Android 虚线实现绘制

前言：

通过view绘制虚实线，采用Android自带API——DashPathEffect。具体使用请参考更多的链接，这里只是讲解。

构造函数

DashPathEffect 的构造函数有两个参数：

DashPathEffect (float[] intervals, float phase)

官方文档解释如下：

The intervals array must contain an even number of entries (>=2), with the even indices specifying the "on" intervals, and the odd indices specifying the "off" intervals. phase is an offset into the intervals array (mod the sum of all of the intervals). The intervals array controls the length of the dashes. The paint's strokeWidth controls the thickness of the dashes. Note: this path effect only affects drawing with the paint's style is set to STROKE or FILL_AND_STROKE. It is ignored if the drawing is done with style == FILL.

翻译成中文如下：

间隔数组必须包含偶数个条目(大于等于2)，偶数索引指定“开”间隔，而奇数索引指定“关”间隔。相位是间隔数组的偏移量(所有间隔的总和)。间隔数组控制了冲线的长度。画笔宽度宽度控制着冲线的厚度。注意：路径效果只对画笔样式为描边或填充和描边有效。如果画笔样式为填充则会忽略它。

下面通过一个示例来研究这两个参数的作用。

代码部分

MainActivity.java

package ivan.rich.patheffect;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

    }

}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout

    xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    tools:context="ivan.rich.patheffect.MainActivity">

    <ivan.rich.patheffect.PathEffectView

        android:layout_width="match_parent"

        android:layout_height="match_parent"/>

</LinearLayout>

PathEffectView.java

package ivan.rich.patheffect;

import android.content.Context;

import android.graphics.Canvas;

import android.graphics.DashPathEffect;

import android.graphics.Paint;

import android.graphics.Path;

import android.support.annotation.Nullable;

import android.util.AttributeSet;

import android.view.View;

public class PathEffectView extends View {

    private int mWidth;

    private int mHeight;

    private Paint mLinePaint;

    private Paint mPaint;

    private Path mPath;

    public PathEffectView(Context context) {

        this(context, null);

    }

    public PathEffectView(Context context, @Nullable AttributeSet attrs) {

        super(context, attrs);

        init();

    }

    private void init() {

        mPaint = new Paint();

        mPaint.setAntiAlias(true);

        mPaint.setStrokeWidth(20);

        mPaint.setColor(getResources().getColor(R.color.colorPrimary));

        mPaint.setStyle(Paint.Style.STROKE);

        mLinePaint = new Paint(Paint.ANTI_ALIAS_FLAG);

        mLinePaint.setStrokeWidth(1);

        mLinePaint.setStyle(Paint.Style.STROKE);

        mLinePaint.setColor(getResources().getColor(R.color.colorAccent));

        mPath = new Path();

    }

    @Override

    protected void onSizeChanged(int w, int h, int oldw, int oldh) {

        super.onSizeChanged(w, h, oldw, oldh);

        mWidth = w;

        mHeight = h;

    }

    @Override

    protected void onDraw(Canvas canvas) {

        super.onDraw(canvas);

        // 绘制一条参考线

        canvas.drawLine(60, 0, 60, mHeight, mLinePaint);

        // 绘制第一条虚线

        DashPathEffect dashPathEffect1 = new DashPathEffect(new float[]{60, 60}, 0);

        mPaint.setPathEffect(dashPathEffect1);

        mPath.reset();

        mPath.moveTo(0, mHeight / 10);

        mPath.lineTo(mWidth, mHeight / 10);

        canvas.drawPath(mPath, mPaint);

        // 绘制第二条虚线

        DashPathEffect dashPathEffect2 = new DashPathEffect(new float[]{60, 60}, 20);

        mPaint.setPathEffect(dashPathEffect2);

        mPath.reset();

        mPath.moveTo(0, mHeight * 2 / 10);

        mPath.lineTo(mWidth, mHeight * 2 / 10);

        canvas.drawPath(mPath, mPaint);

        // 绘制第三条虚线

        DashPathEffect dashPathEffect3 = new DashPathEffect(new float[]{60, 60}, 40);

        mPaint.setPathEffect(dashPathEffect3);

        mPath.reset();

        mPath.moveTo(0, mHeight * 3 / 10);

        mPath.lineTo(mWidth, mHeight * 3 / 10);

        canvas.drawPath(mPath, mPaint);

