上图中的按钮是 iOS 中的自带的开关控件，Android 也有很多优秀的仿这个控件的开源库，自己也是模仿着实现了一下，下面记录一下实现过程。

1 思路

首先还是来进行分解动作，从静态样子来看，这个开关是一个圆角矩形的背景，然后中间有一个圆形的东西（姑且叫做按钮指示器）。点击该控件的时候会像 CheckBox 一样，会在开和关的状态之间切换，这个切换不是瞬间完成的，中间的指示器会有一个从左移动到右或从右移动到左的动画，背景也会慢慢过渡变化。
既然刚才说这个控件像 CheckBox 一样，会在两种状态之间切换，所以我们继承 CheckBox 就好。

2 圆角矩形背景

既然理清了思路，那么开始动手，从背景开始。
绘制一个圆角矩形的背景很简单，调用原生 api 即可：

public class SwitchButton extends android.support.v7.widget.AppCompatCheckBox {    private static final String TAG = "SwitchButton";    private static final int DEFAULT_WIDTH = 200;    private static final int DEFAULT_HEIGHT = DEFAULT_WIDTH / 8 * 5;        private Paint mPaint;    private RectF mRectF;    public SwitchButton(Context context) {        this(context, null);    }    public SwitchButton(Context context, AttributeSet attrs) {        this(context, attrs, 0);    }    public SwitchButton(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);        setButtonDrawable(null);        setBackgroundResource(0);        mPaint = new Paint();        mPaint.setAntiAlias(true);        mRectF = new RectF();    }    @Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        int widthMode = MeasureSpec.getMode(widthMeasureSpec);        int widthSize = MeasureSpec.getSize(widthMeasureSpec);        int heightMode = MeasureSpec.getMode(heightMeasureSpec);        int heightSize = MeasureSpec.getSize(heightMeasureSpec);        int width;        int height;        if (widthMode == MeasureSpec.EXACTLY) {            width = widthSize;        } else {            width =  (getPaddingLeft() + DEFAULT_WIDTH + getPaddingRight());        }        if (heightMode == MeasureSpec.EXACTLY) {            height = heightSize;        } else {            height = (getPaddingTop() + DEFAULT_HEIGHT + getPaddingBottom());        }        setMeasuredDimension(width, height);    }    @Override    protected void onDraw(Canvas canvas) {        super.onDraw(canvas);        mPaint.setStrokeWidth((float) getMeasuredWidth() / 40);        mPaint.setColor(0xFFCCCCCC);        mRectF.set(mPaint.getStrokeWidth()                , mPaint.getStrokeWidth()                , getMeasuredWidth() - mPaint.getStrokeWidth()                , getMeasuredHeight() - mPaint.getStrokeWidth());        canvas.drawRoundRect(mRectF, getMeasuredHeight(), getMeasuredHeight(), mPaint);    }}

效果如下：

3 绘制按钮指示器

按钮指示器就是一个圆形，它的半径可以理解为就是控件高度的一半，不过为了好看一点可以让它有点内边距，多减去一个画笔宽度，在 onDraw 方法中增加如下代码：

mPaint.setColor(0xFFFFFFFF);float radius = (getMeasuredHeight() - mPaint.getStrokeWidth() * 4) / 2;float x = mPaint.getStrokeWidth() + mPaint.getStrokeWidth() + radius;float y = (float) getMeasuredHeight() / 2;canvas.drawCircle(x, y, radius, mPaint);

现在效果如下：

4 动起来

样子有了，现在我们来做功能。在点击这个控件的时候，因为它是继承自 CheckBox 的，所以它的 checked 属性会改变，我们根据这个属性就可以绘制不同 UI。主要是在绘制背景时根据不同状态给画笔设置不同颜色，还有在绘制按钮指示器时根据不同状态，圆心 x 坐标一个在左，一个在右，y 坐标不变，在左边的时候就是半径加上两个画笔宽度，在右边的时候就是控件宽度减去半径，再减去两个画笔宽度而已。
修改 onDraw 方法：

