拖拽的Android实现

- 长按进入拖拽
- 增加拖拽时间监听

```
import android.content.ClipData;
import android.content.ClipDescription;
import android.graphics.Color;
import android.net.Uri;
import android.os.ParcelFileDescriptor;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.util.TypedValue;
import android.view.DragEvent;
import android.view.Gravity;
import android.view.View;
import android.widget.FrameLayout;
import android.widget.TextView;

import java.io.FileDescriptor;
import java.io.FileInputStream;
import java.io.FileNotFoundException;

import static android.view.View.DRAG_FLAG_GLOBAL;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        TextView dragText = findViewById(R.id.text_drag);
        FrameLayout targetFrame = findViewById(R.id.frame_target);

        //Set up drop target listener.
        targetFrame.setOnDragListener(new DropTargetListener(this));

        //Set up draggable item listener.
        dragText.setOnLongClickListener(new TextViewLongClickListener());
    }

    protected class DropTargetListener implements View.OnDragListener {
        private AppCompatActivity mActivity;

        public DropTargetListener(AppCompatActivity mActivity) {
            this.mActivity = mActivity;
        }

        @Override
        public boolean onDrag(View v, DragEvent event) {
            final int action = event.getAction();

            switch(action) {
                case DragEvent.ACTION_DRAG_STARTED:
                    // Limit the types of items this can receive to plain-text and Chrome OS files
                    if (event.getClipDescription().hasMimeType(ClipDescription.MIMETYPE_TEXT_PLAIN)
                            || event.getClipDescription().hasMimeType("application/x-arc-uri-list")) {

                        // Greenify background colour so user knows this is a target.
                        v.setBackgroundColor(Color.argb(55,0,255,0));
                        return true;
                    }

                    //If the dragged item is of an undesired type, report that this is not a valid target
                    return false;

                case DragEvent.ACTION_DRAG_ENTERED:
                    // Increase green background colour when item is over top of target.
                    v.setBackgroundColor(Color.argb(150,0,255,0));
                    return true;

                case DragEvent.ACTION_DRAG_LOCATION:
                    return true;

                case DragEvent.ACTION_DRAG_EXITED:
                    // Less intense green background colour when item not over target.
                    v.setBackgroundColor(Color.argb(55,0,255,0));
                    return true;

                case DragEvent.ACTION_DROP:
                    requestDragAndDropPermissions(event); //Allow items from other applications
                    ClipData.Item item = event.getClipData().getItemAt(0);

                    if (event.getClipDescription().hasMimeType(ClipDescription.MIMETYPE_TEXT_PLAIN)) {
                        //If this is a text item, simply display it in a new TextView.
                        FrameLayout frameTarget = (FrameLayout) v;
                        frameTarget.removeAllViews();

                        TextView dropped_text = new TextView(mActivity);
                        FrameLayout.LayoutParams params = new FrameLayout.LayoutParams(FrameLayout.LayoutParams.WRAP_CONTENT, FrameLayout.LayoutParams.WRAP_CONTENT);
                        params.gravity = Gravity.CENTER;
                        dropped_text.setLayoutParams(params);

                        dropped_text.setTextSize(TypedValue.COMPLEX_UNIT_SP,30);
                        dropped_text.setText(item.getText());

                        frameTarget.addView(dropped_text);

                    } else if (event.getClipDescription().hasMimeType("application/x-arc-uri-list")) {
                        //If a file, read the first 200 characters and output them in a new TextView.

                        //Note the use of ContentResolver to resolve the ChromeOS content URI.
                        Uri contentUri = item.getUri();
                        ParcelFileDescriptor parcelFileDescriptor;
                        try {
                            parcelFileDescriptor = getContentResolver().openFileDescriptor(contentUri, "r");
                        } catch (FileNotFoundException e) {
                            e.printStackTrace();
                            Log.e("DRAGTEST", "File not found.");
                            return false;
                        }

                        FileDescriptor fileDescriptor = parcelFileDescriptor.getFileDescriptor();

                        final int MAX_LENGTH = 5000;
                        byte[] bytes = new byte[MAX_LENGTH];

                        try {
                            FileInputStream in = new FileInputStream(fileDescriptor);
                            try {
                                in.read(bytes,0, MAX_LENGTH);
                            } finally {
                                in.close();
                            }
                        } catch (Exception ex) {}
                        String contents = new String(bytes);

                        final int CHARS_TO_READ = 200;
                        int content_length = (contents.length() > CHARS_TO_READ) ? CHARS_TO_READ : 0;

                        FrameLayout frameTarget = (FrameLayout) v;
                        frameTarget.removeAllViews();

                        TextView dropped_text = new TextView(mActivity);
                        FrameLayout.LayoutParams params = new FrameLayout.LayoutParams(FrameLayout.LayoutParams.WRAP_CONTENT, FrameLayout.LayoutParams.WRAP_CONTENT);
                        params.gravity = Gravity.CENTER;
                        dropped_text.setLayoutParams(params);

                        dropped_text.setTextSize(TypedValue.COMPLEX_UNIT_SP,10);
                        dropped_text.setText(contents.substring(0,content_length));

                        frameTarget.addView(dropped_text);

                    } else {
                        return false;
                    }
                    return true;

                case DragEvent.ACTION_DRAG_ENDED:
                    // Restore background colour to transparent.
                    v.setBackgroundColor(Color.argb(0,255,255,255));
                    return true;

                default:
                    Log.e("DragDrop Example", "Unknown action type received by DropTargetListener.");
                    return false;
            }
        }
    }

    protected class TextViewLongClickListener implements View.OnLongClickListener {
        @Override
        public boolean onLongClick(View v) {
            TextView thisTextView = (TextView) v;
            String dragContent = "Dragged Text: " + thisTextView.getText();

            //Set the drag content and type.
            ClipData.Item item = new ClipData.Item(dragContent);
            ClipData dragData = new ClipData(dragContent,
                    new String[] {ClipDescription.MIMETYPE_TEXT_PLAIN}, item);

            //Set the visual look of the dragged object.
            //Can be extended and customized. Default is used here.
            View.DragShadowBuilder dragShadow = new View.DragShadowBuilder(v);

            // Starts the drag, note: global flag allows for cross-application drag.
            v.startDragAndDrop(dragData, dragShadow, null, DRAG_FLAG_GLOBAL);

            return false;
        }
    }

}

```