【朝花夕拾】Android自定义View篇之（九）多点触控（下）实践出真知

前言

       在上一篇文章中，已经总结了MotionEvent以及多点触控相关的基础理论知识和常用的函数。本篇将通过实现单指拖动图片，多指拖动图片的实际案例来进行练习并实现一些效果，来理解前面的理论知识。要理解本文的代码，需要先掌握上一篇的理论知识，事件处理基础，以及一定的自定义View基础，这些我也在本系列文章的前几篇中讲过，有兴趣的可以按照本系列的顺序依次阅读学习，相信您一定会有不小的收获。。

 

一、实现单指拖动图片

       要实现单指拖动图片，大致思路就是监控手指的ACTION_MOVE事件。手指移动过程中，获取事件的坐标，让图片根据坐标的变化来进行移动。

代码实现如下，先自定义一个View，在其中处理单指拖动逻辑。

 1 public class SingleTouchDragView extends View {
 2     private static final String TAG = "songzheweiwang";
 3     private Bitmap mBitmap;
 4     private RectF mRectF;
 5     private Matrix mMatrix;
 6     private Paint mPaint;
 7     private PointF mLstPointF;
 8     private boolean mCanDrag = false;
 9 
10     public SingleTouchDragView(Context context, @Nullable AttributeSet attrs) {
11         super(context, attrs);
12         init();
13     }
14 
15     private void init() {
16         mPaint = new Paint();
17         mBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.dog);
18         mRectF = new RectF(0, 0, mBitmap.getWidth(), mBitmap.getHeight());
19         mMatrix = new Matrix();
20         mLstPointF = new PointF();
21     }
22     
23     @Override
24     public boolean onTouchEvent(MotionEvent event) {
25         switch (event.getAction()) {
26             case MotionEvent.ACTION_DOWN:
27                 //判断按下位置是否在图片区域内
28                 if (mRectF.contains(event.getX(), event.getY())) {
29                     mCanDrag = true;
30                     mLstPointF.set(event.getX(), event.getY());
31                 }
32                 break;
33             case MotionEvent.ACTION_UP:
34                 mCanDrag = false;
35             case MotionEvent.ACTION_MOVE:
36                 if (mCanDrag) {
37                     //移动图片
38                     mMatrix.postTranslate(event.getX() - mLstPointF.x, event.getY() - mLstPointF.y);
39                     //更新触摸位置
40                     mLstPointF.set(event.getX(), event.getY());
41                     // 更新图片区域
42                     mRectF = new RectF(0, 0, mBitmap.getWidth(), mBitmap.getHeight());
43                     mMatrix.mapRect(mRectF);
44                     //刷新
45                     invalidate();
46                 }
47                 break;
48         }
49         //注意这里需要返回true，因为当前自定义view继承自基类View，默认是无法消费触摸事件的
50         return true;
51     }
52 
53     @Override
54     protected void onDraw(Canvas canvas) {
55         super.onDraw(canvas);
56         canvas.drawBitmap(mBitmap, mMatrix, mPaint);
57     }
58 }

自定义View使用的布局如下：

1 <?xml version="1.0" encoding="utf-8"?>
2 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
3     android:layout_width="match_parent"
4     android:layout_height="match_parent">
5 
6     <com.example.demos.customviewdemo.SingleTouchDragView
7         android:layout_width="match_parent"
8         android:layout_height="match_parent" />
9 </RelativeLayout>

用一根手指在图片上进行拖动，效果如下左图所示：

                

       在单指操作的情况下，可以正常被拖动。但是如果是多指操作的时候，就会混乱了。右图为两根手指滑动的图片的效果，因为两根手指都在移动， 导致ACTION_MOVE事件中，一会儿以第一根手指的触摸点为坐标，一会儿又以第二根手指的触摸点为坐标，这就导致图片闪动。

 

二、实现多指操作时只有第一根手指可以拖动图片

 1 public class MultiTouchDragView extends View {
 2     private static final String TAG = "songzheweiwang";
 3     private Bitmap mBitmap;
 4     private RectF mRectF;
 5     private Matrix mMatrix;
 6     private Paint mPaint;
 7     private PointF mLstPointF;
 8     private boolean mCanDrag = false;
 9 
10     public MultiTouchDragView(Context context, @Nullable AttributeSet attrs) {
11         super(context, attrs);
12         init();
13     }
14 
15     private void init() {
16         mPaint = new Paint();
17         mBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.dog);
18         mRectF = new RectF(0, 0, mBitmap.getWidth(), mBitmap.getHeight());
19         mMatrix = new Matrix();
20         mLstPointF = new PointF();
21     }
22 
23     @RequiresApi(api = Build.VERSION_CODES.KITKAT)
24     @Override
25     public boolean onTouchEvent(MotionEvent event) {
26         Log.i(TAG, ""   MotionEvent.actionToString(event.getAction())   ";mCanDrag="   mCanDrag   ";actionIndex="   event.getActionIndex()   ";action="   event.getAction());
27         switch (event.getActionMasked()) {
28             case MotionEvent.ACTION_DOWN:
29             case MotionEvent.ACTION_POINTER_DOWN:
30                 //pointerId为0的手指（即我们定义的第一根手指）按下在指定区域内
31                 if (event.getPointerId(event.getActionIndex()) == 0 && mRectF.contains(event.getX(), event.getY())) {
32                     mCanDrag = true;
33                     //getX()和getY()没有传入参数时，默认传入的0
34                     mLstPointF.set(event.getX(), event.getY());
35                 }
36                 break;
37             case MotionEvent.ACTION_MOVE:
38                 if (mCanDrag) {
39                     int pointerIndex = event.findPointerIndex(0);
40                     //这里需要注意，多手指频繁按下和抬起时可能会出现pointerIndex为-1的情况,如不处理，后面会报错
41                     if (pointerIndex == -1) {
42                         break;
43                     }
44                     mMatrix.postTranslate(event.getX(pointerIndex) - mLstPointF.x, event.getY(pointerIndex) - mLstPointF.y);
45                     mLstPointF.set(event.getX(), event.getY());
46                     mRectF = new RectF(0, 0, mBitmap.getWidth(), mBitmap.getHeight());
47                     mMatrix.mapRect(mRectF);
48                     invalidate();
49                 }
50                 break;
51             case MotionEvent.ACTION_POINTER_UP:
52             case MotionEvent.ACTION_UP:
53                 if (event.getPointerId(event.getActionIndex()) == 0) {
54                     mCanDrag = false;
55                 }
56                 break;
57         }
58         return true;
59     }
60 
61     @Override
62     protected void onDraw(Canvas canvas) {
63         super.onDraw(canvas);
64         canvas.drawBitmap(mBitmap, mMatrix, mPaint);
65     }
66 }

