Android:ImageView图片缩放、居中

几个重要知识点：

1、布局文件中android:scaleType="matrix" 设置图片动态缩放

2、matrix.postTranslate(dx,dy); 平移图片 matrix.postScale(sx,sy,p.x,p.y); 缩放图片

3、两点的中点、距离计算方式，限制缩放范围函数，在指定区域内移动图片方法

(以下代码网上很多地方有类似的，仅供参考)

public class Main extends Activity
{
	private static final int NONE = 0;
	private static final int DRAG = 1;
	private static final int ZOOM = 2;

	private int mode = NONE;
	private float oldDist;
	private Matrix matrix = new Matrix();
	private Matrix savedMatrix = new Matrix();
	private PointF start = new PointF();
	private PointF mid = new PointF();
	
	ImageView view;
	Bitmap bitmap;
	DisplayMetrics dm;
	float minScaleR=0.1f;  //最少缩放比例
	static final float MAX_SCALE = 4f; //最大缩放比例
	float dist = 1f;

	@Override
	public void onCreate(Bundle savedInstanceState)
	{
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
		view = (ImageView) findViewById(R.id.image_view);
		bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.test);
		
		dm = new DisplayMetrics();
		getWindowManager().getDefaultDisplay().getMetrics(dm); //获取分辨率
		
		
		matrix.setScale(minScaleR, minScaleR); //开始先缩小
		matrix.postTranslate(120, 120);		//图片的位置相对于imageview的左上角往右往下各偏移120个像素
		view.setImageMatrix(matrix);
		
		view.setOnTouchListener(new OnTouchListener()
		{
			@Override
			public boolean onTouch(View v, MotionEvent event)
			{
				ImageView view = (ImageView) v;
				switch (event.getAction() & MotionEvent.ACTION_MASK)
				{
				case MotionEvent.ACTION_DOWN:  //单点
					Toast.makeText(Main.this, bitmap.getWidth()+"*"+bitmap.getHeight(), Toast.LENGTH_LONG).show();
					savedMatrix.set(matrix);
					start.set(event.getX(), event.getY());
					mode = DRAG;
					break;
				case MotionEvent.ACTION_UP:  //单点弹起
				case MotionEvent.ACTION_POINTER_UP:  //多点弹起
					mode = NONE;
					break;
				case MotionEvent.ACTION_POINTER_DOWN:  //多点
					oldDist = spacing(event);
					if (oldDist > 10f)
					{
						savedMatrix.set(matrix);
						midPoint(mid, event);
						mode = ZOOM;
					}
					break;
				case MotionEvent.ACTION_MOVE: //按下且在拖动
					if (mode == DRAG)
					{
						matrix.set(savedMatrix);
						matrix.postTranslate(event.getX() - start.x,event.getY() - start.y);  //xy方向都可以拖动
						//matrix.postTranslate(event.getX() - start.x,0); //只在x轴方向拖动 即左右拖动  上下不动
						//matrix.postTranslate(0,event.getY() - start.y);  //只在y轴方向拖动 即上下拖动  左右不动
					}
					else if (mode == ZOOM)
					{
						float newDist = spacing(event);
						if (newDist > 10f)
						{
							matrix.set(savedMatrix);
							float scale = newDist / oldDist;
							matrix.postScale(scale, scale, mid.x, mid.y);
						}
					}
					break;
				}
				
				view.setImageMatrix(matrix);
				
				CheckScale();  //限制缩放范围
				center();  //居中控制
				
				return true;
			}

			
			//两点的距离
			private float spacing(MotionEvent event)
			{
				float x = event.getX(0) - event.getX(1);
				float y = event.getY(0) - event.getY(1);
				return FloatMath.sqrt(x * x + y * y);
			}
			
			//两点的中点
			private void midPoint(PointF point, MotionEvent event)
			{
				float x = event.getX(0) + event.getX(1);
				float y = event.getY(0) + event.getY(1);
				point.set(x / 2, y / 2);
			}
		});
	}
	
	//限制最大最小缩放比例  
	protected void CheckScale()
	{
		float p[] = new float[9];
		matrix.getValues(p);
		if (mode == ZOOM)
		{
			if (p[0] < minScaleR)
			{
				matrix.setScale(minScaleR, minScaleR);
			}
			if (p[0] > MAX_SCALE)
			{
				matrix.set(savedMatrix);
			}
		}
	}

	//自动居中  左右及上下都居中
	protected void center()
	{
		center(true,true);
	}

	private void center(boolean horizontal, boolean vertical)
	{
		Matrix m = new Matrix();
		m.set(matrix);
		RectF rect = new RectF(0, 0, bitmap.getWidth(), bitmap.getHeight());
		m.mapRect(rect);
		float height = rect.height();
		float width = rect.width();
		float deltaX = 0, deltaY = 0;
		if (vertical)
		{
			//int screenHeight = dm.heightPixels;  //手机屏幕分辨率的高度
			int screenHeight = 400;
			if (height < screenHeight)
			{
				deltaY = (screenHeight - height)/2 - rect.top;
			}else if (rect.top > 0)
			{
				deltaY = -rect.top;
			}else if (rect.bottom < screenHeight)
			{
				deltaY = view.getHeight() - rect.bottom;
			}
		}
		
		if (horizontal)
		{
			//int screenWidth = dm.widthPixels;  //手机屏幕分辨率的宽度
			int screenWidth = 400;
			if (width < screenWidth)
			{
				deltaX = (screenWidth - width)/2 - rect.left;
			}else if (rect.left > 0)
			{
				deltaX = -rect.left;	
			}else if (rect.right < screenWidth)
			{
				deltaX = screenWidth - rect.right;
			}
		}
		matrix.postTranslate(deltaX, deltaY);
	}
}