我是初学者，所以请清楚解释一下你的答案。感谢
编辑2 我现在有
public class MainActivity extends Activity {
//other declarations
public ReadTask readTask;
protected void onCreate(Bundle savedInstanceState){
readTask = new ReadTask();
private OnClickListener test = new OnClickListener()
{
@Override
public void onClick (View V)
{
readTask.execute();
//new ReadTask().execute();
}
};
private OnClickListener cancel = new OnClickListener()
{
@Override
public void onClick (View V)
{
readTask.cancel(true);
results1.setText("");
}
检查编辑1是否存在上述代码（编辑2）抛出的错误; 感谢
我在应用程序中有一个取消按钮。点击这个，就是这个 发生;
public void onClick (View V)
{
InputMethodManager inputManager = (InputMethodManager)
getSystemService(Context.INPUT_METHOD_SERVICE);
inputManager.hideSoftInputFromWindow(getCurrentFocus().getWindowToken(),
InputMethodManager.HIDE_NOT_ALWAYS);
new ReadTask().onCancelled();
new ReadTask().cancel(true);
results1.setText("Cancelled");
}
ReadTask像这样扩展了AsyncTask;
public class ReadTask extends AsyncTask<Void, String, Void>
{
BufferedReader reader = null;
public volatile boolean isCancelled=false;
//other declarations
protected void onCancelled() {
super.onCancelled();
isCancelled = true;
new ReadTask().cancel(true);
}
@Override
protected Void doInBackground(Void... params)
{
// isCancelled=false;
try {
for(int k=x1[3];k<=x2[3];k++)
{
//bunch of code
Log.w("5.newString", newString);
String isC1 = String.valueOf(isCancelled);
Log.w("5.Is cancelled?", isC1); // at this point, VALUE IS ALWAYS FALSE !
if (isCancelled() || isCancelled == true) {Log.d("Entered","WHY");break;}
//bunch of code
} //bunch of catches
这里尝试了两种方法，一种是布尔变量isCancelled，另一种是ReadTask（）。cancel（true）; - 两个都不起作用。为什么呢？
我是新手，所以请清楚解释一下你的答案。感谢
**************编辑1 - Logcat输出*******
11-27 17:18:56.626: W/dalvikvm(28775): threadid=1: thread exiting with uncaught exception (group=0x41bcee48)
11-27 17:18:56.636: E/AndroidRuntime(28775): FATAL EXCEPTION: main
11-27 17:18:56.636: E/AndroidRuntime(28775): Process: com.example.pingtest, PID: 28775
11-27 17:18:56.636: E/AndroidRuntime(28775): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.pingtest/com.example.pingtest.MainActivity}: java.lang.NullPointerException
11-27 17:18:56.636: E/AndroidRuntime(28775): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2202)
11-27 17:18:56.636: E/AndroidRuntime(28775): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2252)
11-27 17:18:56.636: E/AndroidRuntime(28775): at android.app.ActivityThread.access$800(ActivityThread.java:139)
11-27 17:18:56.636: E/AndroidRuntime(28775): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1200)
11-27 17:18:56.636: E/AndroidRuntime(28775): at android.os.Handler.dispatchMessage(Handler.java:102)
11-27 17:18:56.636: E/AndroidRuntime(28775): at android.os.Looper.loop(Looper.java:136)
11-27 17:18:56.636: E/AndroidRuntime(28775): at android.app.ActivityThread.main(ActivityThread.java:5103)
11-27 17:18:56.636: E/AndroidRuntime(28775): at java.lang.reflect.Method.invokeNative(Native Method)
11-27 17:18:56.636: E/AndroidRuntime(28775): at java.lang.reflect.Method.invoke(Method.java:515)
11-27 17:18:56.636: E/AndroidRuntime(28775): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
11-27 17:18:56.636: E/AndroidRuntime(28775): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:606)
11-27 17:18:56.636: E/AndroidRuntime(28775): at dalvik.system.NativeStart.main(Native Method)
11-27 17:18:56.636: E/AndroidRuntime(28775): Caused by: java.lang.NullPointerException
