android 的线程模型：当一个 android 的应用运行后，就会有一个 UI 的 main 线程启动 , 这是一个非常重要的线程，它负责把事件分派到相应的控件，其中就包括屏幕绘图事件，它同样是用户与 android 控件 交互的线程。比如，当你在屏幕上的 EditText 上输入文字， UI 线程会把这个事件分发给刚输入文字的 EditText ，紧接会向事件队列发送一个更新 （ invalidate ）请求。 UI 线程会把这个请求移出事件队列并通知 EditText 在屏幕上重新绘制自身。
这种单线线程模型就会使得 android 的应用程序性能低下， 如果在这个单线程里执行一些耗时的操作， 比如访问数据库， 或是从网络端下载图片， 就会会阻塞整个用户界面。 比如如下操作：
[java]
Bitmap b =loadImageFromNetwork();
Bitmap b =loadImageFromNetwork();这个操作非常耗时， 在这种情况下你会发现 , 界面僵死在那里并且 android 在系统 5 秒中后没有反应，会显示一个关闭或等待的错误。
也许我们可以使用一个新的 Thread 来解决它
[java]
new Thread(new Runnable() {
public void run() {
Bitmap b = loadImageFromNetwork();
mImageView.setImageBitmap( b );
}
}).start();
new Thread(new Runnable() {
public void run() {
Bitmap b = loadImageFromNetwork();
mImageView.setImageBitmap( b );
}
}).start(); 但这样会发生一些很难察觉的错误， 因为我们知道 UI 线程不是线程安全的。当然有很多种方法来处理这个问题：
android 提供了几种在其他线程中访问 UI 线程的方法。
Activity.runOnUiThread( Runnable )
View.post( Runnable )
View.postDelayed( Runnable, long )
Hanlder
[java]
new Thread( new Runnable() {
public void run() {
final Bitmap b = loadImageFromNetwork();
mImageView.post( new Runnable() {
mImageView.setImageBitmap( b );
});
}
}).start();
new Thread( new Runnable() {
public void run() {
final Bitmap b = loadImageFromNetwork();
mImageView.post( new Runnable() {
mImageView.setImageBitmap( b );
});
}
}).start();这种方法比较繁琐，同时当你需要实现一些很复杂的操作并需要频繁地更新UI 时这会变得更糟糕。为了解决这个问题，android 提供了一个工具类：AsyncTask ，它使创建需要与用户界面交互的长时间运行的任务变得更简单。
就拿加载网络图片举个例子：
[java]
ublic class CanvasImageTask extends AsyncTask<ImageView, Void, Bitmap>{
private ImageView gView ;
protected Bitmap doInBackground(ImageView... views) {
Bitmap bmp = null ;
ImageView view = views[0];
// 根据iconUrl获取图片并渲染，iconUrl的url放在了view的tag中。
if (view.getTag() != null) {
try {
URL url = new URL(view.getTag().toString());
HttpURLConnection conn = (HttpURLConnection)url.openConnection();
conn.setDoInput(true);
conn.connect();
InputStream stream = conn.getInputStream();
bmp = BitmapFactory.decodeStream(stream);
stream.close();
} catch (Exception e) {
Log.v("img", e.getMessage());
return null;
}
}
this.gView = view;
return bmp;
}
protected void onPostExecute(Bitmap bm) {
if (bm != null) {
this.gView.setImageBitmap(bm);
this.gView = null ;
}
}
}
在Activity中直接调用
if(!img.isDrawingCacheEnabled() || !holder.image.getTag().equals(imgpath)){
img.setImageResource(R.drawable.icon_app);
img.setTag(imgpath);
try{
new CanvasImageTask().execute(img);
img.setDrawingCacheEnabled(true);
}catch (Exception e) {
Log.e("error", "RejectedExecutionException in content_img: " +imgpath);
ublic class CanvasImageTask extends AsyncTask<ImageView, Void, Bitmap>{
private ImageView gView ;
protected Bitmap doInBackground(ImageView... views) {
Bitmap bmp = null ;
ImageView view = views[0];
// 根据iconUrl获取图片并渲染，iconUrl的url放在了view的tag中。
if (view.getTag() != null) {
try {
URL url = new URL(view.getTag().toString());
HttpURLConnection conn = (HttpURLConnection)url.openConnection();
conn.setDoInput(true);
conn.connect();
InputStream stream = conn.getInputStream();
bmp = BitmapFactory.decodeStream(stream);
stream.close();
} catch (Exception e) {
Log.v("img", e.getMessage());
return null;
}
}
this.gView = view;
return bmp;
}
protected void onPostExecute(Bitmap bm) {
if (bm != null) {
this.gView.setImageBitmap(bm);
