Android开发：拖拽
google官网的training和API两个地方都提到了拖拽的实现，两种方法不太一样。
方法一
training（https://developer.android.com/training/gestures/scale.html）中提到的方法是监听onTouchEvent，在ACTION_DOWN的时候记录位置，在ACTION_MOVE的时候获取坐标，改变拖拽的控件位置。
方法二
在android3.0及以上可以使用View.OnDragListener。
下面是我写的简单的demo:
activity_drag_demo.xml
<?xml version="1.0" encoding="utf-8"?> <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" xmlns:app="http://schemas.android.com/apk/res-auto" android:id="@+id/activity_drag_demo" android:layout_width="match_parent" android:layout_height="match_parent" android:paddingBottom="@dimen/activity_vertical_margin" android:paddingLeft="@dimen/activity_horizontal_margin" android:paddingRight="@dimen/activity_horizontal_margin" android:paddingTop="@dimen/activity_vertical_margin" tools:context="com.topsports.testapplication.DragDemoActivity"> <ImageView android:id="@+id/imageView" android:layout_width="100dp" android:layout_height="100dp" android:background="@drawable/c1"/> <FrameLayout android:background="#c7f6ff" android:layout_width="500dp" android:layout_height="500dp" android:id="@+id/content_fragment" android:layout_alignParentBottom="true"></FrameLayout> <FrameLayout android:background="#dce9c1" android:layout_width="200dp" android:layout_height="200dp" android:id="@+id/content_fragment2" android:layout_alignParentRight="true" android:layout_alignParentBottom="true"></FrameLayout> </RelativeLayout>
在官网示例基础上修改的DragDemoActivity:package com.topsports.testapplication;
import android.content.ClipData; import android.content.ClipDescription; import android.graphics.Canvas; import android.graphics.Color; import android.graphics.Point; import android.graphics.drawable.ColorDrawable; import android.graphics.drawable.Drawable; import android.support.v7.app.AppCompatActivity; import android.os.Bundle; import android.util.Log; import android.view.DragEvent; import android.view.View; import android.view.ViewGroup; import android.widget.FrameLayout; import android.widget.ImageView; import android.widget.Toast; public class DragDemoActivity extends AppCompatActivity { private ImageView imageView; private FrameLayout frameLayout; private FrameLayout frameLayout2; private static final String IMAGEVIEW_TAG = "icon bitmap"; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_drag_demo); imageView=(ImageView)findViewById(R.id.imageView); frameLayout=(FrameLayout)findViewById(R.id.content_fragment); frameLayout2=(FrameLayout)findViewById(R.id.content_fragment2); imageView.setTag(IMAGEVIEW_TAG); imageView.setOnLongClickListener(new View.OnLongClickListener() { public boolean onLongClick(View v) { v.getBackground().setAlpha(100); ClipData.Item item = new ClipData.Item(v.getTag().toString());
ClipData dragData = new ClipData(v.getTag().toString(),new String[]{ClipDescription.MIMETYPE_TEXT_PLAIN},item); // Instantiates the drag shadow builder. View.DragShadowBuilder myShadow = new View.DragShadowBuilder(v); v.startDrag(dragData, // the data to be dragged myShadow, // the drag shadow builder null, // no need to use local data 0 // flags (not currently used, set to 0) ); return true; } }); frameLayout.setOnDragListener(new myDragEventListener()); frameLayout2.setOnDragListener(new myDragEventListener()); imageView.setOnDragListener(new myDragEventListener()); }protected class myDragEventListener implements View.OnDragListener { // This is the method that the system calls when it dispatches a drag event to the // listener. public boolean onDrag(View v, DragEvent event) { // Defines a variable to store the action type for the incoming event final int action = event.getAction(); // Handles each of the expected events switch(action) { case DragEvent.ACTION_DRAG_STARTED: // Determines if this View can accept the dragged data if (event.getClipDescription().hasMimeType(ClipDescription.MIMETYPE_TEXT_PLAIN)) { v.invalidate(); // returns true to indicate that the View can accept the dragged data. return true; } // Returns false. During the current drag and drop operation, this View will // not receive events again until ACTION_DRAG_ENDED is sent. return false; case DragEvent.ACTION_DRAG_ENTERED: // Applies a green tint to the View. Return true; the return value is ignored // Invalidate the view to force a redraw in the new tint v.invalidate(); return true; case DragEvent.ACTION_DRAG_LOCATION: // Ignore the event return true; case DragEvent.ACTION_DRAG_EXITED: // Re-sets the color tint to blue. Returns true; the return value is ignored. // Invalidate the view to force a redraw in the new tint v.invalidate(); return true; case DragEvent.ACTION_DROP: // Gets the item containing the dragged data ClipData.Item item = event.getClipData().getItemAt(0); // Gets the text data from the item. String dragData = item.getText().toString(); // Displays a message containing the dragged data. Toast.makeText(DragDemoActivity.this, "Dragged data is " + dragData, Toast.LENGTH_LONG).show(); // Turns off any color tints if(v instanceof FrameLayout){ ((ViewGroup)imageView.getParent()).removeView(imageView); ((FrameLayout)v).addView(imageView); } // Invalidates the view to force a redraw v.invalidate(); // Returns true. DragEvent.getResult() will return true. return true; case DragEvent.ACTION_DRAG_ENDED: // Turns off any color tinting // Invalidates the view to force a redraw v.invalidate(); // Does a getResult(), and displays what happened. if (event.getResult()) { Toast.makeText(DragDemoActivity.this, "The drop was handled.", Toast.LENGTH_LONG).show(); } else { Toast.makeText(DragDemoActivity.this, "The drop didn't work.", Toast.LENGTH_LONG).show(); } imageView.getBackground().setAlpha(255); // returns true; the value is ignored. return true; // An unknown action type was received. default: Log.e("DragDrop Example","Unknown action type received by OnDragListener."); break; } return false; } } }
实现的是将图片拖进两个layout中，并在两个layout间来回拖动。
代码做简单的解释
在长按imageview的时候开始拖拽事件
ClipData.Item item = new ClipData.Item(v.getTag().toString());
ClipData dragData = new ClipData(v.getTag().toString(),new String[]{ClipDescription.MIMETYPE_TEXT_PLAIN},item);
这两句是为拖动的view创建传递的数据(可以省略)
View.DragShadowBuilder myShadow = new View.DragShadowBuilder(v)
这句根据imageview(v)创建一个拖拽的阴影，就像在桌面上拖动应用图标一样，产生一个view的图像副本，产生拖动效果。
frameLayout.setOnDragListener(new myDragEventListener());
frameLayout2.setOnDragListener(new myDragEventListener());
imageView.setOnDragListener(new myDragEventListener());
设置拖拽的事件监听（拖动的对象和被放置的对象）实现View.OnDragListener的onDrag方法，判断event.getAction()的值进行不同的操作。
