android fragment更新ui 安卓更新ui

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新新人类 2023-08-06 18:10:11

文章标签 android方法更新 ide UI 子线程 文章分类 Android 移动开发

第一种场景：

在UI线程中更新UI，这种是最简单的，直接更新UI即可。

代码如下

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");
}
});
}
}

第二种场景：

从子线程中更新UI

代码如下

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();
}
});
}
}

当点击按钮更新UI的时候就会发现报了异常，异常如下

android fragment更新ui 安卓更新ui_UI

图片.png

这个异常证明了子线程不能直接更新UI，解决方案如下

(1)通过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();
}
});
}
}

我们来深入源码/**

* 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();
}
}

源码的意思是说，如果当前线程不是UI线程，那么执行mHandler.post(action);

否则直接执行run。

这个结论直接告诉了我们，Handler的post方法也能做到从子线程更新UI。

(2)通过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线程中new了一个Handler对象，在子线程中用这个对象来调用post方法。

我们来深入源码/**

* 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对象中，有一个post方法，分析源码得知，这个方法将形参r封装到一个消息里面，再利用sendMessageDelayed方法将消息发送(添加)到消息队列。(注：理解这句话需要对Handler机制有一定的了解)

我们得出结论，通过handler发送消息也能实现子线程更新UI。

(3)通过handler发送消息来实现子线程更新

UIpublic 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)通过view的post方法实现

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();
}
});
}
}

我们来深入源码/**

* 
Causes the Runnable to be added to the message queue.
* The runnable will be run on the user interface thread.
*
* @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;
}

其实最终也调用了mHandler.post(action)方法。

第二种场景总结：

(1)Android从子线程更新UI就是通过Handler来实现的，官方发明Handler主要就是给我们更新UI用的。

其实吧，一些脑洞大开的猿类动物偏不按照常理出牌：(1)在子线程中他偏偏不用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));//耗时操作
if(i==499999){
tv_text.setText("22222222");
}
}
}
});
thread.start();
}
}

这个例子是从onCreate方法中的子线程更新UI，其中有耗时操作

android fragment更新ui 安卓更新ui_UI_02

图片.png

上述的例子依然报错，那么怎么才能不让他报错呢，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();
}
}

当我去除耗时操作时，就不会报这个错误了，那么为什么呢？

我们来翻看源码

在ViewRootImpl类中找到了这个方法，这个方法就是之所以报错的根本

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

而ViewRootImpl对象是在执行到onResume才创建时的，所以得出结论，onCreate中的子线程如果不是耗时操作，基本都是可以更新UI的，但不能保证。因为一个是UI线程，一个是子线程，我们谁也不知道哪个线程更快一些。(2)把消息从UI线程发送到子线程？

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();
}
}

作者：NoBugException

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