android 大图片拖拽并缩放实现原理

public class SpacePageActivity extends Activity {private LinearLayout linnerLayout_spacepage;private RelativeLayout relativeLayout_spacepage;private Button btn_spacepage_back;private Button btn_spacepage_save;private static Boolean isTrue = false;private static String IMAGE_MIME_TYPE;private ImageView image_spacePage;private ProgressDialog dialog;private Bitmap bitmap;@Overrideprotected void onCreate(Bundle savedInstanceState) {// TODO Auto-generated method stubsuper.onCreate(savedInstanceState);requestWindowFeature(Window.FEATURE_NO_TITLE);// 隐藏标题setContentView(R.layout.spacepage);setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);initHead();if (isNetworkAvailable()) {image_spacePage = (ImageView) findViewById(R.id.image_spacepage);btn_spacepage_save = (Button) findViewById(R.id.btn_spacepage_save);image_spacePage.setOnTouchListener(new MulitPointTouchListener(image_spacePage));image_spacePage.setScaleType(ScaleType.CENTER_INSIDE);// 加载成功String urls = "http://epaper.yzdsb.com.cn/201201/04/yz2104_7.jpg";bitmap = returnBitMap(urls);if (null != bitmap) {image_spacePage.setImageBitmap(bitmap);}}}public Bitmap returnBitMap(String url) {if (null == url || "".equals(url)) {return null;}URL myFileUrl = null;Bitmap bitmap = null;try {myFileUrl = new URL(url);} catch (MalformedURLException e) {e.printStackTrace();}try {HttpURLConnection conn = (HttpURLConnection) myFileUrl.openConnection();conn.setConnectTimeout(2000);conn.setDoInput(true);conn.connect();InputStream is = conn.getInputStream();bitmap = BitmapFactory.decodeStream(is);is.close();} catch (IOException e) {e.printStackTrace();}return bitmap;}public class MulitPointTouchListener implements OnTouchListener {Matrix matrix = new Matrix();Matrix savedMatrix = new Matrix();public ImageView image;static final int NONE = 0;static final int DRAG = 1;static final int ZOOM = 2;int mode = NONE;PointF start = new PointF();PointF mid = new PointF();float oldDist = 1f;public MulitPointTouchListener(ImageView image) {super();this.image = image;}@Overridepublic boolean onTouch(View v, MotionEvent event) {this.image.setScaleType(ScaleType.MATRIX);ImageView view = (ImageView) v;// dumpEvent(event);switch (event.getAction() & MotionEvent.ACTION_MASK) {case MotionEvent.ACTION_DOWN:Log.w("FLAG", "ACTION_DOWN");matrix.set(view.getImageMatrix());savedMatrix.set(matrix);start.set(event.getX(), event.getY());mode = DRAG;break;case MotionEvent.ACTION_POINTER_DOWN:Log.w("FLAG", "ACTION_POINTER_DOWN");oldDist = spacing(event);if (oldDist > 10f) {savedMatrix.set(matrix);midPoint(mid, event);mode = ZOOM;}break;case MotionEvent.ACTION_UP:Log.w("FLAG", "ACTION_UP");case MotionEvent.ACTION_POINTER_UP:Log.w("FLAG", "ACTION_POINTER_UP");mode = NONE;break;case MotionEvent.ACTION_MOVE:Log.w("FLAG", "ACTION_MOVE");if (mode == DRAG) {matrix.set(savedMatrix);matrix.postTranslate(event.getX() - start.x, event.getY()- start.y);} else if (mode == ZOOM) {float newDist = spacing(event);if (newDist > 10f) {matrix.set(savedMatrix);float scale = newDist / oldDist;matrix.postScale(scale, scale, mid.x, mid.y);}}break;}view.setImageMatrix(matrix);return true;}private float spacing(MotionEvent event) {float x = event.getX(0) - event.getX(1);float y = event.getY(0) - event.getY(1);return FloatMath.sqrt(x * x + y * y);}private void midPoint(PointF point, MotionEvent event) {float x = event.getX(0) + event.getX(1);float y = event.getY(0) + event.getY(1);point.set(x / 2, y / 2);}}private void initHead() {linnerLayout_spacepage = (LinearLayout) findViewById(R.id.linnerLayout_spacepage);relativeLayout_spacepage = (RelativeLayout) findViewById(R.id.relativeLayout_spacepage);btn_spacepage_back = (Button) findViewById(R.id.btn_spacepage_back);btn_spacepage_back.setOnClickListener(new OnClickListener() {@Overridepublic void onClick(View v) {// TODO Auto-generated method stubfinish();}});btn_spacepage_save = (Button) findViewById(R.id.btn_spacepage_save);}protected boolean isNetworkAvailable() {ConnectivityManager connectivity = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);if (connectivity == null) {Log.i("NetWorkState", "Unavailabel");return false;} else {NetworkInfo[] info = connectivity.getAllNetworkInfo();if (info != null) {for (int i = 0; i < info.length; i++) {if (info[i].getState() == NetworkInfo.State.CONNECTED) {Log.i("NetWorkState", "Availabel");return true;}}}}return false;}@Overrideprotected void onStop() {// TODO Auto-generated method stubif (bitmap != null) {if (!bitmap.isRecycled()) {bitmap.recycle(); // 回收图片所占的内存bitmap = null;System.gc(); // 提醒系统及时回收}}super.onStop();}@Overrideprotected void onDestroy() {// TODO Auto-generated method stubsuper.onDestroy();if (bitmap != null) {if (!bitmap.isRecycled()) {bitmap.recycle(); // 回收图片所占的内存bitmap = null;System.gc(); // 提醒系统及时回收}}}}

因为是大图片，在onDestroy的时候要recycle掉，这样系统才会定时回收掉，System.gc();是提醒系统回收，虽然java这种垃圾回收机制不用我们自己关心对象的回收，但是这也给android开发人员带来了不好的一面，如果系统回收不及时很可能就造成了内存溢出了。什么时候手机不担心内存使用了就好了。

详细出处参考：http://www.jb51.net/article/32972.htm

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