this.gView = null ;
}
}
}
在Activity中直接调用
if(!img.isDrawingCacheEnabled() || !holder.image.getTag().equals(imgpath)){
img.setImageResource(R.drawable.icon_app);
img.setTag(imgpath);
try{
new CanvasImageTask().execute(img);
img.setDrawingCacheEnabled(true);
}catch (Exception e) {
Log.e("error", "RejectedExecutionException in content_img: " +imgpath);这样图片加载使用异步线程便不会进行堵塞发生错误，我们还可以使用 callback 在图片加载完后进行回调
[java]
public class CanvasImageTaskCall extends AsyncTask<ImageView, Void, Bitmap> implements Callback{
private ImageView gView ;
protected Bitmap doInBackground(ImageView... views) {
Bitmap bmp = null ;
ImageView view = views[0];
// 根据iconUrl获取图片并渲染，iconUrl的url放在了view的tag中。
if (view.getTag() != null) {
try {
URL url = new URL(view.getTag().toString());
HttpURLConnection conn = (HttpURLConnection)url.openConnection();
conn.setDoInput(true);
conn.connect();
InputStream stream = conn.getInputStream();
bmp = BitmapFactory.decodeStream(stream);
stream.close();
} catch (Exception e) {
e.printStackTrace();
Log.v("img", e.getMessage());
Message msg = new Message();
msg.what = 0;
handleMessage(msg);
return null;
}
}
this.gView = view;
return bmp;
}
protected void onPostExecute(Bitmap bm) {
if (bm != null) {
this.gView.setImageBitmap(bm);
this.gView.setTag(bm);
this.gView = null ;
Message msg = new Message();
msg.what = 1;
handleMessage(msg);
}
}
public boolean handleMessage(Message msg) {
// TODO Auto-generated method stub
return false;
}
}
public class CanvasImageTaskCall extends AsyncTask<ImageView, Void, Bitmap> implements Callback{
private ImageView gView ;
protected Bitmap doInBackground(ImageView... views) {
Bitmap bmp = null ;
ImageView view = views[0];
// 根据iconUrl获取图片并渲染，iconUrl的url放在了view的tag中。
if (view.getTag() != null) {
try {
URL url = new URL(view.getTag().toString());
HttpURLConnection conn = (HttpURLConnection)url.openConnection();
conn.setDoInput(true);
conn.connect();
InputStream stream = conn.getInputStream();
bmp = BitmapFactory.decodeStream(stream);
stream.close();
} catch (Exception e) {
e.printStackTrace();
Log.v("img", e.getMessage());
Message msg = new Message();
msg.what = 0;
handleMessage(msg);
return null;
}
}
this.gView = view;
return bmp;
}
protected void onPostExecute(Bitmap bm) {
if (bm != null) {
this.gView.setImageBitmap(bm);
this.gView.setTag(bm);
this.gView = null ;
Message msg = new Message();
msg.what = 1;
handleMessage(msg);
}
}
public boolean handleMessage(Message msg) {
// TODO Auto-generated method stub
return false;
}
}在 Activity 中直接调用
[java]
new CanvasImageTaskCall(){
@Override
public boolean handleMessage(Message msg) {
switch (msg.what) {
case 0:
Log.i("test", "图片加载失败");
break;
case 1:
Log.i("test", "图片加载成功");
break;
default:
break;
}
saveButton.setTextColor(Color.WHITE);
saveButton.setClickable(true);
bitmap = (Bitmap) imageView.getTag();
return super.handleMessage(msg);
}
}.execute(img);
new CanvasImageTaskCall(){
@Override
public boolean handleMessage(Message msg) {
switch (msg.what) {
case 0:
Log.i("test", "图片加载失败");
break;
case 1:
Log.i("test", "图片加载成功");
break;
default:
break;
}
saveButton.setTextColor(Color.WHITE);
saveButton.setClickable(true);
bitmap = (Bitmap) imageView.getTag();
return super.handleMessage(msg);
}
}.execute(img);
