下载应用到SD卡并安装

更新软件时要用到将软件下载到SD卡并主动弹出安装界面，就想安卓市场一样，下面总结一个简单的实训过程：
1、检测某个应用是否已经安装了：

public boolean isPkgInstalled(String packageName) {
		PackageManager pm = getPackageManager();
		try {
			pm.getPackageInfo(packageName, 0);
		} catch (Exception e) {
			return false;
		}
		return true;
	}

只需要一个包名就可以了。如果是更新软件本身就没必要检测安装，只检测版本号就可以了。

2、判断sd卡是否存在：

private boolean isSDcard() {
		if (android.os.Environment.getExternalStorageState().equals(
				android.os.Environment.MEDIA_MOUNTED)) {
			return true;
		} else {
			return false;
		}
	}

如果不存在就没必要下载了。

3、下载并保存到sd卡：

	public void down_file(String url) {
		try {
			String path = "/sdcard/download/";
			filename = url.substring(url.lastIndexOf("/") + 1);

			URL myURL = new URL(url);
			URLConnection conn = myURL.openConnection();
			conn.connect();
			InputStream is = conn.getInputStream();
			fileSize = conn.getContentLength();
			if (fileSize <= 0)
				throw new RuntimeException(getString(R.string.download_unknow_size));
			if (is == null)
				throw new RuntimeException("stream is null");
			FileOutputStream fos = new FileOutputStream(path + filename);
			byte buf[] = new byte[1024];
			downLoadFileSize = 0;
			sendMsg(0);
			do {
				int numread = is.read(buf);
				if (numread == -1) {
					break;
				}
				fos.write(buf, 0, numread);
				downLoadFileSize += numread;
				sendMsg(1);
			} while (true);
			
			sendMsg(2);
			is.close();
		} catch (Exception ex) {
			Log.e("tag", "error: " + ex.getMessage(), ex);
		}
	}

4、等下载完成了就要启动安装界面：

	private void installAPK() {
		String fileName = getSDPath() +"/download/"+filename;
		Intent intent = new Intent(Intent.ACTION_VIEW);
		intent.setDataAndType(Uri.fromFile(new File(fileName)), "application/vnd.android.package-archive");
		startActivity(intent);
	}

其中的sendMsg()方法是用于更新handler的。