圖片拖拽，放大，縮小

佈局：

 <ImageView
        android:id="@+id/imageView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:scaleType="matrix"
        android:src="@drawable/i" />

mainactivity：

public class MainActivity extends AppCompatActivity {

    private final int MODE_DRAG = 1;
    private final int MODE_ZOOM = 2;
    private final int MODE_NONE = 0;
    private ImageView imageView;
    private int currentMode = MODE_NONE;
    private double distanceStart;
    private float xl;
    private float yl;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        imageView = (ImageView) findViewById(R.id.imageView);
        initView();
    }

    Matrix matrix = new Matrix();
    Matrix matrixNew = new Matrix();

    private void initView() {
        imageView.setOnTouchListener(new View.OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                switch (event.getAction() & MotionEvent.ACTION_MASK) {
                    case MotionEvent.ACTION_DOWN:
                        currentMode = MODE_DRAG;
                        xl = event.getX();
                        yl = event.getY();
                        matrix.set(imageView.getImageMatrix());
                        break;
                    case MotionEvent.ACTION_POINTER_DOWN:
                        distanceStart = spacing(event);
                        if (distanceStart > 20) {
                            currentMode = MODE_ZOOM;
                            matrix.set(imageView.getImageMatrix());
                        }
                        break;
                    case MotionEvent.ACTION_MOVE:
                        if (currentMode == MODE_DRAG) {
                            matrixNew.set(matrix);
                            float xn = event.getX();
                            float yn = event.getY();
                            //matrix.setTranslate(xn,yn);
                            matrixNew.postTranslate(xn - xl, yn - yl);
                        } else if (currentMode == MODE_ZOOM) {
                            double distance = spacing(event);
                            float scale = (float) (distance / distanceStart);
                            PointF pointF = midPoint(event);
                            matrixNew.postScale(scale, scale, pointF.x, pointF.y);
                        }
                        break;
                    case MotionEvent.ACTION_UP:
                        currentMode = MODE_NONE;
                        break;
                }
                imageView.setImageMatrix(matrixNew);
                return true;
            }
        });
    }

    //計算兩點之間的距離
    private double spacing(MotionEvent event) {
        float x = 0;
        float y = 0;
        try {
            x = event.getX(0) - event.getX(1);
            y = event.getY(0) - event.getY(1);
        } catch (IllegalArgumentException e) {
        }
        return Math.sqrt(x * x + y * y);
    }

    public PointF midPoint(MotionEvent event) {
        float x0 = event.getX(0);
        float x1 = event.getX(1);
        float y0 = event.getX(0);
        float y1 = event.getX(1);

        float x = (x0 + x1) / 2;
        float y = (y0 + y1) / 2;

        PointF pointF = new PointF(x, y);
        return pointF;
    }

}