Android实现图片拖动效果

要求：

1.通过手指移动来拖动图片 

2.控制图片不能超出屏幕显示区域

技术点:

1.motionevent处理

2.对view进行动态定位(layout)

activity_main.xml：

<relativelayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent" >
 <imageview
  android:id="@+id/iv_main"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:src="@drawable/test"/>
</relativelayout>

mainactivity:

public class mainactivity extends activity implements ontouchlistener {
 private imageview iv_main;
 private relativelayout parentview;
 @override
 protected void oncreate(bundle savedinstancestate) {
  super.oncreate(savedinstancestate);
  setcontentview(r.layout.activity_main);
  iv_main = (imageview) findviewbyid(r.id.iv_main);
  parentview = (relativelayout) iv_main.getparent();
  /*
  int right = parentview.getright(); //0
  int bottom = parentview.getbottom(); //0
  toast.maketext(this, right+"---"+bottom, 1).show();
  */
  //设置touch监听
  iv_main.setontouchlistener(this);
 }
 private int lastx;
 private int lasty;
 private int maxright;
 private int maxbottom;
 @override
 public boolean ontouch(view v, motionevent event) {
  //得到事件的坐标
  int eventx = (int) event.getrawx();
  int eventy = (int) event.getrawy();
  switch (event.getaction()) {
  case motionevent.action_down:
   //得到父视图的right/bottom
   if(maxright==0) {//保证只赋一次值
    maxright = parentview.getright();
    maxbottom = parentview.getbottom();
   }
   //第一次记录lastx/lasty
   lastx =eventx;
   lasty = eventy;
   break;
  case motionevent.action_move:
   //计算事件的偏移
   int dx = eventx-lastx;
   int dy = eventy-lasty;
   //根据事件的偏移来移动imageview
   int left = iv_main.getleft()+dx;
   int top = iv_main.gettop()+dy;
   int right = iv_main.getright()+dx;
   int bottom = iv_main.getbottom()+dy;
   //限制left >=0
   if(left<0) {
    right += -left;
    left = 0;
   }
   //限制top
   if(top<0) {
    bottom += -top;
    top = 0;
   }
   //限制right <=maxright
   if(right>maxright) {
    left -= right-maxright;
    right = maxright;
   }
   //限制bottom <=maxbottom
   if(bottom>maxbottom) {
    top -= bottom-maxbottom;
    bottom = maxbottom;
   }
   iv_main.layout(left, top, right, bottom);
   //再次记录lastx/lasty
   lastx = eventx;
   lasty = eventy;
   break;
  default:
   break;
  }
  return true;//所有的motionevent都交给imageview处理
 }
}

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