效果图如下，用两根手指来依次按下并拖动图片：

我们发现，只有第一根手指在滑动时，图片才会跟着移动，第二根手指（右边的手指）的滑动无效。

 

三、实现两根手指轮流拖动图片

 1 public class MultiTouchDragView2 extends View {
 2     private static final String TAG = "songzheweiwang";
 3     private Bitmap mBitmap;
 4     private RectF mRectF;
 5     private Matrix mMatrix;
 6     private Paint mPaint;
 7     private PointF mLstPointF;
 8     private boolean mCanDrag = false;
 9     private int mActivePointerId;
10     private final int INVALID_POINTER = -1;
11 
12     public MultiTouchDragView2(Context context, @Nullable AttributeSet attrs) {
13         super(context, attrs);
14         init();
15     }
16 
17     private void init() {
18         mPaint = new Paint();
19         mBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.dog);
20         mRectF = new RectF(0, 0, mBitmap.getWidth(), mBitmap.getHeight());
21         mMatrix = new Matrix();
22         mLstPointF = new PointF();
23     }
24 
25     @RequiresApi(api = Build.VERSION_CODES.KITKAT)
26     @Override
27     public boolean onTouchEvent(MotionEvent event) {
28         Log.i(TAG, ""   MotionEvent.actionToString(event.getAction())   ";mCanDrag="   mCanDrag   ";actionIndex="   event.getActionIndex()   ";action="   event.getAction());
29         int actionIndex = event.getActionIndex();
30         switch (event.getActionMasked()) {
31             case MotionEvent.ACTION_DOWN:
32                 //getX()和getY()没有传入参数时，默认传入的0
33                 if (mRectF.contains(event.getX(), event.getY())) {
34                     mActivePointerId = 0; //第一根手指按下时，pointerId和pointerIndex都为0
35                     mCanDrag = true;
36                     mLstPointF.set(event.getX(), event.getY());
37                 }
38                 break;
39             case MotionEvent.ACTION_POINTER_DOWN:
40                 //有新落下的手指,则将新落下的手指作为活动手指
41                 mActivePointerId = event.getPointerId(actionIndex);
42                 mLstPointF.set(event.getX(actionIndex), event.getY(actionIndex));
43                 break;
44             case MotionEvent.ACTION_MOVE:
45                 if (mActivePointerId == INVALID_POINTER) {
46                     break;
47                 }
48                 if (mCanDrag) {
49                     int pointerIndex = event.findPointerIndex(mActivePointerId);
50                     //这里需要注意，多手指频繁按下和抬起时可能会出现pointerIndex为-1的情况,如不处理，后面会报错
51                     if (pointerIndex == -1) {
52                         break;
53                     }
54                     mMatrix.postTranslate(event.getX(pointerIndex) - mLstPointF.x, event.getY(pointerIndex) - mLstPointF.y);
55                     mLstPointF.set(event.getX(pointerIndex), event.getY(pointerIndex));
56                     mRectF = new RectF(0, 0, mBitmap.getWidth(), mBitmap.getHeight());
57                     mMatrix.mapRect(mRectF);
58                     invalidate();
59                 }
60                 break;
61             case MotionEvent.ACTION_POINTER_UP:
62                 //如果当前抬起的手指为活动手指
63                 if (mActivePointerId == event.getPointerId(actionIndex)) {
64                     int newPointerIndex = actionIndex == 0 ? 1 : 0;
65                     mActivePointerId = event.getPointerId(newPointerIndex);
66                     mLstPointF.set(event.getX(newPointerIndex), event.getY(newPointerIndex));
67                 }
68                 break;
69             case MotionEvent.ACTION_UP:
70                 //最后一根手指也抬起来了
71                 mActivePointerId = INVALID_POINTER;
72                 mCanDrag = false;
73                 break;
74         }
75         return true;
76     }
77 
78     @Override
79     protected void onDraw(Canvas canvas) {
80         super.onDraw(canvas);
81         canvas.drawBitmap(mBitmap, mMatrix, mPaint);
82     }
83 }

依然用两根手指依次拖动图片，效果如下所示：

现在可以看到，两根手指正常拖动图片了，毫无违和感。