Android异步任务（AsyncTask）的设计思想(二)

private void doHardWork() {
runInBackend(url); // added url
}
};
}

protected void runInBackend(String url) { // added url
}

...
}

而调用类的代码变成这样：

public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

String url = "http://path/to/file";
new SubHandlerAndThread().doInBackground(url); //added url
}

class SubHandlerAndThread extends HandlerAndThread {
@Override
protected void runInBackend(String url) { // added url
System.out.println("Start download from url:" + url);
try {
Thread.sleep(10 * 1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}

@Override
protected void runInFrontend() {
System.out.println("finish download");
}
}
}

假如是下一个文件呢，我们是不是加一个进度更新的方法呢，于是又变成这样：

public class HandlerAndThread {
private Handler handler = new Handler() {
public void handleMessage(Message msg) {
switch (msg.what) { // added
case 0:
runInFrontend();
break;

case 1:
runInFrontendProgress(msg.arg1);
break;
}
}
};

...

final protected void publishProgress(int progress) { // added
handler.obtainMessage(1, progress, 0);
}

protected void runInFrontendProgress(int progress) { // added
}
}

public class MainActivity extends Activity {
...

class SubHandlerAndThread extends HandlerAndThread {
@Override
protected void runInBackend(String url) {
System.out.println("Start download from url:" + url);
for (int i = 0; i < 10; ++i) {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrack();
}
publishProgress(i*10); // added
}
}

...

@Override
protected void runInFrontendProgress(int progress) { // added
System.out.println("Progress: " + progress);
}
}
}

你可能已经没有耐心一版一版的进化了，那我就跳跃一下吧，一次多加几条需要：一、我们下载完了可能要得到文件的路径，所以我们给runInFrontend方法加一个输入参数filePath表示路径；二、把子类必须实现的方法改成抽象方法，类也改成抽象方法；我把代码中的一些方法名改一下，使其更好理解，把doInBackground改为execute，把runInFrontend改为onPostExecute，把runInFrontendProgress改为onProgressUpdate。最终版如下：

public abstract class HandlerAndThread {
private Handler handler = new Handler() {
public void handleMessage(Message msg) {
switch (msg.what) {
case 0:
onPostExecute((String) msg.obj);
break;

case 1:
onProgressUpdate(msg.arg1);
break;
}
}
};

public void doInBackground(final String url) {
new Thread() {
public void run() {
String result = runInBackend(url);
handler.obtainMessage(0, result);
}

};
}

final protected void publishProgress(int progress) {
handler.obtainMessage(1, progress, 0);
}

abstract protected String runInBackend(String url);
protected void onPostExecute(String filePath) { }
protected void onProgressUpdate(int progress) { }
}
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

String url = "ht