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AsyncTask的注意事項(xiàng)

asyncTask的執(zhí)行順序

在execute執(zhí)行的參數(shù)，傳遞進(jìn)來的參數(shù)作為doInBackground,String 傳遞進(jìn)來的第一個(gè)參數(shù)，

先調(diào)用PreExcute的操作，然后調(diào)用doInBackground的方法開始真正的異步操作原理，最后返回指定的類（onPostexecute）里面.

doBackground 開啟異步線程進(jìn)行操作的，基本屬于耗時(shí)的操作

onPreExecute 線程開啟之前的操作

前面設(shè)置的返回值設(shè)置的參數(shù)為Bitmap<String ,Void,Bitmap>,前面調(diào)用doInBackground后返回Bitmap的值,所以在onPostExcute返回的值為Bitmap,可以去設(shè)置UI。

1.獲取傳遞進(jìn)來的參數(shù) String url=params[0]只獲取第一位參數(shù)

2.網(wǎng)絡(luò)的操作，BitmapFactory.decodeStream 將輸入流解析成Bitmap

3.將Bitmap作為返回值

AsynckTask 加載圖片

他的執(zhí)行順序

onPreExcute執(zhí)行前的方法

doInBackground? 執(zhí)行中的方法

onPostExcute 執(zhí)行后的方法

AsynckTask的異步參數(shù),和四種方法

//Main.java:

public?class?Main?extends?AppCompatActivity?{
????ImageView?imageView;
????ProgressBar?progressBar;
????ProgressBar?progressBar2;
????TextView?jishu,?hint;
????static?Handler?myHandler;
????ImageLoad?i1;
????Button?btn;

????@Override
????protected?void?onCreate(@Nullable?Bundle?savedInstanceState)?{
????????super.onCreate(savedInstanceState);
????????setContentView(R.layout.activity_main);
????????imageView?=?(ImageView)?findViewById(R.id.imageView);
????????progressBar?=?(ProgressBar)?findViewById(R.id.progressBar);
????????progressBar2?=?(ProgressBar)?findViewById(R.id.progressBar2);
????????jishu?=?(TextView)?findViewById(R.id.textView);
????????hint?=?(TextView)?findViewById(R.id.tv);
????????i1?=?new?ImageLoad(imageView,?progressBar,?progressBar2);
????????btn?=?(Button)?findViewById(R.id.button);
????}

????@Override
????protected?void?onPause()?{
????????super.onPause();
????????if?(i1?!=?null?&&?i1.getStatus()?==?AsyncTask.Status.RUNNING)?{
????????????i1.cancel(true);
????????}
????}

????public?void?startload(View?v)?{
????????btn.setEnabled(false);
????????jishu.setVisibility(View.VISIBLE);
????????myHandler?=?new?Handler()?{
????????????@Override
????????????public?void?handleMessage(Message?msg)?{
????????????????super.handleMessage(msg);
????????????????jishu.setText("Loading:"?+?msg.arg1?+?"%");
????????????????if?(msg.arg1?==?100)?{
????????????????????jishu.setVisibility(View.INVISIBLE);
????????????????????btn.setVisibility(View.INVISIBLE);
????????????????????hint.setVisibility(View.VISIBLE);
????????????????}
????????????}
????????};
????????i1.execute("https://ss0.bdstatic.com/70cFvHSh_Q1YnxGkpoWK1HF6hhy/it
????????/u=3651717973,1621948329&fm=26&gp=0.jpg");
????}

}


//ImageLoad.java:

public?class?ImageLoad?extends?AsyncTask<String,?Integer,?Bitmap>?{
????ImageView?imageView;
????ProgressBar?progressBar;
????ProgressBar?progressBar2;
????static?Message?msg;

????public?ImageLoad(ImageView?imageView,?ProgressBar?progressBar,?ProgressBar?progressBar2)?{
????????this.imageView?=?imageView;
????????this.progressBar?=?progressBar;
????????this.progressBar2?=?progressBar2;
????}

????@Override
????protected?void?onPreExecute()?{
????????super.onPreExecute();
????????progressBar.setVisibility(View.VISIBLE);
????????progressBar2.setVisibility(View.VISIBLE);
????}

