AsyncTask 如何将一个进程工作到另一个进程？

Posted 2023-05-07

【中文标题】AsyncTask 如何将一个进程工作到另一个进程？

【英文标题】：How does AsyncTask work one process to another one?

【发布时间】：2020-09-27 14:51:19

【问题描述】：

我目前正在自己​​学习 android，并且对 java 很陌生。我想知道 AsyncTask 是如何工作的：onPreExecute() -> doInBackground() -> onPostExecute()。当我查看其他人定义他们的 AsynTask 时，似乎在他们的代码中只声明了方法，而没有调用该方法。我无法弄清楚doInBackground() 是如何在onPreExecute() 之后出现的，而没有链接两者的代码，例如：

onPreExecute()   ~~~~~ call doInBackground()

我的意思是，当AsyncTask.execute() 被调用时，onPreExecute() 被调用，然后是doInBackground()，最后是onPostExecute()。我在库中找不到任何将这些实际连接在一起的代码。我能找到的只有这个：

@MainThread
    public final AsyncTask<Params, Progress, Result> execute(Params... params) 
        return executeOnExecutor(sDefaultExecutor, params);

@MainThread
    public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
            Params... params) 
        if (mStatus != Status.PENDING) 
            switch (mStatus) 
                case RUNNING:
                    throw new IllegalStateException("Cannot execute task:"
                            + " the task is already running.");
                case FINISHED:
                    throw new IllegalStateException("Cannot execute task:"
                            + " the task has already been executed "
                            + "(a task can be executed only once)");
            
        

        mStatus = Status.RUNNING;

        onPreExecute();

        mWorker.mParams = params;
        exec.execute(mFuture);

        return this;
    

这里当AsyncTask.execute()被调用时，onPreExecute()被调用。但是没有与doInBackground 的任何连接，该任务就可以正常工作。我觉得我缺少 java 或 android 的一些基本逻辑或过程。请帮我解决这个未解决的问题。示例代码如下所示。提前谢谢你。

    @Override
    protected void onPreExecute() 
        super.onPreExecute();
        mLoadingIndicator.setVisibility(View.VISIBLE);
    

    @Override
    protected String[] doInBackground(String... params) 

        /* If there's no zip code, there's nothing to look up. */
        if (params.length == 0) 
            return null;
        

        String location = params[0];
        URL weatherRequestUrl = NetworkUtils.buildUrl(location);

        try 
            String jsonWeatherResponse = NetworkUtils
                    .getResponseFromHttpUrl(weatherRequestUrl);

            String[] simpleJsonWeatherData = OpenWeatherJsonUtils
                    .getSimpleWeatherStringsFromJson(MainActivity.this, jsonWeatherResponse);

            return simpleJsonWeatherData;

         catch (Exception e) 
            e.printStackTrace();
            return null;
        
    

    @Override
    protected void onPostExecute(String[] weatherData) 
        // COMPLETED (19) As soon as the data is finished loading, hide the loading indicator
        mLoadingIndicator.setVisibility(View.INVISIBLE);
        if (weatherData != null) 
            // COMPLETED (11) If the weather data was not null, make sure the data view is visible
            showWeatherDataView();
            /*
             * Iterate through the array and append the Strings to the TextView. The reason why we add
             * the "\n\n\n" after the String is to give visual separation between each String in the
             * TextView. Later, we'll learn about a better way to display lists of data.
             */
            for (String weatherString : weatherData) 
                mWeatherTextView.append((weatherString) + "\n\n\n");
            
         else 
            // COMPLETED (10) If the weather data was null, show the error message
            showErrorMessage();
        

【参考方案1】：

我想你不应该在 AsyncTask 上浪费时间，因为它是 deprecated。

相反，您应该专注于谷歌推荐的协程 here 或其他一些最先进的框架来实现您想要的（例如 rx java）

【参考方案2】：

是的，你是对的。逻辑是 onPreExecute() -> doInBackground() -> onPostExecute()

Synchronous VS asynchronous

您可以阅读this 文章以获得更好的理解，即使它使用 javascript 来解释它。