夢想會被現實磨滅，希望我能堅持的長久！

1升級原理

build.gradle 中 versionCode 1 ， versionName “1.0.0” 是升級的關鍵，versionCode是個int,versionName是個String，其中versionCode每次要升級版本都需要+1，VersionName是給使用者看的，讓使用者知道當前版本。
升級原理：
從伺服器獲取升級資訊，包括versionCode，versionName，升級下載app的地址url，是否強制升級，升級的資訊等，然後通過比較本地app的versionCode,決定是否下載app進行升級。

2一個簡單的升級示例

//獲取versionCode
  public static int getVersionCode(Context context) {
        int version = 0;
        try {
            PackageManager packageManager = context.getPackageManager();
            PackageInfo packInfo = packageManager.getPackageInfo(context.getPackageName(), 0);
            version = packInfo.
 
versionCode;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return version;
    }

public class UpdateHelper {
    private static final int ALREADY_NEW = 1;
    private static final int SHOWUPDATE = 2;
    private static final int INSTALLUPDATE = 3;

    private
 
 final String savePath = Environment.getExternalStorageDirectory()
            .getPath() + "/updateload";

    private UpdateInfoData mUpdateInfoData;
    private File mDownloadedFile = null;
    private final String defaultPackageName = "testapp.apk";
    private Context mContext = null;
    private boolean mShowToast = false;

    public UpdateHelper(Context context, boolean showToast) {
        mContext = context;
        mShowToast = showToast;
        mUpdateInfoData = new UpdateInfoData("0",0,"0","","");
    }

    private Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case ALREADY_NEW:
                    if ( mDownloadedFile != null ) {
                    //刪除上次已下載的檔案
                        if ( mDownloadedFile.exists() ) {
                            if (! mDownloadedFile.delete() ) {
                                mDownloadedFile.deleteOnExit();
                            }
                        }
                    }
                    if (mShowToast) {
                        Toast.makeText(mContext,
                                "當前已是最新版本.", Toast.LENGTH_SHORT)
                                .show();
                    }
                    break;
                case SHOWUPDATE:
                    showUpdateDialog();
                    break;
                case INSTALLUPDATE:
                    installUpdate();
                    break;
            }
        }
    };

    public void checkUpdate(final boolean reportResult) {
        try {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    boolean isNewVersionAvailable = false;
                    if (getServerVersionInfo()) {
                        String lastestVersionCode = "0";
                        if ( mUpdateInfoData != null ) {
                            if (mUpdateInfoData.getLastestVersionCode() != null) {
                                lastestVersionCode = mUpdateInfoData.getLastestVersionCode();
                            }
                            if (Integer.parseInt(lastestVersionCode) > UpdateUtil.getVersionCode(mContext))
                                isNewVersionAvailable = true;
                        }
                    }

                    if (isNewVersionAvailable) {
                        Message msg = new Message();
                        msg.what = SHOWUPDATE;
                        handler.sendMessage(msg);
                    } else {
                        if (reportResult) {
                            Message msg = new Message();
                            msg.what = ALREADY_NEW;
                            handler.sendMessage(msg);
                        }
                    }
                }
            }).start();
        } catch (NumberFormatException e) {
            e.printStackTrace();
        }
    }

    public boolean getServerVersionInfo() {
        //是否有可用版本；
        //從網路獲取升級資訊

        Call<UpdateResultData> call = null ;//ServiceFactory.newApiService().getUpdateInfo(paramsMap);
        try {
            Response<UpdateResultData> updateInfoResponse = call.execute();
            if (updateInfoResponse != null && updateInfoResponse.isSuccessful()) {
                UpdateResultData resultData = updateInfoResponse.body();
                if(resultData != null && resultData.getErrno() == 0){
                    UpdateInfoData info = updateInfoResponse.body().getResult();
                    if (info != null) {
                        mUpdateInfoData = info;
                    }else{
                        return false;
                    }
                }

            } else {
                return false;
            }

        } catch (IOException e) {
            e.printStackTrace();
            return false;
        }
        return true;
    }

