我知道你首先要做的就是……到底为什么你要使用 AsyncTask。
所以这是我的问题,我正在开发一些 Android 应用程序(适用于 android 2.1 或更高版本的 API 7),并且我正在模拟器上进行测试,一切都很好,所以然后我在 HTC Sensation 上进行了测试,它显示 NetworkOnMainThreadExeption!
我正在下载一些图片,然后在地图上画画。
因此,为了解决这个问题,在这种情况下,每次(互联网连接)下载图片时,我必须将其放在 AsyncTask 上才能工作。
所以我需要一种方法来知道所有图片何时完成,以便我可以开始绘画..
我尝试了很多,但没有结果,我不知道。我得到了一个带有处理程序的解决方案,但如果在较慢的网络上运行,我会得到空指针(因为图片未下载)。
所以请帮助我。
编辑:
这个想法是这样的:
Bitmap bubbleIcon ;
onCreate(){
...
// i am making call for Async
new ImgDown().execute(url);
//and then i calling functions and classes to draw with that picture bubbleIcon !
DrawOnMap(bubbleIcon);
}
//THIS IS ASYNC AND FOR EX. SUPPOSE I NEED TO DOWNLOAD THE PIC FIRST
class ImgDown extends AsyncTask<String, Void, Bitmap> {
private String url;
public ImgDown() {
}
@Override
protected Bitmap doInBackground(String... params) {
url = params[0];
try {
return getBitmapFromURL(url);
} catch (Exception err) {
}
return null;
}
@Override
protected void onPostExecute(Bitmap result) {
bubbleIcon = result;
bubbleIcon = Bitmap
.createScaledBitmap(bubbleIcon, 70, 70, true);
}
public Bitmap getBitmapFromURL(String src) {
try {
Log.e("src", src);
URL url = new URL(src);
HttpURLConnection connection = (HttpURLConnection) url
.openConnection();
connection.setDoInput(true);
connection.connect();
InputStream input = connection.getInputStream();
// /tuka decode na slika vo pomalecuk kvalitet!
BitmapFactory.Options options = new BitmapFactory.Options();
options.inSampleSize = 3;
Bitmap myBitmap = BitmapFactory
.decodeStream(new FlushedInputStream(input));
Log.e("Bitmap", "returned");
return myBitmap;
} catch (IOException e) {
e.printStackTrace();
Log.e("getBitmapFromURL", e.getMessage());
return null;
}
}
class FlushedInputStream extends FilterInputStream {
public FlushedInputStream(InputStream inputStream) {
super(inputStream);
}
public long skip(long n) throws IOException {
long totalBytesSkipped = 0L;
while (totalBytesSkipped < n) {
long bytesSkipped = in.skip(n - totalBytesSkipped);
if (bytesSkipped == 0L) {
int byteValue = read();
if (byteValue < 0) {
break; // we reached EOF
} else {
bytesSkipped = 1; // we read one byte
}
}
totalBytesSkipped += bytesSkipped;
}
return totalBytesSkipped;
}
}
}
我希望现在更清楚了。
class OpenWorkTask extends AsyncTask {
@Override
protected Boolean doInBackground(String... params) {
// do something
return true;
}
@Override
protected void onPostExecute(Boolean result) {
// The results of the above method
// Processing the results here
myHandler.sendEmptyMessage(0);
}
}
Handler myHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case 0:
// calling to this function from other pleaces
// The notice call method of doing things
break;
default:
break;
}
}
};
您可以使用 OOP 原则编写自己的委托来委托有关完成任务的信息:
task_delegate.java
public interface TaskDelegate {
void TaskCompletionResult(String result);
}
main_activity.java
public class MainActivity extends Activity implements TaskDelegate {
//call this method when you need
private void startAsynctask() {
myAsyncTask = new MyAsyncTask(this);
myAsyncTask.execute();
}
//your code
@Override
public void TaskCompletionResult(String result) {
GetSomethingByResult(result);
}
}
my_asynctask.java
public class MyAsyncTask extends AsyncTask<Void, Integer, String> {
private TaskDelegate delegate;
protected MyAsyncTask(TaskDelegate delegate) {
this.delegate = delegate;
}
//your code
@Override
protected void onPostExecute(String result) {
delegate.TaskCompletionResult(result);
}
}
class openWorkTask extends AsyncTask<String, String, Boolean> {
@Override
protected Boolean doInBackground(String... params) {
//do something
return true;
}
@Override
protected void onPostExecute(Boolean result) {
// The results of the above method
// Processing the results here
}
}
如果我是你,我会使用进度对话框。这样用户就可以在 ASyncTask 下载图片时看到正在发生的事情。在 PostExecute 上,从主代码中调用一个方法来检查图片是否为空。请记住,您无法在 doInBackground 方法中更新 UI,因此请在 onPreExecute 或 onPostExecute 中执行任何 UI 工作
private class DownloadPictures extends AsyncTask<String, Void, String>
{
ProgressDialog progressDialog;
@Override
protected String doInBackground(String... params)
{
//Download your pictures
return null;
}
@Override
protected void onPostExecute(String result)
{
progressDialog.cancel();
//Call your method that checks if the pictures were downloaded
}
@Override
protected void onPreExecute() {
progressDialog = new ProgressDialog(
YourActivity.this);
progressDialog.setMessage("Downloading...");
progressDialog.setCancelable(false);
progressDialog.show();
}
@Override
protected void onProgressUpdate(Void... values) {
// Do nothing
}
}