@Overrideprotected void onDraw(Canvas canvas) {    super.onDraw(canvas);    mPaint.setStrokeWidth((float) getMeasuredWidth() / 40);    if (isChecked()) {        mPaint.setColor(0xFF6495ED);    } else {        mPaint.setColor(0xFFCCCCCC);    }    mRectF.set(mPaint.getStrokeWidth()            , mPaint.getStrokeWidth()            , getMeasuredWidth() - mPaint.getStrokeWidth()            , getMeasuredHeight() - mPaint.getStrokeWidth());    canvas.drawRoundRect(mRectF, getMeasuredHeight(), getMeasuredHeight(), mPaint);    mPaint.setColor(0xFFFFFFFF);    float radius = (getMeasuredHeight() - mPaint.getStrokeWidth() * 4) / 2;    float x;    float y;    if (isChecked()) {        x = getMeasuredWidth() - radius - mPaint.getStrokeWidth() - mPaint.getStrokeWidth();    } else {        x = mPaint.getStrokeWidth() + radius + mPaint.getStrokeWidth();    }    y = (float) getMeasuredHeight() / 2;    canvas.drawCircle(x, y, radius, mPaint);}

在构造方法中添加一个 OnClickListener，在控件被点击时重新绘制 UI：

setOnClickListener(new OnClickListener() {    @Override    public void onClick(View v) {        invalidate();    }});

现在效果如下：

如果不考虑动画的话，这个开关控件的基本效果已经有了。

5 动画

动画主要分两个部分，一个是按钮指示器位置（圆心）切换的动画，另一个就是背景颜色的过渡动画。这两个动画我都选择用属性动画来做。
我们可以计算按钮指示器在左右两边时的圆心的 x 坐标的差，然后通过属性动画让这个值慢慢变为 0，这个值可以看作是 x 坐标的偏移量，从左到右时 x 坐标减去这个偏移量，从右到左时 x 坐标加上这个偏移量。
背景颜色的过渡从度娘上找了一个方法：

private int getCurrentColor(float fraction, int startColor, int endColor) {    int redStart = Color.red(startColor);    int blueStart = Color.blue(startColor);    int greenStart = Color.green(startColor);    int alphaStart = Color.alpha(startColor);    int redEnd = Color.red(endColor);    int blueEnd = Color.blue(endColor);    int greenEnd = Color.green(endColor);    int alphaEnd = Color.alpha(endColor);    int redDifference = redEnd - redStart;    int blueDifference = blueEnd - blueStart;    int greenDifference = greenEnd - greenStart;    int alphaDifference = alphaEnd - alphaStart;    int redCurrent = (int) (redStart + fraction * redDifference);    int blueCurrent = (int) (blueStart + fraction * blueDifference);    int greenCurrent = (int) (greenStart + fraction * greenDifference);    int alphaCurrent = (int) (alphaStart + fraction * alphaDifference);    return Color.argb(alphaCurrent, redCurrent, greenCurrent, blueCurrent);}

这个方法可以获取一个过渡期中当前颜色，fraction 为过渡系数，取值范围 0f-1f,值越接近 1，颜色就越接近 endColor。我们仍然通过属性动画来修改过渡系数即可。
主要代码如下：