    //可以換成retrofit 下載更簡單
    public boolean downloadFile(ProgressDialog pd) {
        URL url;
        try {
            String urlStr = "";
            if (mUpdateInfoData != null && mUpdateInfoData.getUrl() != null) {
                urlStr = mUpdateInfoData.getUrl();
            }
            url = new URL(urlStr);
            HttpURLConnection conn;

            conn = (HttpURLConnection) url.openConnection();
            conn.setConnectTimeout(10000);
            pd.setMax(conn.getContentLength());
            InputStream is = conn.getInputStream();
            String packageName = urlStr.substring(urlStr.lastIndexOf("/"));
            if (!packageName.endsWith("apk")) {
                packageName = defaultPackageName;
            }

            mDownloadedFile = new File(savePath, packageName);
            FileOutputStream fos = new FileOutputStream(mDownloadedFile);
            BufferedInputStream bis = new BufferedInputStream(is);
            byte[] buffer = new byte[1024];
            int len;
            int total = 0;
            while ((len = bis.read(buffer)) != -1) {
                fos.write(buffer, 0, len);
                total += len;
                //update progress
                pd.setProgress(total);
            }
            fos.close();
            bis.close();
            is.close();
            return true;

        } catch (MalformedURLException e) {
            e.printStackTrace();
            return false;
        } catch (IOException e) {
            e.printStackTrace();
            return false;
        }

    }

    public void InstallAPK(Context context) {

        if ( context == null ) {
            return;
        }

        Intent intent = new Intent(Intent.ACTION_VIEW);
        intent.setDataAndType(Uri.fromFile(this.mDownloadedFile), "application/vnd.android.package-archive");
        context.startActivity(intent);
    }

    //顯示更新dialog
    private void showUpdateDialog() {
        try {
            Builder builder = new Builder(mContext);
            builder.setIcon(R.mipmap.ic_launcher);
            builder.setTitle("發現新版本");
            builder.setMessage(mUpdateInfoData.getContent().replaceAll("\\\\n","\n"));
            builder.setCancelable(false);
            builder.setPositiveButton(
                    "立即更新",
                    new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int which) {
                            final ProgressDialog pd = new ProgressDialog(mContext);
                            try {
                                pd.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
                                pd.setCancelable(false);
                                pd.setMessage("正在下載...");
                                pd.show();

                                new Thread(new Runnable() {
                                    @Override
                                    public void run() {
                                        boolean isSuccess = downloadFile(pd);
                                        if (isSuccess) {
                                            pd.dismiss();
                                            Message msg = new Message();
                                            msg.what = INSTALLUPDATE;
                                            handler.sendMessage(msg);
                                        } else {
                                            if (pd != null) {
                                                pd.dismiss();
                                            }
                                            Toast.makeText(mContext, "下載失敗，請檢查你的網路", Toast.LENGTH_SHORT).show();
                                        }
                                    }
                                }).start();
                            } catch (Exception e) {
                                e.printStackTrace();
                            }
                        }
                    }
            );
            builder.setNegativeButton( mUpdateInfoData.getForce() == 1 ? "退出應用":"暫不更新", (( dialog, which ) -> {
                if ( mUpdateInfoData != null ) {
                    if ( mUpdateInfoData.getForce() == 1 ) {
                        Process.killProcess( Process.myPid() );
                    } else {
                        dialog.dismiss();
                    }
                }
            }) );

            AlertDialog dialog = builder.create();
            dialog.show();
            // dialog.getButton(AlertDialog.BUTTON_POSITIVE).setTextColor(Color.BLUE);
            // dialog.getButton(DialogInterface.BUTTON_NEGATIVE).setTextColor(Color.BLACK);

        } catch (Exception e) {
            e.printStackTrace();
        }

    }

    private void installUpdate() {
        InstallAPK(mContext);
    }

    public void destroyHelper() {
        try {
            handler.removeCallbacksAndMessages(null);
            handler = null;
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

public class UpdateInfoData implements Serializable {
	private String lastestVersionName;
	// 1 for force, 0 for not.
	private int force;
	private String lastestVersionCode;
	private String url;
	private String content;
	}

點選之後利用ProgressDialog更新下載進度，之後完成安裝。

如果想更好的控制dialog的生命週期，或者自定義升級提示框的樣式，可以利用dialogFragment實現。

3關於強制升級

強制升級的實現方法：
使用者不升級無法使用，要嗎升級要麼退出應用，實現方式彈出的升級提示框沒有取消升級按鈕，或者有不升級按鈕點選之後是退出應用。
一般不需要強制升級這個功能，但是有時由於後端介面升級或者資料結構改變，或者重大功能上線都可能需要強制升級功能，但儘量平穩的升級。