11-27 17:18:56.636: E/AndroidRuntime(28775): at com.example.pingtest.MainActivity$ReadTask.<init>(MainActivity.java:120)
11-27 17:18:56.636: E/AndroidRuntime(28775): at com.example.pingtest.MainActivity.onCreate(MainActivity.java:39)
11-27 17:18:56.636: E/AndroidRuntime(28775): at android.app.Activity.performCreate(Activity.java:5275)
11-27 17:18:56.636: E/AndroidRuntime(28775): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
11-27 17:18:56.636: E/AndroidRuntime(28775): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2166)
11-27 17:18:56.636: E/AndroidRuntime(28775): ... 11 more
11-27 17:19:05.665: I/Process(28775): Sending signal. PID: 28775 SIG: 9
答案 0 :(得分：1)
您好我在您的代码中做了一些更改，请说明。
public class Asyn extends Activity {
public ReadTask readTask;
// keep global to prevent creating new async task on every time button click
boolean isCancelled = true;
TextView results1;
Button start, end;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.qww);
start = (Button) findViewById(R.id.button1);
end = (Button) findViewById(R.id.button2);
start.setOnClickListener(test);
end.setOnClickListener(cancel);
results1 = (TextView) findViewById(R.id.textView1);
}
private OnClickListener test = new OnClickListener() {
@Override
public void onClick(View V) {
if (isCancelled) { // checked to prevent creating new async task on
// every time button click
Toast.makeText(Asyn.this, "new task started", Toast.LENGTH_LONG)
.show();
readTask = new ReadTask();
readTask.execute();
isCancelled = false;
} else {
Toast.makeText(Asyn.this, "Task already runing",
Toast.LENGTH_LONG).show();
}
}
};
private OnClickListener cancel = new OnClickListener() {
@Override
public void onClick(View V) {
if (null != readTask) {
Toast.makeText(Asyn.this, "Task stop", Toast.LENGTH_LONG)
.show();
readTask.cancel(true);
isCancelled = true;
}
results1.setText("");
}
};
private class ReadTask extends AsyncTask<Void, String, Void> {
protected void onProgressUpdate(String... values) {
super.onProgressUpdate(values);
results1.setText(values[0].toString());
}
@Override
protected Void doInBackground(Void... params) {
try {
for (int k = 0; k <= 10; k++) {
// bunch of code
Log.w("5.newString", k + "");
String isC1 = String.valueOf(isCancelled);
Log.w("5.Is cancelled?", isC1); // at this point, VALUE IS
//below line for instant update as soon as the value is changed
publishProgress(" "+k + "");
// use thread to see the update otherwise remove it
Thread.sleep(2000);
if (isCancelled() || isCancelled == true) {
Log.d("Entered", "WHY");
break;
}
// bunch of code
}
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
@Override
protected void onPostExecute(Void result) {
// TODO Auto-generated method stub
super.onPostExecute(result);
// assign true to isCancelled so that user can start task again
isCancelled = true;
}
}
}
答案 1 :(得分：0)
每次从ReadTask创建新对象时，它都无法正常工作。如果你想取消现有的工作。你应该这样做：
public class MainActivity extends Activity {
ReadTask readTask;
@Override
public void onCreate (Bundle b) {
...
readTask = new ReadTask();
btnStart.setOnClickListener(new OnClickListener() {
@Override
void onClick(View v) {
readTask.execute();
}
}
btnCancel.setOnClickListener(new OnClickListener() {
@Override
void onClick(View v) {
readTask.cancel();
}
}
}
}
答案 2 :(得分：0)
new ReadTask().onCancelled();
new ReadTask().cancel(true);
这部分代码错误，因为它们都引用了ReadTask类的不同实例。 你可以声明一个ReadTask类的全局变量
public ReadTask rt;
并在onCreate()方法上初始化。
rt = new ReadTask();
然后您就可以在活动的每个部分使用它了
使用rt.execute()启动它并使用rt.cancel(true)