catch (JSONException e) {
                            e.printStackTrace();
                        }


                    }
                }.start();

2.主线程

Handler handler=new Handler(){
        @Override
        public void handleMessage(@NonNull Message msg) {
            super.handleMessage(msg);
            switch (msg.what){
                case 1:
                    list.setAdapter((SimpleAdapter)msg.obj);
            }
        }
    };

***************************************主线程向子线程发消息***********
声明全局变量handler
在主线程调用handler.setMessage
在子线程调用handleMessage的前加上Looper.prepare，后加Looper.looper()