android– 按下后退按钮时异步任务崩溃.

概述当调用片段暂停/删除时,异步任务会发生什么？我假设任务完成但是有人可以确认这一点很棒.我试图找到一个修复我的崩溃.我有一个异步任务,是一个递归图像加载器.图像加载器是它为其加载图像的listFragment的内部类.除非在完成加载所有图像之前按下后退按钮,否则图像加载器工作正常.当

当调用片段暂停/删除时,异步任务会发生什么？我假设任务完成但是有人可以确认这一点很棒.我试图找到一个修复我的崩溃.

我有一个异步任务,是一个递归图像加载器.图像加载器是它为其加载图像的ListFragment的内部类.除非在完成加载所有图像之前按下后退按钮,否则图像加载器工作正常.当我在所有图像完成之前按下后退按钮加载应用程序崩溃.我觉得图像加载器失去了对适配器的访问权限,但我不确定我是否正确地攻击了这个问题.我不确定如何在异步任务中修复此问题.如果有人可以帮助我.甚至反省了一个想法让我思考的不同.

任何帮助都是极好的.谢谢你的时间,甚至看.

ImageLoader的

private class ImageLoaderTask extends        AsyncTask<HashMap<String, Object>, VoID, HashMap<String, Object>> {    @OverrIDe    protected HashMap<String, Object> doInBackground(            HashMap<String, Object>... hm) {        inputStream in = null;        String url = (String) hm[0].get("image");        int pos = (Integer) hm[0].get("pos");        url = "http://kzfr.org/u/img/small/" + url;        URL mUrl;        try {            mUrl = new URL(url);            URLConnection urlConnect = (URLConnection) mUrl                    .openConnection();            in = urlConnect.getinputStream();            file cacheDirectory = getSherlockActivity().getBaseContext()                    .getCacheDir();            file tmp = new file(cacheDirectory.getPath() + "/kzfr_small"                    + pos + ".png");            fileOutputStream fOut = new fileOutputStream(tmp);            Bitmap b = BitmapFactory.decodeStream(in);            b.compress(Bitmap.CompressFormat.PNG, 100, fOut);            fOut.flush();            fOut.close();            HashMap<String, Object> hmBmp = new HashMap<String, Object>();            hmBmp.put("blank_start", tmp.getPath());            hmBmp.put("pos", pos);            return hmBmp;        } catch (Exception e) {            e.printstacktrace();        }        return null;    }    @OverrIDe    protected voID onPostExecute(HashMap<String, Object> result) {        // this is the img path that leads to a local image that was just        // saved on the phone.        String imgPath = (String) result.get("blank_start");        // the position of the image in the ListvIEw        int position = (Integer) result.get("pos");        // local adapter        SimpleAdapter adapter = (SimpleAdapter) getlistadapter();        // hashmap entry from the local adapter at the position of the image        // that we just loaded.        HashMap<String, Object> hm = (HashMap<String, Object>) adapter                .getItem(position);        // replace the actual blank image with the image path that we just        // loaded        hm.put("blank_start", imgPath);        // update ListvIEw        adapter.notifyDataSetChanged();        // we are done with the current position increment position and move        // on.        position++;        // this while loop finds the next position in the adapter that does        // not have false for an image path. False indicates that there is        // no image and i should load a default image.        while (position < adapter.getCount()) {            hm = (HashMap<String, Object>) adapter.getItem(position);            imgPath = (String) hm.get("image");            if (!imgPath.equalsIgnoreCase("false")) {                break;            }            position++;        }        // checks to make sure that the position is not out of bounds on the        // adapter. If in bounds then there might be more images to load.        // Start a new ImageLoader. This is a recursive Class.        if (position < adapter.getCount()) {            ImageLoaderTask imageLoader = new ImageLoaderTask();            hm.put("pos", (position));            imageLoader.execute(hm);        }    }

堆栈跟踪

01-14 20:09:28.752: E/AndroIDRuntime(6069): FATAL EXCEPTION: main01-14 20:09:28.752: E/AndroIDRuntime(6069): java.lang.NullPointerException01-14 20:09:28.752: E/AndroIDRuntime(6069):     at com.appdomum.doublea.ListFrag$ImageLoaderTask.onPostExecute(ListFrag.java:272)01-14 20:09:28.752: E/AndroIDRuntime(6069):     at com.appdomum.doublea.ListFrag$ImageLoaderTask.onPostExecute(ListFrag.java:1)01-14 20:09:28.752: E/AndroIDRuntime(6069):     at androID.os.AsyncTask.finish(AsyncTask.java:417)01-14 20:09:28.752: E/AndroIDRuntime(6069):     at androID.os.AsyncTask.access0(AsyncTask.java:127)01-14 20:09:28.752: E/AndroIDRuntime(6069):     at androID.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:429)01-14 20:09:28.752: E/AndroIDRuntime(6069):     at androID.os.Handler.dispatchMessage(Handler.java:99)01-14 20:09:28.752: E/AndroIDRuntime(6069):     at androID.os.Looper.loop(Looper.java:130)01-14 20:09:28.752: E/AndroIDRuntime(6069):     at androID.app.ActivityThread.main(ActivityThread.java:3806)01-14 20:09:28.752: E/AndroIDRuntime(6069):     at java.lang.reflect.Method.invokeNative(Native Method)01-14 20:09:28.752: E/AndroIDRuntime(6069):     at java.lang.reflect.Method.invoke(Method.java:507)01-14 20:09:28.752: E/AndroIDRuntime(6069):     at     com.androID.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)01-14 20:09:28.752: E/AndroIDRuntime(6069):     at   com.androID.internal.os.ZygoteInit.main(ZygoteInit.java:597)01-14 20:09:28.752: E/AndroIDRuntime(6069):     at dalvik.system.NativeStart.main(Native Method)

解决方法:

我想你在这里走在正确的轨道上.

在编写自定义AsyncTasks时,许多开发人员忘记了,有必要处理任务取消.简单地取消任务并不意味着它的代码不会在(例如)可能在当时释放的Activity / Fragment资源上执行.这通常在以下步骤中完成：

>创建任务后,保存对它的引用.
>在活动/片段完成后取消所有正在运行的任务.这通常在Activity.onDestroy()/ Fragment.onDestroy()中完成.删除对已取消任务的所有引用.
>在任务代码中：

>在doInBackground()中定期检查它是否为isCancelled().如果是这样,整理并中止进一步执行.
>最终在onCancelled()中处理取消(在UI线程上运行).
>在onPostExecute()中执行取消任务的任何处理(如果任务被取消,则使用null结果执行).

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