android 异步 执行 任务 例子 附讲解

【实例简介】

• The AsyncTask instance must be created in UI thread. 
• .execute must be invoked on the UI thread.
• Never call  objMyTask.onPreExecute(), objMyTask.doInBackground(), objMyTask.onProgressUpdate(),  objMyTask.onPostExecute manually.
• The AsyncTask can be executed only once (an exception will be thrown if a second execution is attempted.)

1. onPreExecute() 
2. doInBackground()
3. onProgressUpdate()
4. onPostExecute() 

• onPreExecute-This method is called first when you start AsyncTask using objAsync.execute().And mostly this method is use for initializing dialog(ProgressDialog,CustomDialog) and showing.

• doInBackground-The main purpose of AsyncTask is accomplished by this method.Any non-UI thread process is running in this method.Such as Rss Feed Reader,Image and video Uploading and Downloading.You cant handle your View in this method.Because this method is non-UI thread.While any background process is running if you want to handle UI therea are  onProgressUpdate method. after completion of process this method send result to OnPostExecute.

• onProgressUpdate-While backgrounding task is running ,you can handle your UI using this method .Such as status of downloading or uploading task.and this method is called from  doInBackground.Using publishProgress() you can call onProgressUpdate method to update UI while process is running.

• onPostExecute -This method is called after the background computation finishes.The result of background process in passed in this method as parameters.And now you can dismiss progress dialog ,to indicate that background task is completed.

【实例截图】

【核心代码】

package com.samir;

import android.app.Activity;
import android.app.AlertDialog;
import android.app.Dialog;
import android.content.DialogInterface;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.Window;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;

public class MainActivity extends Activity {

	Button btnStart;
	MyTask objMyTask;

	private static final int SLEEP_TIME=((60*1000)/100);
	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);

		btnStart = (Button) findViewById(R.id.btnstart);
		btnStart.setOnClickListener(new OnClickListener() {

			@Override
			public void onClick(View v) {
				objMyTask = new MyTask();
				objMyTask.execute();
			}
		});
	}

	class MyTask extends AsyncTask<Void, Integer, Void> {

		Dialog dialog;
		ProgressBar progressBar;
		TextView tvLoading,tvPer;
		Button btnCancel;

		@Override
		protected void onPreExecute() {
			super.onPreExecute();
			dialog = new Dialog(MainActivity.this);
			dialog.setCancelable(false);
			dialog.requestWindowFeature(Window.FEATURE_NO_TITLE);
			dialog.setContentView(R.layout.progressdialog);

			progressBar = (ProgressBar) dialog.findViewById(R.id.progressBar1);
			tvLoading = (TextView) dialog.findViewById(R.id.tv1);
			tvPer = (TextView) dialog.findViewById(R.id.tvper);
			btnCancel = (Button) dialog.findViewById(R.id.btncancel);

			btnCancel.setOnClickListener(new OnClickListener() {

				@Override
				public void onClick(View v) {
					objMyTask.cancel(true);
					dialog.dismiss();
				}
			});

			dialog.show();
		}

		@Override
		protected Void doInBackground(Void... params) {

			for (int i = 1; i <100; i  ) {
				if (isCancelled()) {
					break;
				} else {
					System.out.println(i);
					publishProgress(i);
					try {
						Thread.sleep(SLEEP_TIME);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
				}
			}

			return null;
		}

		@Override
		protected void onProgressUpdate(Integer... values) {
			super.onProgressUpdate(values);
			progressBar.setProgress(values[0]);
			tvLoading.setText("Loading...  "   values[0]   " %");
			tvPer.setText(values[0] " %");
		}

		@Override
		protected void onPostExecute(Void result) {
			super.onPostExecute(result);

			dialog.dismiss();

			AlertDialog alert = new AlertDialog.Builder(MainActivity.this)
					.create();

			alert.setTitle("Completed!!!");
			alert.setMessage("Your Task is Completed SuccessFully!!!");
			alert.setButton("Dismiss", new DialogInterface.OnClickListener() {

				@Override
				public void onClick(DialogInterface dialog, int which) {
					dialog.dismiss();

				}
			});
			alert.show();
		}
	}

}