Android更新UI的幾種方法

黃三歲大愛(ài)人生 2019-05-07

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第一種場(chǎng)景：

在UI線程中更新UI，這種是最簡(jiǎn)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(chǎng)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(shí)候就會(huì)發(fā)現(xiàn)報(bào)了異常，異常如下

圖片.png

這個(gè)異常證明了子線程不能直接更新UI，解決方案如下

（1）通過(guò)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(lái)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(shuō)，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(gè)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(guò)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(gè)Handler對(duì)象，在子線程中用這個(gè)對(duì)象來(lái)調(diào)用post方法。

我們來(lái)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(duì)象中，有一個(gè)post方法，分析源碼得知，這個(gè)方法將形參r封裝到一個(gè)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(duì)Handler機(jī)制有一定的了解）

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

（3）通過(guò)handler發(fā)送消息來(lái)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(guò)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(lái)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(chǎng)景總結(jié)：

（1）Android從子線程更新UI就是通過(guò)Handler來(lái)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(kāi)的猿類動(dòng)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(shí)操作
                            if(i==499999){
                                tv_text.setText('22222222');
                            }
                        }
                    }
                });
        thread.start();
    }
}

這個(gè)例子是從onCreate方法中的子線程更新UI，其中有耗時(shí)操作

圖片.png

上述的例子依然報(bào)錯(cuò)，那么怎么才能不讓他報(bào)錯(cuò)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(shí)操作時(shí)，就不會(huì)報(bào)這個(gè)錯(cuò)誤了，那么為什么呢？

我們來(lái)翻看源碼

在ViewRootImpl類中找到了這個(gè)方法，這個(gè)方法就是之所以報(bào)錯(cuò)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(duì)象是在執(zhí)行到onResume才創(chuàng)建時(shí)的，所以得出結(jié)論，onCreate中的子線程如果不是耗時(shí)操作，基本都是可以更新UI的，但不能保證。因?yàn)橐粋€(gè)是UI線程，一個(gè)是子線程，我們誰(shuí)也不知道哪個(gè)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(méi)有Looper的，所以必須新建Looper來(lái)輪詢Looper中的消息隊(duì)列。

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