Android Thread的限制!

1. Do not block the UI thread 
2. Do not access the Android UI toolkit from outside the UI thread

由第二點得知只能在UI Thread控制UI所以~~~Android提供了以下簡單方法:

• Activity.runOnUiThread(Runnable)
• View.post(Runnable)
• View.postDelayed(Runnable, long)

這些方法可以讓你在另外一個執行緒控制UI

但是程式中過多這樣的寫法是不好的，於是發展了一個AsyncTask ,可將UI與背景想做的事分開，如此程式維護上比較清晰

要使用AsyncTask  必須Extends AsyncTask 做法如下：已下轉貼Android API低阿~

public void onClick(View v) {
    //你要執行時呼叫execute
    new DownloadImageTask().execute("http://example.com/image.png");
}

private class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {
    /** The system calls this to perform work in a worker thread and
      * delivers it the parameters given to AsyncTask.execute() */
    protected Bitmap doInBackground(String... urls) {
        //這裡做你想在背景做的事，如由網錄下在圖檔等...
return loadImageFromNetwork(urls[0]);
    }
    
    /** The system calls this to perform work in the UI thread and delivers
      * the result from doInBackground() */
    protected void onPostExecute(Bitmap result) {
//這裡做在ＵＩ裡顯示什麼東西!如將網路抓來的圖片顯示在ImageView中
        mImageView.setImageBitmap(result);
    }
}

其他：

• You can specify the type of the parameters, the progress values, and the final value of the task, using generics
• The method doInBackground() executes automatically on a worker thread
• onPreExecute(), onPostExecute(), and onProgressUpdate() are all invoked on the UI thread
• The value returned by doInBackground() is sent to onPostExecute()
• You can call publishProgress() at anytime in doInBackground() to execute onProgressUpdate() on the UI thread
• You can cancel the task at any time, from any thread