上一节（自定义初学5——自定义View显示图片）已经说了如何自定义View显示图片。做android时，加载图片是避免不了的，加载网络图片还需要异步加载，最烦人的就是经常出现OOM，为了避免这样的问题，我们一般这样解决：
- 根据图片控件的大小对图片进行压缩显示。
- 如果图片数量非常多，则会使用LruCache等缓存机制，将所有图片占据的内容维持在一个范围内。
有时加载图片还会遇到特殊情况——就是单个图片非常巨大，还不允许压缩。那么对于这种需求，该如何做呢？
首先不压缩，按照原图尺寸加载，那么屏幕肯定是不够大的，并且考虑到内存的情况，不可能一次性整图加载到内存中，所以肯定是局部加载，而有一个类很好的完成了这个功能，这就是BitmapRegionDecoder
先看下官方的API
写个简单的例子：
public class MainActivity extends Activity {
private ImageView mImageView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mImageView = (ImageView) findViewById(R.id.imageView1);
try {
InputStream inputStream = getAssets().open("dog.jpg");
// 获得图片的宽、高
BitmapFactory.Options tmpOptions = new BitmapFactory.Options();
// tmpOptions.inJustDecodeBounds = true;// 如果设置为true，解码器将返回null
// 设置显示图片的中心区域
BitmapRegionDecoder bitmapRegionDecoder = BitmapRegionDecoder
.newInstance(inputStream, false);
BitmapFactory.Options options = new BitmapFactory.Options();
options.inPreferredConfig = Bitmap.Config.ARGB_8888;
//用给定的Rect和options创建位图
Bitmap bitmap = bitmapRegionDecoder.decodeRegion(new Rect(0, 0,
600, 600), options);
mImageView.setImageBitmap(bitmap);
} catch (IOException e) {
e.printStackTrace();
}
}
}
效果如下：
很明显，BitmapRegionDecoder的功能就是加载图片的部分区域，现在大概原理就清楚了，我们需要定义一个View，然后利用去BitmapRegionDecoder显示一个图片的局部，然后利用手势检测，能够移动图片，好了，开始实现吧。梳理一下我们的View：
- 提供一个设置图片的入口
- 重写onTouchEvent，在里面根据用户移动的手势，去更新显示区域的参数
- 每次更新区域参数后，调用invalidate，onDraw里面去regionDecoder.decodeRegion拿到bitmap，去draw
自定义LargeImageView
public class LargeImageView extends View {
private BitmapRegionDecoder mDecoder;
/**
* 图片的宽度和高度
*/
private int mImageWidth, mImageHeight;
/**
* 绘制的区域
*/
private volatile Rect mRect = new Rect();
private MoveGestureDetector mDetector;
private static final BitmapFactory.Options options = new BitmapFactory.Options();
static {
options.inPreferredConfig = Bitmap.Config.ARGB_8888;
}
public void setInputStream(InputStream is) {
try {
mDecoder = BitmapRegionDecoder.newInstance(is, false);
BitmapFactory.Options tmpOptions = new BitmapFactory.Options();
// 获取图片的宽高
tmpOptions.inJustDecodeBounds = true;
BitmapFactory.decodeStream(is, null, tmpOptions);
mImageWidth = tmpOptions.outWidth;
mImageHeight = tmpOptions.outHeight;
requestLayout();
invalidate();
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
if (is != null)
is.close();
} catch (Exception e) {
}
}
}
public void init() {
mDetector = new MoveGestureDetector(getContext(),
new MoveGestureDetector.SimpleMoveGestureDetector() {
@Override
public boolean onMove(MoveGestureDetector detector) {
int moveX = (int) detector.getMoveX();
int moveY = (int) detector.getMoveY();
if (mImageWidth > getWidth()) {
mRect.offset(-moveX, 0);
checkWidth();
invalidate();
}
if (mImageHeight > getHeight()) {
mRect.offset(0, -moveY);
checkHeight();
invalidate();
}
return true;
}
});
}
private void checkWidth() {
Rect rect = mRect;
int imageWidth = mImageWidth;
if (rect.right > imageWidth) {
rect.right = imageWidth;
rect.left = imageWidth - getWidth();
}
if (rect.left < 0) {
rect.left = 0;
rect.right = getWidth();
}
}
private void checkHeight() {
Rect rect = mRect;
int imageHeight = mImageHeight;
if (rect.bottom > imageHeight) {
rect.bottom = imageHeight;
rect.top = imageHeight - getHeight();
}
if (rect.top < 0) {
rect.top = 0;
rect.bottom = getHeight();
}
}
public LargeImageView(Context context, AttributeSet attrs) {
super(context, attrs);
init();
}
@Override
public boolean onTouchEvent(MotionEvent event) {
mDetector.onToucEvent(event);
return true;
}
@Override
protected void onDraw(Canvas canvas) {
Bitmap bm = mDecoder.decodeRegion(mRect, options);
canvas.drawBitmap(bm, 0, 0, null);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int width = getMeasuredWidth();
int height = getMeasuredHeight();
int imageWidth = mImageWidth;
