Android开发AsyncTask类介绍

释放双眼,带上耳机,听听看~!

AsyncTask类的声明:

public abstract class AsyncTask<Param,Progress,Result>

Param 执行异步任务后,需要参数的数据类型

Progress 执行异步任务过程中,标识进度的数据类型

Result 执行异步任务后,需要返回的结果的数据类型

AsyncTask中的抽象方法: public abstract Result doInBackground(params… params)

让AsyncTask开始工作:

   public final AsyncTask<params,Progress,Result> execute(params…params)

该方法被调用后,会自动开启子线程并调用dnInBackground()方法,该方法必须在UI线程中调用

案例:

布局:

<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="104dp"
android:onClick="doAsyncTask"
android:text="开始" />

MainActivity:

public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
System.out.println("onCreate" + Thread.currentThread().getId());
}
public void doAsyncTask(View view){
new InnerAsyncTask().execute("");
}
private class InnerAsyncTask extends AsyncTask<Object, Object, Object>{
@Override
protected Object doInBackground(Object... params) {
for(int i = 0; i < 30;i++){
System.out.println("InnerAsyncTask" + Thread.currentThread().getId());
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
return null;
}
}
}

AsyncTask更新UI

AsyncTask约定了任务执行完毕后的回调方法,该方法并不是抽象的,开发者可以选择性的实现。

protected void onPostExecute(Result result)

该方法是运行在主线程的方法

实例:

布局:

<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="104dp"
android:onClick="doAsyncTask"
android:text="开始" />
<ImageView
android:id="@+id/imageView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/button1"
android:layout_centerHorizontal="true"
android:layout_marginTop="22dp"
android:src="https://js.aizhan.com/develop/android/@drawable/abs" />

MainActivity:

public class MainActivity extends Activity {
private ImageView image;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
image = (ImageView) findViewById(R.id.imageView1);
//   System.out.println("onCreate" + Thread.currentThread().getId());
}
public void doAsyncTask(View view){
new InnerAsyncTask().execute("");
}
private class InnerAsyncTask extends AsyncTask<String,Integer, Bitmap>{
@Override
protected Bitmap doInBackground(String... params) {
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return BitmapFactory.decodeResource(getResources(), R.drawable.abc);
}
@Override
protected void onPostExecute(Bitmap result) {
image.setImageBitmap(result);
}
}
}

 AsyncTask更新进度

AsyncTask约定了任务执行过程中,更新进度的回调方法,该方法并不是抽象的,开发者可以选择性地实现。

protected void onProgressUpdate(Progress… values)(该方法运行在主线程)

在任务执行过程中,可以调用publishProgress()方法提交进度,使得onProgressUpdate()方法被回调

实例

布局:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<TextView
android:id="@+id/tv_pb"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="100%"
android:visibility="gone"
android:textSize="16sp"/>
<Button
android:id="@+id/btn_update"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="104dp"
android:onClick="doAsyncTask"
android:text="开始" />
<ImageView
android:id="@+id/iv_image"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/btn_update"
android:layout_centerHorizontal="true"
android:layout_marginTop="22dp"
android:src="https://js.aizhan.com/develop/android/@drawable/abs" />
<ProgressBar
android:id="@+id/pb_progress"
style="?android:attr/progressBarStyleHorizontal"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:max="100"
android:visibility="gone"
android:layout_alignRight="@+id/btn_update"
android:layout_marginTop="32dp" />
</RelativeLayout>

LoadImage:

public class LoadImage extends AsyncTask<String, Integer, Object> {
private Context context;
private ImageView imageview;
private Bitmap image;
private Button button;
private ProgressBar pg;
private TextView tv;
public LoadImage(Context context, Button button, ImageView imageview,
ProgressBar pg, TextView tv) {
this.context = context;
this.imageview = imageview;
this.button = button;
this.pg = pg;
this.tv = tv;
}
@Override
protected Object doInBackground(String... params) {
for (int i = 0; i <= 100; i++) {
publishProgress(i);
try {
Thread.sleep(50);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
image = BitmapFactory.decodeResource(context.getResources(),
R.drawable.abc);
return null;
}
@Override
protected void onProgressUpdate(Integer... values) {
// TODO Auto-generated method stub
pg.setProgress(values[0]);
tv.setText(values[0] + "%");
}
@Override
protected void onPostExecute(Object result) {
imageview.setImageBitmap(image);
button.setEnabled(true);
pg.setVisibility(View.GONE);
tv.setVisibility(View.GONE);
}
}

MainActivity:

public class MainActivity extends Activity {
private ImageView image;
private Button button;
private ProgressBar pg;
private TextView tv;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
image = (ImageView) findViewById(R.id.iv_image);
button = (Button) findViewById(R.id.btn_update);
pg = (ProgressBar) findViewById(R.id.pb_progress);
tv = (TextView) findViewById(R.id.tv_pb);
}
public void doAsyncTask(View view){
button.setEnabled(false);
pg.setVisibility(View.VISIBLE);
tv.setVisibility(View.VISIBLE);
new LoadImage(this,button,image,pg,tv).execute("");
}
}

AsyncTask是一个综合了任务的执行、进度更新、结果提交的类,使用AsyncTask

可以集中的编写某个异步任务的全部代码,而不必关心线程间的通信问题,降低了

编码出错几率,并有效的提高了代码的可阅读性、可维护性等。

小案例之异步加载图片

使用到的技术: Canvas(画布)、Paint(画笔)

Canvas(画布):用来决定画布的基础属性,执行绘制

Paint(画笔):设置颜色、设置字体、其他的设置

同一次绘图过程中,可能需要多个画笔对象,或多次调整画笔的属性

使用Canvas:

public Canvas()
public Canvas(Bitmap bitmap)
public void drawRect(float left,float top,float right,float bottom,Paint paint)
public void drawBitmap(Bitmap bitmap,float left,float top,Paint paint)
public void drawText(String text,float x,float y,Paint paint)

 

使用Paint:

public Paint()
public native void setColr(int color)
public native void setAntiAlias(boolean aa)
public native void setTextSize(float textSize)
public void setTextAlign(Align align)
public Xfermode setXfermode(Xfermode xfermode)

 

 

人已赞赏
个人中心
购物车
优惠劵
今日签到
有新私信 私信列表
有新消息 消息中心
搜索