android應用是單線程模式的。
單線程模式需要記住兩條:
一、防止UI線程阻塞
二、確保只在UI線程中訪問Android UI工具包
在開發Android應用時必須遵守單線程模型的原則:Android UI操作並不是線程安全的並且這些操作必須在UI線程中執行。
每個Android應用程序都運行在一個dalvik虛擬機進程中,進程開始的時候會啟動一個主線程(MainThread),主線程負責處理和ui相關的事件,因此主線程通常又叫UI線程。而由於Android采用UI單線程模型,所以只能在主線程中對UI元素進行操作。
開一個線程或者在後台線程中來執行耗時的操作,如下面的例子:
public void onClick( View v ) {
new Thread( new Runnable() {
public void run() {
Bitmap b = loadImageFromNetwork(); //從網絡上下載圖片
mImageView.setImageBitmap( b ); //把圖片設置給ImageView
}
}).start()
}
上面的代碼會報錯,你可能會說邏輯很正確啊,但是它違背了Android單線程模型:Android UI操作並不是線程安全的並且這些操作必須在UI線程中執行.
例如: 如果在非UI線程直接對UI進行了操作,則會報錯:
CalledFromWrongThreadException:only the original thread that created a view hierarchy can touch its views
Android為我息循環們提供了消的機制,我們可以利用這個機制來實現線程間的通信。那麼,我們就可以在非UI線程發送消息到UI線程,最終讓Ui線程來進行ui的操作。
Andriod提供了幾種在其他線程中訪問UI線程的方法:
Activity.runOnUiThread( Runnable )
View.post( Runnable )
View.postDelayed( Runnable, long )
Hanlder
對於運算量較大的操作和IO操作,我們需要新開線程來處理這些繁重的工作,以免阻塞ui線程。
例子:下面我們以獲取CSDN logo的例子,演示如何使用Thread+Handler的方式實現在非UI線程發送消息通知UI線程更新界面
ThradHandlerActivity.java:
[java]
<span style="font-size:18px">package com.example.thread;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import com.example.test.R;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;
public class ThreadHandlerActivity extends Activity{
private static final int MSG_SUCCESS = 0;
private static final int MSG_FAILURE = 1;
private ImageView mImageView;
private Button mButton;
private Thread mThread;
@SuppressLint("HandlerLeak")
private Handler mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_SUCCESS:
mImageView.setImageBitmap((Bitmap)msg.obj);
Toast.makeText(getApplication(), "成功獲取圖片", Toast.LENGTH_LONG).show();
break;
case MSG_FAILURE:
Toast.makeText(getApplication(), "獲取圖片失敗", Toast.LENGTH_LONG).show();
break;
}
super.handleMessage(msg);
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.thread_layout);
mImageView= (ImageView) findViewById(R.id.logo);//顯示圖片的ImageView
mButton = (Button) findViewById(R.id.click);
mButton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
if (mThread == null) {
mThread = new Thread(runnable);
mThread.start();
}else {
Toast.makeText(getApplication(), "線程已經運行", Toast.LENGTH_LONG).show();
}
}
});
}
Runnable runnable = new Runnable() {
@Override
public void run() {
HttpClient hc = new DefaultHttpClient();
HttpGet hg = new HttpGet("http://csdnimg.cn/www/images/csdnindex_logo.gif");
final Bitmap bm;
try {
HttpResponse hr = hc.execute(hg);
bm = BitmapFactory.decodeStream(hr.getEntity().getContent());
} catch (Exception e) {
e.printStackTrace();
mHandler.obtainMessage(MSG_FAILURE).sendToTarget();
return;
}
mHandler.obtainMessage(MSG_SUCCESS, bm).sendToTarget();
// mImageView.setImageBitmap(bm); //出錯!不能在非ui線程操作ui元素
// mImageView.post(new Runnable() {//另外一種更簡潔的發送消息給ui線程的方法。
// @Override
// public void run() {//run()方法會在ui線程執行
// mImageView.setImageBitmap(bm);
// }
// });
}
};
}
</span>
對於上面的方法,我們使用的是handler+Thread來實現更新UI,在裡面也有一條注意的就是
[java]
<span style="font-size:18px">mImageView.setImageBitmap(bm); //出錯!不能在非ui線程操作ui元素</span>
其實我們上面提到一個方法Activity.runOnUiThread( Runnable ),將這個Runnable以UI線程的方式啟動
[java]
<span style="font-size:18px">/**
* 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();
}
}</span>
上面Activity的runOnUiThread(Runnable)方法實現。
利用Activity.runOnUiThread(Runnable)把更新ui的代碼創建在Runnable中,然後在需要更新ui時,把這個Runnable對象傳給Activity.runOnUiThread(Runnable)。 這樣Runnable對像就能在ui程序中被調用。如果當前線程是UI線程,那麼行動是立即執行。如果當前線程不是UI線程,操作是發布到事件隊列的UI線程。
使用示例:
[java]
<span style="font-size:18px">current_activity.this. runOnUiThread(new Runnable()
@Override
public void run() {
// refresh ui 的操作代碼
}
});</span>
這裡需要注意的是runOnUiThread是Activity中的方法,在線程中我們需要告訴系統是哪個activity調用,所以前面顯示的指明了activity.
所以我們修改一下上面的代碼:
[java]
<span style="font-size:18px">package com.example.thread;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import com.example.test.R;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;
public class ThreadHandlerActivity extends Activity{
private static final int MSG_SUCCESS = 0;
private static final int MSG_FAILURE = 1;
private ImageView mImageView;
private Button mButton;
@SuppressLint("HandlerLeak")
private Handler mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_SUCCESS:
mImageView.setImageBitmap((Bitmap)msg.obj);
Toast.makeText(getApplication(), "成功獲取圖片", Toast.LENGTH_LONG).show();
break;
case MSG_FAILURE:
Toast.makeText(getApplication(), "獲取圖片失敗", Toast.LENGTH_LONG).show();
break;
}
super.handleMessage(msg);
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.thread_layout);
mImageView= (ImageView) findViewById(R.id.logo);//顯示圖片的ImageView
mButton = (Button) findViewById(R.id.click);
mButton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
ThreadHandlerActivity.this.runOnUiThread(runnable);
}
});
}
Runnable runnable = new Runnable() {
@Override
public void run() {
HttpClient hc = new DefaultHttpClient();
HttpGet hg = new HttpGet("http://csdnimg.cn/www/images/csdnindex_logo.gif");
final Bitmap bm;
try {
HttpResponse hr = hc.execute(hg);
bm = BitmapFactory.decodeStream(hr.getEntity().getContent());
} catch (Exception e) {
e.printStackTrace();
mHandler.obtainMessage(MSG_FAILURE).sendToTarget();
return;
}
mImageView.setImageBitmap(bm);
}
};
}
</span>
也可以在線程裡面直接更新UI。
有人會說我傳遞一個當前的Activity到一個線程中,然後實現UI更新,那我就是調用的當前的Activity的內容,其實這個也是不對的也會提示
[java]
<span style="font-size:18px">android.view.ViewRoot$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.</span>