一般顶部栏的返回键都是让UI切图，其实还可以自定义View绘制，本篇就闲来蛋疼绘制一个箭头返回键吧，有Material Design和微信风格（就是没有中间那条横线）。
Android 自定义View绘制箭头.png
原理
我们观察一下，控件中点，向右一条横线，再从中心点2个45度角的2条线，如果用数学的惯性思维，会想到使用三角函数，计算出2条45度线终点的x，y坐标。但其实Canvas有rotate旋转的API，就不需要那么多麻烦的计算了，2个45度的角，其实就是一个90度的直角，我们可以先画一个直角，然后再旋转45度，就可以了。
完整代码
- 自定义属性
<declare-styleable name="BackArrowView">
<!-- 箭头颜色 -->
<attr name="bav_color" format="color" />
<!-- 箭头线宽 -->
<attr name="bav_stroke_width" format="integer|float|dimension" />
<!-- 模式 -->
<attr name="bav_arrow_style" format="enum">
<!-- Google Material Design -->
<enum name="material_design" value="1" />
<!-- 微信风格 -->
<enum name="wechat_design" value="2" />
</attr>
</declare-styleable>
- Java代码
public class BackArrowView extends View {
/**
* View默认最小宽度
*/
private static final int DEFAULT_MIN_WIDTH = 100;
/**
* Material Design风格
*/
private static final int ARROW_STYLE_MATERIAL_DESIGN = 1;
/**
* 微信风格
*/
private static final int ARROW_STYLE_WECHAT_DESIGN = 2;
/**
* 控件宽
*/
private int mViewWidth;
/**
* 控件高
*/
private int mViewHeight;
/**
* 箭头开始的距离
*/
private float mArrowStartDistance;
/**
* 箭头的2个边的长度
*/
private float mArrowLineLength;
/**
* 箭头颜色
*/
private int mColor;
/**
* 箭头粗细
*/
private float mArrowStrokeWidth;
/**
* 风格模式
*/
private int mArrowStyle;
/**
* 画笔
*/
private Paint mPaint;
/**
* 箭头Path
*/
private Path mArrowPath;
public BackArrowView(Context context) {
this(context, null);
}
public BackArrowView(Context context, @Nullable AttributeSet attrs) {
this(context, attrs, 0);
}
public BackArrowView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init(context, attrs, defStyleAttr);
}
private void init(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
initAttr(context, attrs, defStyleAttr);
mPaint = new Paint();
mPaint.setColor(mColor);
//使用Path必须使用STROKE，使用FILL是画不了的
mPaint.setStyle(Paint.Style.STROKE);
mPaint.setAntiAlias(true);
mPaint.setStrokeWidth(mArrowStrokeWidth);
//设置拐角形状为圆形，3条线相接处则不会有缝隙了
mPaint.setStrokeJoin(Paint.Join.ROUND);
}
private void initAttr(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
TypedArray array = context.obtainStyledAttributes(attrs, R.styleable.BackArrowView, defStyleAttr, 0);
mColor = array.getColor(R.styleable.BackArrowView_bav_color, Color.argb(255, 0, 0, 0));
mArrowStrokeWidth = array.getDimension(R.styleable.BackArrowView_bav_stroke_width, dip2px(context, 2f));
mArrowStyle = array.getInt(R.styleable.BackArrowView_bav_arrow_style, ARROW_STYLE_MATERIAL_DESIGN);
array.recycle();
}
@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
super.onSizeChanged(w, h, oldw, oldh);
mViewWidth = w;
mViewHeight = h;
//计算半径
float radius = Math.min(mViewWidth, mViewHeight) / 2f;
//计算箭头起始位置
if (mArrowStyle == ARROW_STYLE_MATERIAL_DESIGN) {
mArrowStartDistance = radius / 3f;
} else if (mArrowStyle == ARROW_STYLE_WECHAT_DESIGN) {
mArrowStartDistance = radius / 4f;
}
//计算箭头长度
mArrowLineLength = radius * 0.63f;
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
//将画布中心移动到中心点偏左位置
canvas.translate((mViewWidth / 2f) - mArrowStartDistance, mViewHeight / 2);
//将画布旋转45度，让后面画的直角旋转
canvas.rotate(45);
if (mArrowPath == null) {
mArrowPath = new Path();
}
mArrowPath.reset();
//画第一条线
mArrowPath.lineTo(0, -mArrowLineLength);
//画第二条线
mArrowPath.moveTo(0, 0);
mArrowPath.lineTo(mArrowLineLength, 0);
//Google Material Design风格才有中间的线
if (mArrowStyle == ARROW_STYLE_MATERIAL_DESIGN) {
//画中间的线
mArrowPath.moveTo(0, 0);
mArrowPath.lineTo(mArrowLineLength, -mArrowLineLength);
}
//闭合路径
mArrowPath.close();
//画路径
canvas.drawPath(mArrowPath, mPaint);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
setMeasuredDimension(handleMeasure(widthMeasureSpec), handleMeasure(heightMeasureSpec));
}
/**
* 处理MeasureSpec
*/
private int handleMeasure(int measureSpec) {
int result = DEFAULT_MIN_WIDTH;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
if (specMode == MeasureSpec.EXACTLY) {
result = specSize;
} else {
//处理wrap_content的情况
if (specMode == MeasureSpec.AT_MOST) {
result = Math.min(result, specSize);
}
}
return result;
}
public static int dip2px(Context context, float dipValue) {
final float scale = context.getResources().getDisplayMetrics().density;
return (int) (dipValue * scale + 0.5f);
}
}
- 简单使用
<com.zh.cavas.sample.widget.BackArrowView
android:id="@+id/back_arrow"
android:layout_width="40dp"
android:layout_height="40dp"
android:layout_marginTop="20dp"
app:bav_arrow_style="wechat_design"
app:bav_color="@android:color/black"
app:bav_stroke_width="1.5dp" />
<com.zh.cavas.sample.widget.BackArrowView
android:id="@+id/back_arrow2"
android:layout_width="40dp"
android:layout_height="40dp"
android:layout_marginTop="20dp"
app:bav_arrow_style="material_design"
app:bav_color="@android:color/black"
app:bav_stroke_width="1.5dp" />