int imageHeight = mImageHeight;
// 默认直接显示图片的中心区域，可以自己去调节
mRect.left = imageWidth / 2 - width / 2;
mRect.top = imageHeight / 2 - height / 2;
mRect.right = mRect.left + width;
mRect.bottom = mRect.top + height;
}
}
手势检测抽象类BaseGestureDetector
public abstract class BaseGestureDetector {
protected boolean mGestureInProgress;
protected MotionEvent mPreMotionEvent;
protected MotionEvent mCurrentMotionEvent;
protected Context mContext;
public BaseGestureDetector(Context context)
{
mContext = context;
}
public boolean onToucEvent(MotionEvent event)
{
if (!mGestureInProgress)
{
handleStartProgressEvent(event);
} else
{
handleInProgressEvent(event);
}
return true;
}
protected abstract void handleInProgressEvent(MotionEvent event);
protected abstract void handleStartProgressEvent(MotionEvent event);
protected abstract void updateStateByEvent(MotionEvent event);
protected void resetState()
{
if (mPreMotionEvent != null)
{
mPreMotionEvent.recycle();
mPreMotionEvent = null;
}
if (mCurrentMotionEvent != null)
{
mCurrentMotionEvent.recycle();
mCurrentMotionEvent = null;
}
mGestureInProgress = false;
}
}
手势检测实现类
public class MoveGestureDetector extends BaseGestureDetector {
private PointF mCurrentPointer;
private PointF mPrePointer;
// 用于记录最终结果，并返回
private PointF mExtenalPointer = new PointF();
private OnMoveGestureListener mListenter;
public MoveGestureDetector(Context context, OnMoveGestureListener listener) {
super(context);
mListenter = listener;
}
@Override
protected void handleInProgressEvent(MotionEvent event) {
int actionCode = event.getAction() & MotionEvent.ACTION_MASK;
switch (actionCode) {
case MotionEvent.ACTION_CANCEL:
case MotionEvent.ACTION_UP:
mListenter.onMoveEnd(this);
resetState();
break;
case MotionEvent.ACTION_MOVE:
updateStateByEvent(event);
boolean update = mListenter.onMove(this);
if (update) {
mPreMotionEvent.recycle();
mPreMotionEvent = MotionEvent.obtain(event);
}
break;
}
}
@Override
protected void handleStartProgressEvent(MotionEvent event) {
int actionCode = event.getAction() & MotionEvent.ACTION_MASK;
switch (actionCode) {
case MotionEvent.ACTION_DOWN:
resetState();// 防止没有接收到CANCEL or UP ,保险起见
mPreMotionEvent = MotionEvent.obtain(event);
updateStateByEvent(event);
break;
case MotionEvent.ACTION_MOVE:
mGestureInProgress = mListenter.onMoveBegin(this);
break;
}
}
protected void updateStateByEvent(MotionEvent event) {
final MotionEvent prev = mPreMotionEvent;
mPrePointer = caculateFocalPointer(prev);
mCurrentPointer = caculateFocalPointer(event);
boolean mSkipThisMoveEvent = prev.getPointerCount() != event
.getPointerCount();
mExtenalPointer.x = mSkipThisMoveEvent ? 0 : mCurrentPointer.x
- mPrePointer.x;
mExtenalPointer.y = mSkipThisMoveEvent ? 0 : mCurrentPointer.y
- mPrePointer.y;
}
/**
* 根据event计算多指中心点
*
* @param event
* @return
*/
private PointF caculateFocalPointer(MotionEvent event) {
final int count = event.getPointerCount();
float x = 0, y = 0;
for (int i = 0; i < count; i++) {
x += event.getX(i);
y += event.getY(i);
}
x /= count;
y /= count;
return new PointF(x, y);
}
public float getMoveX() {
return mExtenalPointer.x;
}
public float getMoveY() {
return mExtenalPointer.y;
}
public interface OnMoveGestureListener {
public boolean onMoveBegin(MoveGestureDetector detector);
public boolean onMove(MoveGestureDetector detector);
public void onMoveEnd(MoveGestureDetector detector);
}
public static class SimpleMoveGestureDetector implements
OnMoveGestureListener {
@Override
public boolean onMoveBegin(MoveGestureDetector detector) {
return true;
}
@Override
public boolean onMove(MoveGestureDetector detector) {
return false;
}
@Override
public void onMoveEnd(MoveGestureDetector detector) {
}
}
}
用法不说了，看效果吧：
这只是简单的实现，我们当然还可以实现更多的功能了，我就抛砖引玉吧
源代码
参考：