????@Override
????protected?Bitmap?doInBackground(String...?params)?{
????????Bitmap?bitmap?=?null;
????????HttpURLConnection?conn;
????????InputStream?is;
????????String?url?=?params[0];

????????try?{
????????????conn?=?(HttpURLConnection)?new?URL(url).openConnection();
????????????is?=?conn.getInputStream();
????????????BufferedInputStream?bis?=?new?BufferedInputStream(is);
????????????for?(int?i?=?0;?i?<?100;?i++)?{
????????????????if?(isCancelled())?{
????????????????????break;
????????????????}
????????????????publishProgress(i);
????????????????msg?=?Main.myHandler.obtainMessage();
????????????????msg.arg1?=?i?+?1;
????????????????Main.myHandler.sendMessage(msg);
????????????????Thread.sleep(25);
????????????}
????????????bitmap?=?BitmapFactory.decodeStream(is);
????????????is.close();
????????????bis.close();
????????}?catch?(IOException?e)?{
????????????e.printStackTrace();
????????}?catch?(InterruptedException?e)?{
????????????e.printStackTrace();
????????}
????????return?bitmap;
????}

????@Override
????protected?void?onProgressUpdate(Integer...?values)?{
????????super.onProgressUpdate(values);
????????progressBar2.setProgress(values[0]);
????}

????@Override
????protected?void?onPostExecute(Bitmap?bitmap)?{
????????super.onPostExecute(bitmap);
????????if?(isCancelled())?{
????????????return;
????????}
????????progressBar.setVisibility(View.INVISIBLE);
????????progressBar2.setVisibility(View.INVISIBLE);
????????imageView.setImageBitmap(bitmap);
????}
}

//activity_main.xml:

<?xml?version="1.0"?encoding="utf-8"?>
<LinearLayout?xmlns:android="http://schemas.android.com/apk/res/android"
????xmlns:app="http://schemas.android.com/apk/res-auto"
????xmlns:tools="http://schemas.android.com/tools"
????android:layout_width="match_parent"
????android:layout_height="match_parent"
????android:orientation="vertical"
????android:padding="10dp"
????tools:context=".Main">

????<Button
????????android:id="@+id/button"
????????android:layout_width="match_parent"
????????android:layout_height="wrap_content"
????????android:text="Show?the?Image"
????????android:layout_gravity="center"
????????android:onClick="startload"/>

????<TextView
????????android:id="@+id/tv"
????????android:layout_width="wrap_content"
????????android:layout_height="wrap_content"
????????android:visibility="invisible"
????????android:layout_gravity="center"
????????android:text="已成功顯示"
????????android:textSize="40dp"/>

????<ImageView
????????android:id="@+id/imageView"
????????android:layout_width="wrap_content"
????????android:layout_height="wrap_content"/>

????<ProgressBar
????????android:id="@+id/progressBar"
????????android:layout_width="match_parent"
????????android:layout_height="wrap_content"
????????android:layout_marginTop="100dp"
????????android:visibility="invisible"?/>

????<TextView
????????android:id="@+id/textView"
????????android:layout_width="wrap_content"
????????android:layout_height="wrap_content"
????????android:visibility="invisible"
????????android:textSize="20dp"
????????android:layout_marginTop="30dp"
????????android:layout_gravity="center"
????????android:text=""?/>

????<ProgressBar
????????android:id="@+id/progressBar2"
????????
????????android:layout_width="match_parent"
????????android:layout_height="wrap_content"
????????android:visibility="invisible"?/>

</LinearLayout>

public?void?onPause(){
if?(mtask?!?=?null?||?mtask.getStatus()?==?AsyncTask.Status.RUNNING)?{
????????mtask.cancel(true);
????????//若AsyncTask對(duì)象存在且處于運(yùn)行狀態(tài)，便將此對(duì)象標(biāo)記為
????????//取消狀態(tài)，然后在doInBackground()方法中利用isCancelled()
????????//方法進(jìn)行相應(yīng)操作
}

