AsyncTask

To understand AsyncTask asynchrony, we need to know about asynchronous tasks (multi-threading), what is thread, it can be said that threads are like watching TV while eating. AsyncTask is to facilitate the operation of background threads to update UI, essentially Handler asynchronous message processing mechanism.

Learning AsyncTask requires knowing its parameters < Params, Progress, Result>, the methods it implements, such as onPreExcecute(), onPostExecute(), background thread operation doInBackground(), display progress onProgressUpdate(), etc. I'm familiar with it here. Now I'll talk about how to use it in code.

Here we create a class

public class MainActivity extends AppCompatActivity {
 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

    }
 //Here we create an internal class that inherits AsyncTask
 public class DownloadAsyncTask extends AsyncTask<String,Integer,Boolean>{

 }

}

Let's click here to see the underlying Async Task

public abstract class AsyncTask<Params, Progress, Result> {
//Params is the type parameter, Progress is the progress, and the result is returned.
}

Here we inherit and implement it in the following ways

//Before asynchronous tasks
@Override
protected void onPreExecute() {
     super.onPreExecute();
     // Dead work
           
}

@Override
protected Boolean doInBackground(String... strings){
 //Handling events
}

@Override
protected void onPostExecute(Boolean aBoolean){
 super.onPostExecute(aBoolean);
 //Execution, execution result processing
}

//When our progress is changing
@Override
 protected void onProgressUpdate(Integer... values) {
    super.onProgressUpdate(values);
     //Receive progress, and then process: also in the UI thread.
}

//cancel
/*@Override
protected void onCancelled(Boolean aBoolean) {
     super.onCancelled(aBoolean);
}

@Override
protected void onCancelled() {
    super.onCancelled();
}*/

Here, onPreExcecute (before execution), doInBackground (in execution), onPostExecute (after execution), onProgressUpdate are usually executed.

Here is an example of how to download an apk from the network and request network data to download files. Here I'll explain in more detail how to get url and request the network.

Set up a button to click on the download file

//Define a method
private void setButtonListener(){
 mDownloadButton.setOnClickListener(new View.OnClickListener() {
           @Override
           public void onClick(View v) {
               DownloadAsyncTask asyncTask = new DownloadAsyncTask();
               asyncTask.execute(APK_URL);
            }
    });
}

To explain here, it is understandable that applying for asyncTask application is to create objects. Public class Download AsyncTask extends AsyncTask < String, Integer, Boolean > {...}, then this'execute'has not been mentioned yet, so click to see the source code.~

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

Here's the code in execution

//Explain that Boolean here is defined by this
public class DownloadAsyncTask extends AsyncTask<String,Integer,Boolean>{
}

//<Params, Progress, Result>
//String... strings represent arrays, variable parameters, and may be empty
@Override
protected Boolean doInBackground(String... strings){

}

Before execution

@Override
protected void onpreExecute(){
  super.onPreExecute();
 //Solving UI problems
}

The code in execution is downloaded:

//String... as an array
@Override
protected Boolean doInBackground(String... strings) {
  //Prevention is empty.
  if (strings != null && strings.length>0){
     //The parameter passed in is url, which is
     // DownloadAsyncTask asyncTask = new DownloadAsyncTask();
     //asyncTask.execute(APK_URL);
     //Decision
     String apkurl = strings[0];
     try{
      //Give you a download address url, then you need to create a URL object
      URL url = new URL(apkurl);//String apkurl = strings[0];
      //Construct a URLConnection to open the connection url.openConnection()
      URLConnection urlConnection = url.openConnection();

     //Input stream reads data
     InputStream inputStream = urlConnection.getInputStream();
     //Next, get the total length of the downloaded content
     int contentLength = urlConnction.getContentLength();

     //Next is to find a place to store it, just follow where it is in the directory, as a string.
     String mFilePath=Environment.getExternalStorageDirectory() + File.separator + FILE_NAME；
// Environment. getExternal Storage Directory () is a directory, which is more practical.
// File.separator is equivalent to'/'
// FILE_NAME is the file name and can be changed to "what.apk".

//Processing the download address

File apkFile = new File(mFilePath);
//Next, the file is entered into the directory.
//Read before write
//Judgement file
if(apkFile.exists()){
 boolean result = apkFile.delete();

 if(!result){
  return false;
 }

}

//Document progress bar, how many downloads have been made
int downloadSize = 0;

//Define the length of a read array, and proceed to...
byte[] bytes = new byte[1024];//1024 is not stipulated, you can decide for yourself
//How long to read?
int length;
//Create an output stream, write
OutputStream outputStream = new FileOutputStream(mFilePath);//Write to this path

while(  (length = inputStream.read(bytes)) != -1){
 outputStream.write(bytes,0,length);
 downloadSize+=length;
 //Sending progress
 publishProgress();
}
....
  }
}

After execution

@Override
protected void onPostExecute(Boolean aBoolean) {
 super.onPostExecute(aBoolean);
 //Also in the main thread, executing result processing
 //Processing UI
}

Final progress change

//When our progress is changing
@Override
protected void onProgressUpdate(Integer... values) {
 super.onProgressUpdate(values);
 if (values != null && values.length >0){
 }
}

The key point is to add network permissions and read permissions.

<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

ListView

ListView is a control we often use in Android development to display data. We can see lists of articles everywhere in Wechat lists.

Deepen understanding

public class AppListAdapter extends BaseAdapter{}

public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {}

public interface ListAdapter extends Adapter {}

public interface SpinnerAdapter extends Adapter {}

Create layout

<ListView
 android:id="@+id/main_list_view"
 android:layout_width="match_parent"
 android:layout_height="match_parent">
</ListView>

Implementing control declarations

private ListView mListView;
mListView=findViewById(R.id.main_list_view);

Step: Learn about Adapter

public class AppListAdapter extends BaseAdapter{
}

//Get package name
String packageName = mAppInfos.get(position).activityInfo.packageName;
//// Get the application name
String className = mAppInfos.get(position).activityInfo.name;
//Stitching-Clicking Method
ComponentName componentName = new ComponentName(packageName,className);

final Intent intent = new Intent();
//Setting Component Names through Intent
intent.setComponent(componentName);
startActivity(intent);

The abstract method in BaseAdapter

public int getCount();
public Object getItem(int arg0);
public long getItemId(int arg1);
public View getView(int position, View convertView, ViewGroup parent);

public View getView(int position, View covertView, ViewGroup parent){
 if(converView == null){
   converView = mInflater.inflate(R.layout.list_item, null);
  }
}

static class ViewHolder{
 public ImageView image;
 public TextView title;
}

@Override
public View getView(int position,view convertView, ViewGroup parent){
 ViewHolder holder;

 if(convertView == null){
  holder = new ViewHolder();
  convertView = mInflater.inflate(R.layout.list_item, null);
  holder.
  holder.
   convertView.setTag(holder);
 }else{
   holder=(ViewHolder)convertView.getTag();
 }

}

Editor: Dashu

Location: Sharing Android & Java Knowledge Points