day4.20总结_异步任务案例和弱引用

一、异步任务案例讲解

（1）启动异步任务加载SQLite中的数据

1)AsyncTask.execute(new Runnable{public void run(){}})

2)update UI :runOnUIThread(.....): 不局限于一种写法

例子1：工作线程中更新UI

public class MainActivity extends Activity {   @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main);         new Thread(){ public void run() { setTitle("Title01");    }; }.start();   }   @Override protected void onResume() { // TODO Auto-generated method stub super.onResume();    onResume在onCreat方法之前执行，那时Activity还没建立 new Thread(){         好检查机制 public void run() { setTitle("AAAAAA"); }; }.start(); } public void onClick(View v){ new Thread(){ public void run() { setTitle("CCCCC"); }; }.start(); } }

 

（2）实现图片的高效加载

例子2：图片压缩

public class MainActivity extends Activity {   private ImageView imageView; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); imageView=(ImageView) findViewById(R.id.imageView1); } public void onClick(View v){ File sdcard=Environment.getExternalStorageDirectory(); File pic=new File(sdcard,"ql.JPG"); //可能会有内存溢出 //imageView.setImageURI(Uri.fromFile(pic)); Bitmap bitMap=decodeSampledBitmapFromResource(pic.getPath(), 300, 200); imageView.setImageBitmap(bitMap); } /**对大图片进行压缩*/ public static Bitmap decodeSampledBitmapFromResource(String  filePath, int reqWidth, int reqHeight) {   //加载图片的边界信息(只读取图片的高度，宽度，不读取具体字节) // First decode with inJustDecodeBounds=true to check dimensions final BitmapFactory.Options options =new BitmapFactory.Options(); //将此选项设置true,此时再加载图片就会只读取图片边界信息了。 options.inJustDecodeBounds = true; //加载图片边界，并将边界信息封装到options对象 BitmapFactory.decodeFile(filePath, options);   // Calculate inSampleSize(计算压缩比例) options.inSampleSize =calculateInSampleSize(options,reqWidth, reqHeight);   // Decode bitmap with inSampleSize set options.inJustDecodeBounds = false; return BitmapFactory.decodeFile(filePath, options); } /**计算压缩比例*/ public static int calculateInSampleSize( BitmapFactory.Options options, int reqWidth, int reqHeight) { // Raw height and width of image //获得实际图片的高度，宽度 final int height = options.outHeight; final int width = options.outWidth; int inSampleSize = 1;   //通过实际的高度，宽度与我们需要的高度，宽度进行比例运算得到一个压缩比例值 if (height > reqHeight || width > reqWidth) { if (width > height) { inSampleSize = Math.round((float)height / (float)reqHeight); } else { inSampleSize = Math.round((float)width / (float)reqWidth); } } return inSampleSize;//高度和宽度都按照此比例进行压缩 } }

 

例子3：图片在工作线程加载

public class MainActivity extends Activity {   private ImageView imageView; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main);     imageView=(ImageView) findViewById(R.id.imageView1); } public void onClick(View v){ File sdcard=Environment.getExternalStorageDirectory(); File pic=new File(sdcard,"ql.JPG"); //可能会有内存溢出 //imageView.setImageURI(Uri.fromFile(pic)); new BitmapWorkerTask(imageView).execute(pic.getPath());  调用异步任务 } /**在工作线程加载并压缩图片:  * 记住：以后在activity写内部类时，  * 内部类又是一个线程对象，  * 请尽量使用静态内部类  * */     static class BitmapWorkerTask extends AsyncTask<String, Void, Bitmap>{ private WeakReference<ImageView> wr; public BitmapWorkerTask(ImageView imageView) { //构建一个弱引用对象，引用外部类的中的属性imageView wr=new WeakReference<ImageView>(imageView); } @Override     protected Bitmap doInBackground(String... params) {    压缩方法放在工作线程     return decodeSampledBitmapFromResource(params[0], 300, 200);     }     @Override     protected void onPostExecute(Bitmap result) {     if (wr== null && result==null) return;     ImageView iv=wr.get();         更新UI放在主线程     if(iv!=null){     iv.setImageBitmap(result);     }     } } /**对大图片进行压缩*/ public static Bitmap decodeSampledBitmapFromResource(String  filePath,         int reqWidth, int reqHeight) {   //加载图片的边界信息(只读取图片的高度，宽度，不读取具体字节)     // First decode with inJustDecodeBounds=true to check dimensions     final BitmapFactory.Options options =      new BitmapFactory.Options();     //将此选项设置true,此时再加载图片就会只读取图片边界信息了。     options.inJustDecodeBounds = true;     //加载图片边界，并将边界信息封装到options对象     BitmapFactory.decodeFile(filePath, options);          // Calculate inSampleSize(计算压缩比例)     options.inSampleSize =     calculateInSampleSize(options,     reqWidth, reqHeight);       // Decode bitmap with inSampleSize set     options.inJustDecodeBounds = false;     return BitmapFactory.decodeFile(filePath, options); } /**计算压缩比例*/ public static int calculateInSampleSize(             BitmapFactory.Options options, int reqWidth, int reqHeight) {     // Raw height and width of image //获得实际图片的高度，宽度     final int height = options.outHeight;     final int width = options.outWidth;     int inSampleSize = 1;       //通过实际的高度，宽度与我们需要的高度，宽度进行比例运算得到一个压缩比例值     if (height > reqHeight || width > reqWidth) {         if (width > height) {             inSampleSize = Math.round((float)height / (float)reqHeight);         } else {             inSampleSize = Math.round((float)width / (float)reqWidth);         }     }     return inSampleSize;//高度和宽度都按照此比例进行压缩 } }

