这几天因为项目需求，需要在ImageView上面叠加一层透明圆弧，并且在沿着圆弧的方向显示相应的文字，效果如下图所示：
拿到这个需求，首先想到的是自定义一个ImageView来实现此功能，即在onDraw()中绘制圆弧和文字。同时因为要保证圆弧的位置可以任意摆放，圆弧的颜色、透明度以及文字大小、颜色等都是可控的，所以增加了一些自定义属性。实现代码非常简单，如下：
1.自定义ImageView：
package com.chunk.customviewsdemo.views.ArcImageView;import android.content.Context;import android.content.res.TypedArray;import android.graphics.Canvas;import android.graphics.Paint;import android.graphics.Path;import android.graphics.RectF;import android.util.AttributeSet;import android.widget.ImageView;import com.chunk.customviewsdemo.R;/*** Description:A custom ImageView with circular arc and text* Author: XiaoYu* Date: // :*/public class ArcImageView extends ImageView {/*** The default text size.*/private final float DEFAULT_TEXT_SIZE = ;/*** The default scale value which decides the width of arc.*/private final float DEFAULT_SCALE = .f;/*** The default transparency of arc.*/private final int DEFAULT_ARC_ALPHA =;/*** The default width of arc.*/private final int DEFAULT_ARC_WIDTH =;/*** The default angle that the arc starts with.*/private final int DEFAULT_START_ANGLE = ;/*** The default angle that the arc.*/private final int DEFAULT_SWEEP_ANGLE = ;/*** The default distance along the path to add to the text's starting position.*/private final int DEFAULT_H_OFFSET = ;/*** The default distance above(-) or below(+) the path to position the text.*/private final int DEFAULT_V_OFFSET = ;private Context mContext;/*** The text displayed on ImageView along arc.*/private String mDrawStr;/*** The font size of text.*/private float mTextSize = DEFAULT_TEXT_SIZE;/*** The scale value which decides the width of arc.*/private float mScale = DEFAULT_SCALE;/*** The transparency of arc.*/private int mArcAlpha = DEFAULT_ARC_ALPHA;/*** The width of arc.*/private int mArcWidth = DEFAULT_ARC_WIDTH;/*** The start angle of angle.*/private int mStartAngle = DEFAULT_START_ANGLE;/*** The swept angle of angle.*/private int mSweepAngle = DEFAULT_SWEEP_ANGLE;/*** The default distance along the path to add to the text's starting position.*/private float mHOffset = DEFAULT_H_OFFSET;/*** The default distance above(-) or below(+) the path to position the text.*/private float mVOffset = DEFAULT_V_OFFSET;/*** The style of arc, all styles includes LEFT_TOP, LEFT_BOTTOM, RIGHT_TOP, RIGHT_BOTTOM, CENTER。* of course, you can add your own style according to your demands.*/private int mDrawStyle;/*** The color of arc.*/private int mArcColor;/*** The color of text.*/private int mTextColor;public ArcImageView(Context context) {super(context);this.mContext = context;}public ArcImageView(Context context, AttributeSet attrs) {super(context, attrs);this.mContext = context;obtainAttributes(attrs);}public ArcImageView(Context context, AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr);this.mContext = context;obtainAttributes(attrs);}/*** Set the text that will be drawn on arc.* @param drawStr the text content.*/public void setDrawStr(String drawStr) {this.mDrawStr = drawStr;//refresh this viewinvalidate();}/*** Set the transparency of arc.* @param mArcAlpha the value of transparency.*/public void setArcAlpha(int mArcAlpha) {this.mArcAlpha = mArcAlpha;//refresh this viewinvalidate();}@Overrideprotected void onDraw(Canvas canvas) {super.onDraw(canvas);//draw arcPaint arcPaint = new Paint();arcPaint.setStrokeWidth(mArcWidth);arcPaint.setStyle(Paint.Style.STROKE);arcPaint.setColor(mArcColor);arcPaint.setAlpha(mArcAlpha);int width = getWidth();int height = getHeight();float radius;if (width > height) {radius = mScale * height;} else {radius = mScale * width;}RectF oval = new RectF();int center_x = width;int center_y = height;switch (mDrawStyle) {case :center_x = ;center_y = ;mStartAngle = ;mSweepAngle = -;break;case :center_x = ;center_y = height;mStartAngle = ;mSweepAngle = ;break;case :center_x = width;center_y = ;mStartAngle = ;mSweepAngle = -;break;case :center_x = width;center_y = height;mStartAngle = ;mSweepAngle = ;break;case :center_x = width / ;center_y = height / ;mStartAngle = ;mSweepAngle = ;break;}float left = center_x - radius;float top = center_y - radius;float right = center_x + radius;float bottom = center_y + radius;oval.set(left, top, right, bottom);canvas.drawArc(oval, mStartAngle, mSweepAngle, false, arcPaint);//draw textPaint textPaint = new Paint();textPaint.setTextSize(mTextSize);textPaint.setStyle(Paint.Style.FILL);textPaint.setColor(mTextColor);Path path = new Path();path.addArc(oval, mStartAngle, mSweepAngle);canvas.drawTextOnPath(mDrawStr, path, mHOffset, mVOffset, textPaint);}/*** Obtain custom attributes that been defined in attrs.xml.* @param attrs A collection of attributes.*/private void obtainAttributes(AttributeSet attrs) {TypedArray ta = mContext.obtainStyledAttributes(attrs, R.styleable.ArcImageView);mDrawStr = ta.getString(R.styleable.ArcImageView_drawStr);mTextSize = ta.getDimension(R.styleable.ArcImageView_textSize, DEFAULT_TEXT_SIZE);mArcAlpha = ta.getInteger(R.styleable.ArcImageView_arcAlpha, DEFAULT_ARC_ALPHA);mArcWidth = ta.getInteger(R.styleable.ArcImageView_arcWidth, DEFAULT_ARC_WIDTH);mStartAngle = ta.getInteger(R.styleable.ArcImageView_startAngle, DEFAULT_START_ANGLE);mSweepAngle = ta.getInteger(R.styleable.ArcImageView_startAngle, DEFAULT_SWEEP_ANGLE);mHOffset = ta.getInteger(R.styleable.ArcImageView_hOffset, DEFAULT_H_OFFSET);mVOffset = ta.getInteger(R.styleable.ArcImageView_vOffset, DEFAULT_V_OFFSET);mArcColor = ta.getColor(R.styleable.ArcImageView_arcColor, XCCCCCC);mTextColor = ta.getColor(R.styleable.ArcImageView_textColor, XFFFFFF);mDrawStyle = ta.getInt(R.styleable.ArcImageView_drawStyle, );ta.recycle();}}
2.在values文件夹下的attrs.xml中自定义属性：
<?xml version="." encoding="utf-"?><resources><declare-styleable name="ArcImageView"><attr name="drawStr" format="string" /><attr name="textSize" format="dimension" /><attr name="arcAlpha" format="integer" /><attr name="arcWidth" format="integer" /><attr name="startAngle" format="integer" /><attr name="sweepAngle" format="integer" /><attr name="scale" format="float" /><attr name="hOffset" format="float" /><attr name="vOffset" format="float" /><attr name="drawStyle" format="enum"><enum name="LEFT_TOP" value="" /><enum name="LEFT_BOTTOM" value="" /><enum name="RIGHT_TOP" value="" /><enum name="RIGHT_BOTTOM" value="" /><enum name="CENTER" value="" /></attr><attr name="arcColor" format="color" /><attr name="textColor" format="color" /></declare-styleable></resources>
3.在MainActivity调用ArcImageView，实现代码如下：
package com.chunk.customviewsdemo;import android.os.Bundle;import android.support.v.app.AppCompatActivity;import android.view.View;import android.widget.Button;import com.chunk.customviewsdemo.views.ArcImageView.ArcImageView;public class MainActivity extends AppCompatActivity implements View.OnClickListener {private ArcImageView aiv_one;private ArcImageView aiv_two;private ArcImageView aiv_three;private ArcImageView aiv_four;private Button btn_another_one;private int mGroup = ;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);aiv_one = (ArcImageView) findViewById(R.id.aiv_one);aiv_one.setArcAlpha();aiv_two = (ArcImageView) findViewById(R.id.aiv_two);aiv_two.setArcAlpha();aiv_three = (ArcImageView) findViewById(R.id.aiv_three);aiv_three.setArcAlpha();aiv_four = (ArcImageView) findViewById(R.id.aiv_four);aiv_four.setArcAlpha();btn_another_one = (Button) findViewById(R.id.btn_another_one);btn_another_one.setOnClickListener(this);}@Overridepublic