上一节说了通过自定义来加载超大图片，同时实现手指的拖动效果。不过，另一种情况就是，我们希望应用能够加载整张图片，如果图片太大就先压缩，如果小于屏幕就直接显示，同时用户可以拖拽移动和缩放图片大小，梳理下流程：
1.自定义View继承ImageView，重新onDraw方法
2.在onDraw先画图片，图片大于屏幕就把图片缩小后显示，图片小于屏幕就直接显示，显示之前要计算显示图片的Rect，Rect是其实就是四个坐标，用来控制显示图片的范围，这个Rect是根据图片的长宽比例计算而来，显示在屏幕中间。
3.重写onTouchEvent方法，处理View随着手指移动的相关操作：在touchEvent事件中处理好按下和松开事件之后，剩下的就是移动事件，通过不断的改变Rect的坐标，然后不断的调用invalidate()，该方法用来重新绘制我们的View
看下DragImageView.java
public class DragImageView extends ImageView {
private Drawable mDrawable;
private Rect mDrawableRect;
private Context mContext;
private float mRation_WH = 0;
private float mOldX = 0;
private float mOldY = 0;
private double mD1;
private boolean isFirst = true;
private int NONE = 0;// 无操作
private int SINGAL_MOVE = 1;// 单点移动
private int MUTIL_MOVE = 2;// 双点拖拽
private int mStatus = 0;
public DragImageView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
public DragImageView(Context context, AttributeSet attrs) {
super(context, attrs);
}
public DragImageView(Context context) {
super(context);
this.mContext = context;
mDrawableRect = new Rect();
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
// getIntrinsicHeight为返回对象的实际高度
if (mDrawable == null || mDrawable.getIntrinsicHeight() == 0
|| mDrawable.getIntrinsicWidth() == 0) {
return;
}
setBounds(); // 设置图片
mDrawable.draw(canvas);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
mOldX = event.getX();
mOldY = event.getY();
break;
case MotionEvent.ACTION_UP:
mStatus = NONE;
checkBounds();
break;
case MotionEvent.ACTION_MOVE:
if (event.getPointerCount() == 1)
mStatus = SINGAL_MOVE;
else
mStatus = MUTIL_MOVE;
onTouchMove(event);
break;
default:
break;
}
return true;
}
private void onTouchMove(MotionEvent event) {
// 判断有几个触摸点
if (mStatus == SINGAL_MOVE) {
int offsetWidth = (int) (event.getX() - mOldX);
int offsetHeight = (int) (event.getY() - mOldY);
mOldX = event.getX();
mOldY = event.getY();
mDrawableRect.offset(offsetWidth, offsetHeight);
invalidate();
} else if (mStatus == MUTIL_MOVE) {
float X0 = event.getX(0);
float Y0 = event.getY(0);
float X1 = event.getX(1);
float Y1 = event.getY(1);
double mD2 = Math.sqrt(Math.pow(X0 - X1, 2) + Math.pow(Y0 - Y1, 2));
if (mD1 < mD2) {
// 放大操作
if (mDrawableRect.width() < mContext.getResources()
.getDisplayMetrics().widthPixels * 2) {
int offsetwidth = 10;
int offsettop = (int) (offsetwidth / mRation_WH);
mDrawableRect.set(mDrawableRect.left - offsetwidth,
mDrawableRect.top - offsettop, mDrawableRect.right
+ offsetwidth, mDrawableRect.bottom
+ offsettop);
invalidate();
}
} else {
// 设置为只能缩小为屏幕的1/3,可以根据需要自己调整
if (mDrawableRect.width() > mContext.getResources()
.getDisplayMetrics().widthPixels / 3) {
int offsetwidth = 10;
int offsettop = (int) (offsetwidth / mRation_WH);
mDrawableRect.set(mDrawableRect.left + offsetwidth,
mDrawableRect.top + offsettop, mDrawableRect.right
- offsetwidth, mDrawableRect.bottom
- offsettop);
invalidate();
}
}
mD1 = mD2;
}
}
/**
* 设置mDrawable
*/
public void setBounds() {
if (isFirst) {
// 图片宽高比
mRation_WH = (float) mDrawable.getIntrinsicWidth()
/ (float) mDrawable.getIntrinsicHeight();
// 取屏幕宽和图片宽较小的一个
int px_w = Math.min(getWidth(),
dp2px(mContext, mDrawable.getIntrinsicWidth()));
int px_h = (int) (px_w / mRation_WH);// 获得图片高
int left = (getWidth() - px_w) / 2;
int top = (getHeight() - px_h) / 2;
int right = px_w + left;
int bottom = px_h + top;
mDrawableRect.set(left, top, right, bottom);
isFirst = false;
}
mDrawable.setBounds(mDrawableRect);
}
/**
* 检测边框范围
*/
public void checkBounds() {
int newLeft = mDrawableRect.left;
int newTop = mDrawableRect.top;
boolean isChange = false;
// 向左移动范围<=屏幕宽度
if (newLeft < -mDrawableRect.width()) {
newLeft = -mDrawableRect.width();
isChange = true;
}
// 向上移动范围<=屏幕高度
if (newTop < -mDrawableRect.height()) {
newTop = -mDrawableRect.height();
isChange = true;
}
// 向右移动范围<=屏幕宽度
if (newLeft > getWidth()) {
newLeft = getWidth();
isChange = true;
}
// 向下移动范围<=屏幕高度
if (newTop > getHeight()) {
newTop = getHeight();
isChange = true;
}
if (isChange) {
mDrawableRect.offsetTo(newLeft, newTop);
invalidate();
}
}
public Drawable getmDrawable() {
return mDrawable;
}
public void setmDrawable(Drawable mDrawable) {
this.mDrawable = mDrawable;
}
/**
* 将dp单位换算成px
* @param context
* @param value
* @return
*/
public int dp2px(Context context, int value) {
final float scale = context.getResources().getDisplayMetrics().density;
return (int) (value * scale + 0.5f);
}
}
在java类中直接调用即可
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
requestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.activity_main);
DragImageView mView = new DragImageView(this);
mView.setmDrawable(getResources().getDrawable(R.drawable.bg));
setContentView(mView);
}
效果如下：
