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第一種場景：

在UI線程中更新UI，這種是最簡單的，直接更新UI即可。
代碼如下

public class MainActivity extends AppCompatActivity { private Button bt_click_me; private TextView tv_text; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); bt_click_me = findViewById(R.id.bt_click_me); tv_text = findViewById(R.id.tv_text); bt_click_me.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { tv_text.setText('111111111111111'); } }); }}

第二種場景：

從子線程中更新UI

代碼如下

public class MainActivity extends AppCompatActivity {    private Button bt_click_me;    private TextView tv_text;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        bt_click_me = findViewById(R.id.bt_click_me);        tv_text = findViewById(R.id.tv_text);        bt_click_me.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Thread thread = new Thread(new Runnable() {                    @Override                    public void run() {                        tv_text.setText('111111111111111');                    }                });                thread.start();            }        });    }}

當(dāng)點(diǎn)擊按鈕更新UI的時候就會發(fā)現(xiàn)報了異常，異常如下

圖片.png

這個異常證明了子線程不能直接更新UI，解決方案如下

（1）通過Activity中的runOnUIThread方法

public class MainActivity extends AppCompatActivity { private Button bt_click_me; private TextView tv_text; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); bt_click_me = findViewById(R.id.bt_click_me); tv_text = findViewById(R.id.tv_text); bt_click_me.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { Thread thread = new Thread(new Runnable() { @Override public void run() { runOnUiThread(new Runnable() { @Override public void run() { tv_text.setText('111111111111111'); } }); } }); thread.start(); } }); }}

我們來深入源碼

/** * Runs the specified action on the UI thread. If the current thread is the UI * thread, then the action is executed immediately. If the current thread is * not the UI thread, the action is posted to the event queue of the UI thread. * * @param action the action to run on the UI thread */public final void runOnUiThread(Runnable action) {    if (Thread.currentThread() != mUiThread) {        mHandler.post(action);    } else {        action.run();    }}

源碼的意思是說，如果當(dāng)前線程不是UI線程，那么執(zhí)行

mHandler.post(action);

否則直接執(zhí)行run。

這個結(jié)論直接告訴了我們，Handler的post方法也能做到從子線程更新UI。

（2）通過Handler的post方法

public class MainActivity extends AppCompatActivity {    private Handler handler = new Handler();    private Button bt_click_me;    private TextView tv_text;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        bt_click_me = findViewById(R.id.bt_click_me);        tv_text = findViewById(R.id.tv_text);        bt_click_me.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Thread thread = new Thread(new Runnable() {                    @Override                    public void run() {                        handler.post(new Runnable() {                            @Override                            public void run() {                                tv_text.setText('111111111111111');                            }                        });                    }                });                thread.start();            }        });    }}

我在UI線程中new了一個Handler對象，在子線程中用這個對象來調(diào)用post方法。

我們來深入源碼

/** * Causes the Runnable r to be added to the message queue. * The runnable will be run on the thread to which this handler is * attached. * * @param r The Runnable that will be executed. * * @return Returns true if the Runnable was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */public final boolean post(Runnable r){ return sendMessageDelayed(getPostMessage(r), 0);}private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; return m;}

在Handler對象中，有一個post方法，分析源碼得知，這個方法將形參r封裝到一個消息里面，再利用sendMessageDelayed方法將消息發(fā)送（添加）到消息隊(duì)列。（注：理解這句話需要對Handler機(jī)制有一定的了解）

我們得出結(jié)論，通過handler發(fā)送消息也能實(shí)現(xiàn)子線程更新UI。

（3）通過handler發(fā)送消息來實(shí)現(xiàn)子線程更新UI

public class MainActivity extends AppCompatActivity {    private Handler handler = new Handler(){        @Override        public void handleMessage(Message msg) {            super.handleMessage(msg);            switch (msg.what){                case 1:                    tv_text.setText('111111111111111');                    break;            }        }    };    private Button bt_click_me;    private TextView tv_text;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        bt_click_me = findViewById(R.id.bt_click_me);        tv_text = findViewById(R.id.tv_text);        bt_click_me.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Thread thread = new Thread(new Runnable() {                    @Override                    public void run() {                        Message message = Message.obtain();                        message.what = 1;                        handler.sendMessage(message);                    }                });                thread.start();            }        });    }}

