android實現程序的後台運行

	/**
	 * 改寫返回鍵事件監聽，使得back鍵功能類似home鍵，讓Acitivty退至後台時不被系統銷毀
	 */
	@Override
	public boolean onKeyDown(int keyCode, KeyEvent event) {
		PackageManager pm = getPackageManager();
		ResolveInfo homeInfo = pm.resolveActivity(
				new Intent(Intent.ACTION_MAIN)
						.addCategory(Intent.CATEGORY_HOME), 0);
		if (keyCode == KeyEvent.KEYCODE_BACK) {
			ActivityInfo ai = homeInfo.activityInfo;
			Intent startIntent = new Intent(Intent.ACTION_MAIN);
			startIntent.addCategory(Intent.CATEGORY_LAUNCHER);
			startIntent
					.setComponent(new ComponentName(ai.packageName, ai.name));
			startActivitySafely(startIntent);
			return true;
		} else
			return super.onKeyDown(keyCode, event);
	}

	private void startActivitySafely(Intent intent) {
		intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
		try {
			startActivity(intent);
		} catch (ActivityNotFoundException e) {
			Toast.makeText(this, "null", Toast.LENGTH_SHORT).show();
		} catch (SecurityException e) {
			Toast.makeText(this, "null", Toast.LENGTH_SHORT).show();
		}
	}