当我们加载一张图片的时候,加载的过程中我们想要取消操作,该怎么办呢?调用Asynctask的 cancel) 方法就可以了,我们看代码:
先看一个例子:
MainAciticty.java
package cn.lixyz.asynctest; import android.app.Activity; import android.graphics.Bitmap; import android.graphics.BitmapFactory; import android.os.AsyncTask; import android.os.Bundle; import android.util.Log; import android.view.View; import android.widget.Button; import android.widget.ImageView; import android.widget.ProgressBar; import java.io.BufferedInputStream; import java.io.IOException; import java.io.InputStream; import java.net.URL; import java.net.URLConnection; public class MainActivity extends Activity { private Button btStart; private Button btStop; private ImageView imageView; private ProgressBar progressBar; private String URL = "http://ww3.sinaimg.cn/bmiddle/612c96afjw1ewwftl3uqkj20u00koq48.jpg"; @Override protected void onCreateBundle savedInstanceState) { super.onCreatesavedInstanceState); setContentViewR.layout.activity_main); //获取组件 btStart = Button) findViewByIdR.id.btStart); btStop = Button) findViewByIdR.id.btStop); imageView = ImageView) findViewByIdR.id.imageView); progressBar = ProgressBar) findViewByIdR.id.progressBar); //给button设置点击事件,点击按钮,启动异步任务 MyOnClickListener moc = new MyOnClickListener); btStart.setOnClickListenermoc); btStop.setOnClickListenermoc); } class MyOnClickListener implements View.OnClickListener { LoadImage li = new LoadImage); @Override public void onClickView v) { switch v.getId)){ case R.id.btStart: li.executeURL); break; case R.id.btStop: li.canceltrue); break; } } } class LoadImage extends AsyncTask<String, Integer, Bitmap> { //开始执行耗时操作,连接网络获取图片,并且将Bitmap返回 @Override protected Bitmap doInBackgroundString... params) { int i = 0; while true) { if i > 100) { break; } i++; try { Thread.sleep100); } catch InterruptedException e) { e.printStackTrace); } Log.d"TTTT","" + i); publishProgressi); } String url = params[0]; Bitmap bitmap = null; URLConnection connection; InputStream is; try { connection = new URLurl).openConnection); is = connection.getInputStream); BufferedInputStream bis = new BufferedInputStreamis); bitmap = BitmapFactory.decodeStreambis); is.close); bis.close); } catch IOException e) { e.printStackTrace); } return bitmap; } //onPreExcute方法是在doInBackGround方法前执行,用于做一些初始化操作,这里将progressBar显示出来 @Override protected void onPreExecute) { super.onPreExecute); progressBar.setVisibilityView.VISIBLE); } //doInBackGround方法执行后,会自动执行该方法,获取到doInBackGround返回的对象,然后更改UI @Override protected void onPostExecuteBitmap bitmap) { super.onPostExecutebitmap); progressBar.setVisibilityView.GONE); imageView.setImageBitmapbitmap); } @Override protected void onProgressUpdateInteger... values) { super.onProgressUpdatevalues); progressBar.setProgressvalues[0]); } } }
acticity_main.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical"> <RelativeLayout android:layout_width="match_parent" android:layout_height="match_parent" android:layout_weight="4"> <ImageView android:id="@+id/imageView" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_centerInParent="true"/> <ProgressBar android:id="@+id/progressBar" android:visibility="gone" android:layout_centerInParent="true" style="@style/Widget.AppCompat.ProgressBar.Horizontal" android:layout_width="wrap_content" android:layout_height="wrap_content" /> </RelativeLayout> <LinearLayout android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_weight="1"> <Button android:id="@+id/btStart" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_weight="1" android:layout_gravity="center" android:text="点击载入" /> <Button android:id="@+id/btStop" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_weight="1" android:text="点击退出"/> </LinearLayout> </LinearLayout>
执行结果:
点击退出按钮之后,进度条停止加载了,似乎已经成功了,但是我们看log:
后台进度依旧在更新着,AsyncTask实际上并没有停止,原来,调用了cancel之后,并不是真正的退出,而是将AsyncTask标记为Canceled,你需要在doInBackground中随时查看状态,如果是canceled,就退出:
package cn.lixyz.asynctest; import android.app.Activity; import android.graphics.Bitmap; import android.graphics.BitmapFactory; import android.os.AsyncTask; import android.os.Bundle; import android.util.Log; import android.view.View; import android.widget.Button; import android.widget.ImageView; import android.widget.ProgressBar; import java.io.BufferedInputStream; import java.io.IOException; import java.io.InputStream; import java.net.URL; import java.net.URLConnection; public class MainActivity extends Activity { private Button btStart; private Button btStop; private ImageView imageView; private ProgressBar progressBar; private String URL = "http://ww3.sinaimg.cn/bmiddle/612c96afjw1ewwftl3uqkj20u00koq48.jpg"; @Override protected void onCreateBundle savedInstanceState) { super.onCreatesavedInstanceState); setContentViewR.layout.activity_main); //获取组件 btStart = Button) findViewByIdR.id.btStart); btStop = Button) findViewByIdR.id.btStop); imageView = ImageView) findViewByIdR.id.imageView); progressBar = ProgressBar) findViewByIdR.id.progressBar); //给button设置点击事件,点击按钮,启动异步任务 MyOnClickListener moc = new MyOnClickListener); btStart.setOnClickListenermoc); btStop.setOnClickListenermoc); } class MyOnClickListener implements View.OnClickListener { LoadImage li = new LoadImage); @Override public void onClickView v) { switch v.getId)){ case R.id.btStart: li.executeURL); break; case R.id.btStop: li.canceltrue); break; } } } class LoadImage extends AsyncTask<String, Integer, Bitmap> { //开始执行耗时操作,连接网络获取图片,并且将Bitmap返回 @Override protected Bitmap doInBackgroundString... params) { int i = 0; while true) { if i > 100) { break; } if isCancelled)){ break; } i++; try { Thread.sleep100); } catch InterruptedException e) { e.printStackTrace); } Log.d"TTTT","" + i); publishProgressi); } String url = params[0]; Bitmap bitmap = null; URLConnection connection; InputStream is; try { connection = new URLurl).openConnection); is = connection.getInputStream); BufferedInputStream bis = new BufferedInputStreamis); bitmap = BitmapFactory.decodeStreambis); is.close); bis.close); } catch IOException e) { e.printStackTrace); } return bitmap; } //onPreExcute方法是在doInBackGround方法前执行,用于做一些初始化操作,这里将progressBar显示出来 @Override protected void onPreExecute) { super.onPreExecute); progressBar.setVisibilityView.VISIBLE); } //doInBackGround方法执行后,会自动执行该方法,获取到doInBackGround返回的对象,然后更改UI @Override protected void onPostExecuteBitmap bitmap) { super.onPostExecutebitmap); progressBar.setVisibilityView.GONE); imageView.setImageBitmapbitmap); } @Override protected void onProgressUpdateInteger... values) { super.onProgressUpdatevalues); progressBar.setProgressvalues[0]); } } }
cancel