（4）通過view的post方法實(shí)現(xiàn)

public class MainActivity extends AppCompatActivity { private Button bt_click_me; private TextView tv_text; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); bt_click_me = findViewById(R.id.bt_click_me); tv_text = findViewById(R.id.tv_text); bt_click_me.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { Thread thread = new Thread(new Runnable() { @Override public void run() { bt_click_me.post(new Runnable() { @Override public void run() { tv_text.setText('111111111111111'); } }); } }); thread.start(); } }); }}

我們來深入源碼

/** * <p>Causes the Runnable to be added to the message queue. * The runnable will be run on the user interface thread.</p> * * @param action The Runnable that will be executed. * * @return Returns true if the Runnable was successfully placed in to the *         message queue.  Returns false on failure, usually because the *         looper processing the message queue is exiting. * * @see #postDelayed * @see #removeCallbacks */public boolean post(Runnable action) {    final AttachInfo attachInfo = mAttachInfo;    if (attachInfo != null) {        return attachInfo.mHandler.post(action);    }    // Postpone the runnable until we know on which thread it needs to run.    // Assume that the runnable will be successfully placed after attach.    getRunQueue().post(action);    return true;}

其實(shí)最終也調(diào)用了mHandler.post(action)方法。

第二種場景總結(jié)：

（1）Android從子線程更新UI就是通過Handler來實(shí)現(xiàn)的，官方發(fā)明Handler主要就是給我們更新UI用的。

其實(shí)吧，一些腦洞大開的猿類動物偏不按照常理出牌：

（1）在子線程中他偏偏不用Handler更新UI？

public class MainActivity extends AppCompatActivity { private TextView tv_text; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); tv_text = findViewById(R.id.tv_text); tv_text.setText('1111111'); Thread thread = new Thread(new Runnable() { @Override public void run() { for (int i=0;i<500000;i ){ Log.e('aa', String.valueOf(i));//耗時操作 if(i==499999){ tv_text.setText('22222222'); } } } }); thread.start(); }}

這個例子是從onCreate方法中的子線程更新UI，其中有耗時操作

圖片.png

上述的例子依然報錯，那么怎么才能不讓他報錯呢，往下看

public class MainActivity extends AppCompatActivity {    private TextView tv_text;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        tv_text = findViewById(R.id.tv_text);        tv_text.setText('1111111');        Thread thread = new Thread(new Runnable() {                    @Override                    public void run() {                        tv_text.setText('22222222');                    }                });        thread.start();    }}

當(dāng)我去除耗時操作時，就不會報這個錯誤了，那么為什么呢？

我們來翻看源碼

在ViewRootImpl類中找到了這個方法，這個方法就是之所以報錯的根本

void checkThread() { if (mThread != Thread.currentThread()) { throw new CalledFromWrongThreadException( 'Only the original thread that created a view hierarchy can touch its views.'); }}

而ViewRootImpl對象是在執(zhí)行到onResume才創(chuàng)建時的，所以得出結(jié)論，onCreate中的子線程如果不是耗時操作，基本都是可以更新UI的，但不能保證。因?yàn)橐粋€是UI線程，一個是子線程，我們誰也不知道哪個線程更快一些。

（2）把消息從UI線程發(fā)送到子線程？

public class MainActivity extends AppCompatActivity {    private Button bt_click_me;    private Handler handler;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        bt_click_me = findViewById(R.id.bt_click_me);        bt_click_me.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Message message = Message.obtain();                message.what = 1;                handler.sendMessage(message);            }        });        Thread thread = new Thread(new Runnable() {            @Override            public void run() {                Looper.prepare();                Looper looper = Looper.myLooper();                handler = new Handler(looper){                    @Override                    public void handleMessage(Message msg) {                        super.handleMessage(msg);                        Log.d('aa', '11111');                    }                };                Looper.loop();            }        });        thread.start();    }}

UI線程本身就有Looper，但是子線程是沒有Looper的，所以必須新建Looper來輪詢Looper中的消息隊(duì)列。

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第一種場景：

第二種場景：

第二種場景總結(jié)：

（1）在子線程中他偏偏不用Handler更新UI？

（2）把消息從UI線程發(fā)送到子線程？

（2）把消息從UI線程發(fā)送到子線程？