 

例子4：图片缓存在内存(LruCache)

public class MainActivity extends Activity {   private ImageView imageView; /**定义一个缓存对象，将加载的图片的位图对象进行缓存(缓存到内存)*/ private static LruCache<String,Bitmap> lruCache; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); imageView=(ImageView) findViewById(R.id.imageView1);         //获取Activity管理器 ActivityManager aManager=(ActivityManager)getSystemService(Context.ACTIVITY_SERVICE);   int memorySize=aManager.getMemoryClass(); 系统分配的内存32M int maxSize=memorySize*1024*1024/8;    转化为字节单位 Log.i("TAG", "memorySize="+memorySize); lruCache=new LruCache<String,Bitmap>(maxSize){ //计算缓存的每个对象占有的字节数 @Override protected int sizeOf(String key, Bitmap value) { return value.getByteCount(); } }; } public void onClick(View v){ File sdcard=Environment.getExternalStorageDirectory(); File pic=new File(sdcard,"ql.JPG"); //在此应用中key为图片的地址 Bitmap bitMap=lruCache.get(pic.getPath()); if(bitMap!=null){ Log.i("TAG", "缓存中的图片="+bitMap); imageView.setImageBitmap(bitMap); return; } Log.i("TAG", "启动异步任务加载图片!"); new BitmapWorkerTask(imageView).execute(pic.getPath()); } /**在工作线程加载并压缩图片:  * 记住：以后在activity写内部类时，  * 内部类又是一个线程对象，  * 请尽量使用静态内部类  * */ static class BitmapWorkerTask extends AsyncTask<String, Void, Bitmap>{ private WeakReference<ImageView> wr; public BitmapWorkerTask(ImageView imageView) { //构建一个弱引用对象，引用外部类的中的属性imageView wr=new WeakReference<ImageView>(imageView); } @Override protected Bitmap doInBackground(String... params) { //加载并压缩大图片 Bitmap bitMap=decodeSampledBitmapFromResource( params[0], 300, 200); //缓存图片 lruCache.put(params[0], bitMap); return bitMap; } @Override protected void onPostExecute(Bitmap result) { if (wr== null && result==null) return; ImageView iv=wr.get(); if(iv!=null){ iv.setImageBitmap(result); } } }     /**对大图片进行压缩*/ public static Bitmap decodeSampledBitmapFromResource(String  filePath, int reqWidth, int reqHeight) {   //加载图片的边界信息(只读取图片的高度，宽度，不读取具体字节) // First decode with inJustDecodeBounds=true to check dimensions final BitmapFactory.Options options = new BitmapFactory.Options(); //将此选项设置true,此时再加载图片就会只读取图片边界信息了。 options.inJustDecodeBounds = true; //加载图片边界，并将边界信息封装到options对象 BitmapFactory.decodeFile(filePath, options);   // Calculate inSampleSize(计算压缩比例) options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);   // Decode bitmap with inSampleSize set options.inJustDecodeBounds = false; return BitmapFactory.decodeFile(filePath, options); } /**计算压缩比例*/ public static int calculateInSampleSize( BitmapFactory.Options options, int reqWidth, int reqHeight) { // Raw height and width of image //获得实际图片的高度，宽度 final int height = options.outHeight; final int width = options.outWidth; int inSampleSize = 1;   //通过实际的高度，宽度与我们需要的高度，宽度进行比例运算得到一个压缩比例值 if (height > reqHeight || width > reqWidth) { if (width > height) { inSampleSize = Math.round((float)height / (float)reqHeight); } else { inSampleSize = Math.round((float)width / (float)reqWidth); } } return inSampleSize;//高度和宽度都按照此比例进行压缩 }     }

 

4)图片缓存在外存(DiskLruCache):????????

 

二、扩展：WeakReferences

1)强引用(Strong References):强引用引用的对象不可被销毁

2)弱引用(WeakReferences):弱引用引用的对象可以被销毁

3)软引用(SoftReferences):软引用引用的对象可以在内存不足时被销毁。

例子5：java代码实现弱引用

class ImageView{} class Activity{ ImageView imageView=new ImageView(); public void onClick(){ new WorkThread(imageView).start(); } static class WorkThread extends Thread{ private WeakReference<ImageView> wr; public WorkThread(ImageView imageView) { 用弱引用关联强引用 wr=new WeakReference<ImageView>(imageView); } @Override public void run() { while(true){ //System.out.println(imageView); System.out.println("imageView="+wr.get());获取强引用，输出为null try{sleep(1000);}catch(Exception e){} } } } @Override protected void finalize() throws Throwable {回收垃圾时执行 System.out.println("finalize()"); } }   public class WeekReferencesDemo01 { public static void main(String[] args) { Activity a=new Activity(); a.onClick(); a=null; while(true){ System.gc();//通知垃圾回收器，回收垃圾 } } } /**  * 1)当一个对象被一个强引用引用时，此对象是不会被销毁的  * 2)当一个对象被一个弱引用引用时，此对象可以被销毁。  * */