void onClick(View v) {switch (v.getId()) {case R.id.btn_another_one:if (mGroup == ) {aiv_one.setDrawStr("苹果");aiv_one.setBackgroundResource(R.drawable.apple);aiv_two.setDrawStr("柚子");aiv_two.setBackgroundResource(R.drawable.pineapple);aiv_three.setDrawStr("香蕉");aiv_three.setBackgroundResource(R.drawable.banana);aiv_four.setDrawStr("菠萝");aiv_four.setBackgroundResource(R.drawable.pineapple);mGroup = ;} else {aiv_one.setDrawStr("牛排");aiv_one.setBackgroundResource(R.drawable.steak);aiv_two.setDrawStr("海鲜");aiv_two.setBackgroundResource(R.drawable.seafood);aiv_three.setDrawStr("奶酪");aiv_three.setBackgroundResource(R.drawable.cheese);aiv_four.setDrawStr("烧烤");aiv_four.setBackgroundResource(R.drawable.barbecue);mGroup = ;}break;}}}
4.MainActivity的布局文件如下：
<LinearLayoutxmlns:android="http://schemas.android.com/apk/res/android"xmlns:custom="http://schemas.android.com/apk/res-auto"android:layout_width="match_parent"android:layout_height="match_parent"android:layout_marginTop="dp"android:layout_marginBottom="dp"android:orientation="vertical" ><Buttonandroid:id="@+id/btn_another_one"android:layout_width="wrap_content"android:layout_height="wrap_content"android:text="换一组" /><LinearLayoutandroid:layout_width="match_parent"android:layout_height="dp"android:layout_weight=""android:orientation="horizontal" ><RelativeLayoutandroid:layout_width="dp"android:layout_weight=""android:layout_height="match_parent" ><com.chunk.customviewsdemo.views.ArcImageView.ArcImageViewandroid:id="@+id/aiv_one"android:layout_width="match_parent"android:layout_height="match_parent"android:background="@drawable/steak"custom:drawStyle="RIGHT_BOTTOM"custom:drawStr="牛排"custom:arcAlpha=""custom:arcColor="@color/gray"custom:textColor="@color/black"custom:textSize="sp" /></RelativeLayout><RelativeLayoutandroid:layout_width="dp"android:layout_weight=""android:layout_height="match_parent" ><com.chunk.customviewsdemo.views.ArcImageView.ArcImageViewandroid:id="@+id/aiv_two"android:layout_width="match_parent"android:layout_height="match_parent"android:background="@drawable/seafood"custom:drawStyle="LEFT_BOTTOM"custom:drawStr="海鲜"custom:arcAlpha=""custom:arcColor="@color/gray"custom:textColor="@color/black"custom:textSize="sp" /></RelativeLayout></LinearLayout><LinearLayoutandroid:layout_width="match_parent"android:layout_height="dp"android:layout_weight=""android:orientation="horizontal" ><RelativeLayoutandroid:layout_width="dp"android:layout_weight=""android:layout_height="match_parent" ><com.chunk.customviewsdemo.views.ArcImageView.ArcImageViewandroid:id="@+id/aiv_three"android:layout_width="match_parent"android:layout_height="match_parent"android:background="@drawable/cheese"custom:drawStyle="RIGHT_TOP"custom:drawStr="奶酪"custom:arcAlpha=""custom:arcColor="@color/gray"custom:textColor="@color/black"custom:textSize="sp" /></RelativeLayout><RelativeLayoutandroid:layout_width="dp"android:layout_weight=""android:layout_height="match_parent" ><com.chunk.customviewsdemo.views.ArcImageView.ArcImageViewandroid:id="@+id/aiv_four"android:layout_width="match_parent"android:layout_height="match_parent"android:background="@drawable/barbecue"custom:drawStyle="LEFT_TOP"custom:drawStr="烧烤"custom:arcAlpha=""custom:arcColor="@color/gray"custom:textColor="@color/black"custom:textSize="sp" /></RelativeLayout></LinearLayout></LinearLayout>
注意，在布局文件中引入自定义属性时需要加入一行代码：xmlns:custom=”http://schemas.android.com/apk/res-auto”。
好了，需求搞定，剩下的就是搬到实际的项目当中去了。实现效果如下：
总结一下，自定义View一般就是通过重写onDraw、onMeasure()、onLayout()等方法来进行测量、绘制，绘制的时候一般会用到Canvas、Paint、Bitmap等类，测量和绘制的过程其实就是对现实生活中绘图工作的抽象和实现，我们利用面向对象的思想将画板、画纸、画笔等工具以及绘画的动作用一行行代码加以描述就OK啦！
由于实现过程比较简单，我就不贴源码了，大家如果对2D绘图还不是很了解，可以去搜一下相关资料或查阅相关书籍！
一个真正的人对困难的回答是战斗，