---轉(zhuǎn)
1.protected?Bitmap?doInBackground(String...?params)傳進(jìn)來的是一個(gè)可變長的數(shù)組。<br><br>
2.String?url?=?params[0];取出對(duì)應(yīng)的URL<br><br>
3.URLConnection?connection：定義網(wǎng)絡(luò)連接對(duì)象<br><br>
4.InputStream?is?：用于獲取數(shù)據(jù)的輸入流。<br><br>
5.connection?=?new?URL(url).openConnection():獲取網(wǎng)絡(luò)連接對(duì)象。<br><br>
6.?is?=?connection.getInputStream():獲取輸入流<br><br>
7.bitmap?=?BitmapFactory.decodeStream();將輸入流解析成bitmap<br><br>
8.is.close();關(guān)閉輸入流
9.bis.close()關(guān)閉輸入流
<br><br>
通過這些方式就將url所對(duì)應(yīng)的圖片解析成了bitmap

AsyncTask<String,Void,Bitmap>三個(gè)參數(shù)分別為：url類型，進(jìn)度值類型，返回值類型。
這里的例子暫時(shí)不設(shè)置進(jìn)度值，url設(shè)置為String類型，又因?yàn)槲覀兗虞d的是一張Bitmap，所以返回的參數(shù)類型設(shè)置為?Bitmap。
1.?doInBackground(String...params)傳進(jìn)來的是一個(gè)可變長數(shù)組，也就是說，我們可以傳進(jìn)不止一個(gè)參數(shù)（通過execute()傳進(jìn)來），這些參數(shù)依次存在于這個(gè)數(shù)組中?，F(xiàn)在只有一個(gè)參數(shù)，所以只要寫個(gè)params[0]取出對(duì)應(yīng)的URL即可。

回調(diào)方法的調(diào)用順序

構(gòu)建AsyncTask子類的回調(diào)方法

AsyncTask子類的參數(shù)

1. 必須在ui線程中創(chuàng)建asynctask實(shí)例，并調(diào)用execute方法

2. 重寫的方法是系統(tǒng)自動(dòng)調(diào)用 的，也就是我們只能重寫，不能選擇何時(shí)調(diào)用
   

3. 每個(gè)asynctask只能執(zhí)行一次，多次調(diào)用會(huì)引發(fā)運(yùn)行時(shí)異常

4. 只有doinbackground才是運(yùn)行在其他線程（非主線程）更新ui是在主線程
   

asynasynctask異步操作的示例

繼承asynctask<string, void ,bitmap>

string?啟動(dòng)任務(wù)時(shí)輸入的參數(shù)類型

void后臺(tái)執(zhí)行任務(wù)時(shí)返回的? 進(jìn)度值類型

bitmap?后臺(tái)執(zhí)行任務(wù)后返回? 結(jié)果的類型

1. 子類調(diào)用execute方法，以URL為傳進(jìn)去的參數(shù)

2. 調(diào)用onpreexecute，進(jìn)行初始化操作，比如將進(jìn)度條可見

3. 調(diào)用doinbackground方法進(jìn)行真正的異步操作處理，例如耗時(shí)操作，返回一個(gè)類型給onpostexecute方法

4. onpostexecute方法（運(yùn)行在主線程，可進(jìn)行ui操作，顯示圖片）
   

asynctask?<params,progress,result>?

params:啟動(dòng)任務(wù)時(shí)輸入的參數(shù)類型

progress:后臺(tái)執(zhí)行任務(wù)時(shí)返回的? 進(jìn)度值類型

result ：后臺(tái)執(zhí)行任務(wù)后返回? 結(jié)果的類型

asynctask子類必須回調(diào)的方法及順序：

2、doinbackground：必須重寫，在里面執(zhí)行了publishprogress方法，系統(tǒng)才回調(diào)3、onprogressupdate方法

1、onpreexecute：后臺(tái)耗時(shí)任務(wù)操作前被調(diào)用，用于初始化操作

4、onpostexecute：當(dāng)doinbackground完成后返回值，系統(tǒng)自動(dòng)調(diào)用onpostexecute，接收值