Android PathMeasure的使用详解

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在上一篇的​​Android Path的使用详解​​中分析了Path类的一些常用的方法，下面顺便看一下和他有关联的一个类PathMeasure，顾名思义，就是测量Path的，来先看一下注释

/**
     * Create an empty PathMeasure object. To uses this to measure the length
     * of a path, and/or to find the position and tangent along it, call
     * setPath.
     *
     * Note that once a path is associated with the measure object, it is
     * undefined if the path is subsequently modified and the the measure object
     * is used. If the path is modified, you must call setPath with the path.
     */
    public PathMeasure() {
        mPath = null;
        native_instance = native_create(0, false);
    }

为了测量他的长度或切线需要先调用setPath方法。如果path修改了，需要测量，则必须再次调用setPath方法。再来看一下另外的一个构造方法

/**
     * Create a PathMeasure object associated with the specified path object
     * (already created and specified). The measure object can now return the
     * path's length, and the position and tangent of any position along the
     * path.
     *
     * Note that once a path is associated with the measure object, it is
     * undefined if the path is subsequently modified and the the measure object
     * is used. If the path is modified, you must call setPath with the path.
     *
     * @param path The path that will be measured by this object
     * @param forceClosed If true, then the path will be considered as "closed"
     *        even if its contour was not explicitly closed.
     */
    public PathMeasure(Path path, boolean forceClosed) {
        // The native implementation does not copy the path, prevent it from being GC'd
        mPath = path;
        native_instance = native_create(path != null ? path.ni() : 0,
                                        forceClosed);
    }

其中forceClosed表示测量的时候是否强制关闭，只是作为一个测量的值，但并不会关闭path，看一下代码

package test.view;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Paint.Style;
import android.graphics.Path;
import android.graphics.PathMeasure;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;

public class PathView extends View {
  private Paint mPaint;
  private Path mPath;

  public PathView(Context context, AttributeSet attrs) {
    super(context, attrs);
    init();
  }

  private void init() {
    mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
    mPaint.setColor(Color.RED);
    mPaint.setStyle(Style.STROKE);
    mPaint.setStrokeWidth(8);
    mPath = new Path();
    mPath.moveTo(100, 100);
    mPath.lineTo(400, 400);
    mPath.lineTo(600, 200);
    Log.d("wld_____", new PathMeasure(mPath, false).getLength() + "");
  }

  @Override
  protected void onDraw(Canvas canvas) {
    canvas.drawPath(mPath, mPaint);

  }
}

看一下运行结果

再看一下打印log

修改一下

Log.d("wld_____", new PathMeasure(mPath, true).getLength() + "");

看一下结果

在看一下打印log

可以看到两个图图形虽然没有变，但测量长度变了。

setPath(Path path, boolean forceClosed)方法和上面的构造方法其实差不多。

getLength()返回path当前轮廓的长度。

getPosTan(float distance, float pos[], float tan[])其中的distance(0<=distance<=getLength())，pos是在长度为distance的点的坐标，其中(x==[0], y==[1])，tan是在那个点的切线的值，其中(x==[0], y==[1])，返回true数据会存入 pos 和 tan 中，返回false，pos 和 tan 不会改变

getMatrix(float distance, Matrix matrix, int flags)得到路径上某一长度的位置以及该位置正切值的矩阵，flags有两种类型

public static final int POSITION_MATRIX_FLAG = 0x01;    // must match flags in SkPathMeasure.h
    public static final int TANGENT_MATRIX_FLAG  = 0x02;    // must match flags in SkPathMeasure.h

一个表示位置，一个表示正切。

getSegment(float startD, float stopD, Path dst, boolean startWithMoveTo)根据开始和结束的长度得到path中的一个片段，startWithMoveTo为true表示截取到的位置不会改变，为false表示截取到的会连接在一起，看一下代码。

package test.view;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Paint.Style;
import android.graphics.Path;
import android.graphics.PathMeasure;
import android.util.AttributeSet;
import android.view.View;

public class PathView extends View {
  private Paint mPaint;;
  private Path dst;

  public PathView(Context context, AttributeSet attrs) {
    super(context, attrs);
    init();
  }

  private void init() {
    mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
    mPaint.setStyle(Style.STROKE);
    mPaint.setStrokeWidth(12);
    mPaint.setColor(Color.RED);
    dst = new Path();

    Path mPath1 = new Path();
    mPath1.addRect(200, 200, 600, 600, Path.Direction.CW);
    PathMeasure measure1 = new PathMeasure(mPath1, false);
    measure1.getSegment(200, 600, dst, true);

    Path mPath2 = new Path();
    mPath2.addRect(200, 1000, 600, 1400, Path.Direction.CW);
    PathMeasure measure = new PathMeasure(mPath2, false);
    measure.getSegment(200, 900, dst, true);

  }

  @Override
  protected void onDraw(Canvas canvas) {
    canvas.drawPath(dst, mPaint);
  }
}

看一下运行结果

截取的两段是互不干涉的，下面修改一下

Path mPath1 = new Path();
    mPath1.addRect(200, 200, 600, 600, Path.Direction.CW);
    PathMeasure measure1 = new PathMeasure(mPath1, false);
    measure1.getSegment(200, 600, dst, false);

    Path mPath2 = new Path();
    mPath2.addRect(200, 1000, 600, 1400, Path.Direction.CW);
    PathMeasure measure = new PathMeasure(mPath2, false);
    measure.getSegment(200, 900, dst, false);

看一下结果

他会找到上一个截取的来进行连接，第一个没有上一个所以连接的为初始位置（0,0）。
isClosed()返回这个路径是否是关闭的。

nextContour()移动到下一个轮廓，因为path可能会有多个轮廓。