        // 绘制第四条虚线

        DashPathEffect dashPathEffect4 = new DashPathEffect(new float[]{60, 60}, 60);

        mPaint.setPathEffect(dashPathEffect4);

        mPath.reset();

        mPath.moveTo(0, mHeight * 4 / 10);

        mPath.lineTo(mWidth, mHeight * 4 / 10);

        canvas.drawPath(mPath, mPaint);

        // 绘制第五条虚线

        DashPathEffect dashPathEffect5 = new DashPathEffect(new float[]{60, 60, 30, 30}, 0);

        mPaint.setPathEffect(dashPathEffect5);

        mPath.reset();

        mPath.moveTo(0, mHeight * 6 / 10);

        mPath.lineTo(mWidth, mHeight * 6 / 10);

        canvas.drawPath(mPath, mPaint);

        // 绘制第六条虚线

        DashPathEffect dashPathEffect6 = new DashPathEffect(new float[]{60, 30, 30, 60}, 0);

        mPaint.setPathEffect(dashPathEffect6);

        mPath.reset();

        mPath.moveTo(0, mHeight * 7 / 10);

        mPath.lineTo(mWidth, mHeight * 7 / 10);

        canvas.drawPath(mPath, mPaint);

        // 绘制第七条虚线

        DashPathEffect dashPathEffect7 = new DashPathEffect(new float[]{30, 60, 60, 30}, 0);

        mPaint.setPathEffect(dashPathEffect7);

        mPath.reset();

        mPath.moveTo(0, mHeight * 8 / 10);

        mPath.lineTo(mWidth, mHeight * 8 / 10);

        canvas.drawPath(mPath, mPaint);

        // 绘制第八条虚线

        DashPathEffect dashPathEffect8 = new DashPathEffect(new float[]{30, 30, 60, 60}, 0);

        mPaint.setPathEffect(dashPathEffect8);

        mPath.reset();

        mPath.moveTo(0, mHeight * 9 / 10);

        mPath.lineTo(mWidth, mHeight * 9 / 10);

        canvas.drawPath(mPath, mPaint);

    }

}

运行结果

参数分析

1.首先看截图上半部分的四条虚线段，四条虚线对应的DashPathEffect如下：

DashPathEffect dashPathEffect1 = new DashPathEffect(new float[]{60, 60}, 0);

DashPathEffect dashPathEffect2 = new DashPathEffect(new float[]{60, 60}, 20);

DashPathEffect dashPathEffect3 = new DashPathEffect(new float[]{60, 60}, 40);

DashPathEffect dashPathEffect4 = new DashPathEffect(new float[]{60, 60}, 60);

这四个DashPathEffect的区别在于第二个参数phase值不同，以参考线为基准可以清晰地看到phase参数的作用是将整个View向“左”移动phase。什么意思呢，左移为0：不移动；左移为20：移动20个单位长度；左移为40：移动40个单位长度。因此，左移60的时候，第四根线条相对第一根线条，左移了一个实线段。

2.然后看截图下半部分的四条虚线段，这四条虚线段对应的DashPathEffect如下：

DashPathEffect dashPathEffect5 = new DashPathEffect(new float[]{60, 60, 30, 30}, 0);

DashPathEffect dashPathEffect6 = new DashPathEffect(new float[]{60, 30, 30, 60}, 0);

DashPathEffect dashPathEffect7 = new DashPathEffect(new float[]{30, 60, 60, 30}, 0);

DashPathEffect dashPathEffect8 = new DashPathEffect(new float[]{60, 60, 40, 40, 20, 20}, 0);

从效果图可以看出间隔数组的偶数索引处数组值对应的是实线宽度，奇数索引处数组值对应的是实线之后空白线的宽度。前面已经提到过数组必须包含偶数个条目，所以“on”和“off”值是对应的。在绘制View时系统遍历当前间隔数组，依次绘制第一个“on”和第一个“off”值，第二个“on”和第二个“off"值。。。，照此类推直至绘制完所有对应的"on"和"off”值，然后按照此方法循环遍历间隔数组直至View的绘制完成。用代码概括来说就是：

for (i=0 ; i<intervals.length; i+=2) {

    // 实线宽度on

    mLineWidth = intervals[i];

    // 实线之间空白线宽度off

    mBlankSpace = intervals[i+1];

}

总结

构造函数

DashPathEffect(float intervals[], float phase)

参数含义

intervals：控制实线和实线之后空白线的宽度（数组长度必须为偶数）
phase：将View向”左“偏移phase

用法

Paint.setPathEffect(DashPathEffect dashPathEffect);