Android的拖放技术
在android3.0以前的版本，拖放一个视图需要使用触摸事件，而且拖动到指定区域还需要判断坐标是否落到这一区域，比较麻烦。从Android3.0以后，android SDK开始支持视图的拖放操作。
建立拖动阴影，可以直接使用View.DragShadowBuilder类，也可以继承View.DragShadowBuilder类，实现自定义的拖动阴影类。
package com.example.firstdragdrop_01;
import android.annotation.SuppressLint;
import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Point;
import android.graphics.Rect;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.ColorDrawable;
import android.graphics.drawable.Drawable;
import android.view.View;
import android.widget.ImageView;
@SuppressLint("NewApi")
public class MyGragShadowBuilder extends View.DragShadowBuilder{
//拖动阴影的区域
private static Drawable shadow;
//存储新绘制的拖动阴影图像
private static Bitmap newbitmap;
public MyGragShadowBuilder(View view){
super(view);
shadow = new ColorDrawable(Color.LTGRAY);
}
@SuppressLint("NewApi")
//在这个方法中绘制拖动阴影图像，也就是实例化newBitmap变量
@Override
public void onProvideShadowMetrics(Point shadowSize, Point shadowTouchPoint) {
// TODO Auto-generated method stub
//设置拖动阴影的宽度和高度，放大为原来的1.5倍
int width,height;
width = (int) (getView().getWidth()*1.5);
height = (int) (getView().getHeight()*1.5);
//设置拖动阴影图像绘制的区域
shadow.setBounds(width, height, width, height);
//设置拖动阴影的高度和宽度
shadowSize.set(width, height);
//设置手指和鼠标在拖动阴影的位置
shadowTouchPoint.set(width/2,height/2);
if(getView() instanceof ImageView){
//getView方法返回的值就是构造方法传入的v的值
ImageView imageView = (ImageView) getView();
//获取imageView控件的Drawable对象
Drawable drawable = imageView.getDrawable();
//获取ImageView的图像资源（Bitmap对象）
Bitmap bitmap = ((BitmapDrawable)drawable).getBitmap();
//根据拖动阴影图像的尺寸创建一个新的可绘制的Bitmap图像
newbitmap = Bitmap.createBitmap(width, height, Config.ARGB_8888);
//将Canvas与Bitmap关联在一起
Canvas canvas = new Canvas(newbitmap);
//将图像绘制在画布上（图像放大50%）。注意，这里的画布只是与newBitmap绑定的完全独立的，现在还没有正式将图像绘制在拖动
//阴影图像上。实际上就是将newBitmap放大50%，然后放置在newBitmap上
canvas.drawBitmap(bitmap, new Rect(0, 0, bitmap.getWidth(), bitmap.getHeight()),
new Rect(0, 0, width, height), null);
}
super.onProvideShadowMetrics(shadowSize, shadowTouchPoint);
}
@Override
public void onDrawShadow(Canvas canvas) {
// TODO Auto-generated method stub
//将图像正式绘制在拖动阴影图像上
canvas.drawBitmap(newbitmap, 0, 0, new Paint());
super.onDrawShadow(canvas);
}
}现在来实现主窗口，实现OnDragListener类。
package com.example.firstdragdrop_01;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.os.Bundle;
import android.view.DragEvent;
import android.view.Menu;
import android.view.View;
import android.view.View.OnDragListener;
import android.widget.FrameLayout;
import android.widget.FrameLayout.LayoutParams;
import android.widget.ImageView;
@SuppressLint("NewApi")
public class MainActivity extends Activity implements OnDragListener{
private FrameLayout dragdropRegin;
private ImageView imageView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
dragdropRegin = (FrameLayout) findViewById(R.id.fragment_dragdrop_regin);
imageView = (ImageView) findViewById(R.id.imageView);
//为目标区域设置拖动监听器
dragdropRegin.setOnDragListener(this);
//为要拖动的视图设置长按单击事件监听器
imageView.setOnLongClickListener(new View.OnLongClickListener() {
//长按ImageView控件后开始拖动
@Override
public boolean onLongClick(View v) {
// TODO Auto-generated method stub
//创建拖动阴影对象
View.DragShadowBuilder myShadow = new MyGragShadowBuilder(imageView);
//开始拖动
v.startDrag(null, myShadow, null, 0);
return true;
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@SuppressLint("NewApi")
@Override
public boolean onDrag(View v, DragEvent event) {
// TODO Auto-generated method stub
int action = event.getAction();
switch (action) {
case DragEvent.ACTION_DRAG_STARTED:
System.out.println("drag_started");
break;
case DragEvent.ACTION_DRAG_ENTERED:
System.out.println("drag_entered");
break;
case DragEvent.ACTION_DRAG_LOCATION:
System.out.println("drag_location x="+event.getX()+"y="+event.getY());
break;
case DragEvent.ACTION_DRAG_EXITED:
System.out.println("drag_exited");
break;
case DragEvent.ACTION_DROP:
System.out.println("drag_drop");
//当到达目标区域时，创建一个新的ImagmeView 对象
ImageView imageView =(ImageView) getLayoutInflater().inflate(R.layout.image, null);
//由于FrameLayout不能直接使用坐标设置子视图的位置，所以这里使用左边距和顶边距，注意引包要引入 android.widget.FrameLayout.LayoutParams
LayoutParams layoutParams =new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
layoutParams.leftMargin = (int) (event.getX());
layoutParams.topMargin = (int) (event.getY());
dragdropRegin.addView(imageView, layoutParams);
break;
case DragEvent.ACTION_DRAG_ENDED:
System.out.println("drag_ended");
break;
default:
return false;
}
return true;
}
}
布局文件：fragment_dragdrop_regin。xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<FrameLayout
android:id="@+id/fragment_dragdrop_regin"
android:layout_width="fill_parent"
android:layout_height="300dp"
android:background="#F00"
/>
<ImageView
android:id="@+id/imageView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="100dp"
android:layout_gravity="center"
android:src="@drawable/ic_launcher"
/>
</LinearLayout>
<ImageView
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:layout_marginTop="50dp"
android:src="@drawable/ic_launcher"/>