@Override    protected void onDraw(Canvas canvas) {        super.onDraw(canvas);        mPaint.setStrokeWidth((float) getMeasuredWidth() / 40);        if (isChecked()) {            mPaint.setColor(getCurrentColor(mColorGradientFactor, 0xFFCCCCCC, 0xFF6495ED));        } else {            mPaint.setColor(getCurrentColor(mColorGradientFactor, 0xFF6495ED, 0xFFCCCCCC));        }        mRectF.set(mPaint.getStrokeWidth()                , mPaint.getStrokeWidth()                , getMeasuredWidth() - mPaint.getStrokeWidth()                , getMeasuredHeight() - mPaint.getStrokeWidth());        canvas.drawRoundRect(mRectF, getMeasuredHeight(), getMeasuredHeight(), mPaint);        mPaint.setColor(0xFFFFFFFF);        float radius = (getMeasuredHeight() - mPaint.getStrokeWidth() * 4) / 2;        float x;        float y;        if (isChecked()) {//            x = getMeasuredWidth() - radius - mPaint.getStrokeWidth() - mPaint.getStrokeWidth();            x = getMeasuredWidth() - radius - mPaint.getStrokeWidth() - mPaint.getStrokeWidth() - mButtonCenterXOffset;        } else {//            x = radius + mPaint.getStrokeWidth() + mPaint.getStrokeWidth();            x = radius + mPaint.getStrokeWidth() + mPaint.getStrokeWidth() + mButtonCenterXOffset;        }        y = (float) getMeasuredHeight() / 2;        canvas.drawCircle(x, y, radius, mPaint);    }    private void startAnimate() {        // 计算开关指示器的半径        float radius = (getMeasuredHeight() - mPaint.getStrokeWidth() * 4) / 2;        // 计算开关指示器的 X 坐标的总偏移量        float centerXOffset = getMeasuredWidth() - mPaint.getStrokeWidth() - mPaint.getStrokeWidth() - radius                - (mPaint.getStrokeWidth() + mPaint.getStrokeWidth() + radius);        AnimatorSet animatorSet = new AnimatorSet();        // 偏移量逐渐变化到 0        ObjectAnimator objectAnimator = ObjectAnimator.ofFloat(this, "buttonCenterXOffset", centerXOffset, 0);        objectAnimator.setDuration(2000);        objectAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {            @Override            public void onAnimationUpdate(ValueAnimator animation) {                invalidate();            }        });        // 背景颜色过渡系数逐渐变化到 1        ObjectAnimator objectAnimator2 = ObjectAnimator.ofFloat(this, "colorGradientFactor", 0, 1);        objectAnimator2.setDuration(2000);        // 同时开始修改开关指示器 X 坐标偏移量的动画和修改背景颜色过渡系数的动画        animatorSet.play(objectAnimator).with(objectAnimator2);        animatorSet.start();    }    private int getCurrentColor(float fraction, int startColor, int endColor) {        int redStart = Color.red(startColor);        int blueStart = Color.blue(startColor);        int greenStart = Color.green(startColor);        int alphaStart = Color.alpha(startColor);        int redEnd = Color.red(endColor);        int blueEnd = Color.blue(endColor);        int greenEnd = Color.green(endColor);        int alphaEnd = Color.alpha(endColor);        int redDifference = redEnd - redStart;        int blueDifference = blueEnd - blueStart;        int greenDifference = greenEnd - greenStart;        int alphaDifference = alphaEnd - alphaStart;        int redCurrent = (int) (redStart + fraction * redDifference);        int blueCurrent = (int) (blueStart + fraction * blueDifference);        int greenCurrent = (int) (greenStart + fraction * greenDifference);        int alphaCurrent = (int) (alphaStart + fraction * alphaDifference);        return Color.argb(alphaCurrent, redCurrent, greenCurrent, blueCurrent);    }

点击事件要修改成调用 startAnimate 方法，现在效果如下：

为了演示，所以把动画时长设置为 2000ms，实际上 300ms 的效果挺好的，然后上面只是部分代码，大家主要看思路。完整代码在最后贴出，我们还可以考虑封装一些 api 供外部方便的使用，如设置按钮颜色，背景颜色，动画时长等。

6 总结

这个控件在我刚开始工作的时候就差不多看到了，当时为了效果好一点，还找了好多类似的来优中选优，现在终于轮到自己实现一个。其实这也是我后来一直的习惯，当看到一个好看的控件或者优秀的框架时，不光是会拿来用，还要分析别人的思路，然后看看自己能不能实现，这样出了问题，可能更容易改。慢慢的最后就会变成接到一个新需求时，不再是去网上找第三方库，而是自己去实现。

7 完整代码

package com.qinshou.switchbuttondemo;import android.animation.AnimatorSet;import android.animation.ObjectAnimator;import android.animation.ValueAnimator;import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.graphics.RectF;import android.util.AttributeSet;import android.view.View;/** * Author: QinHao * Email:cqflqinhao@126.com * Date: 2019/6/1 16:56 * Description:仿 iOS 风格的开关按钮 */public class SwitchButton extends android.support.v7.widget.AppCompatCheckBox {    private static final String TAG = "SwitchButton";    /**     * 控件默认宽度     */    private static final int DEFAULT_WIDTH = 200;    /**     * 控件默认高度     */    private static final int DEFAULT_HEIGHT = DEFAULT_WIDTH / 8 * 5;    /**     * 画笔     */    private Paint mPaint;    /**     * 控件背景的矩形范围     */    private RectF mRectF;    /**     * 开关指示器按钮圆心 X 坐标的偏移量     */    private float mButtonCenterXOffset;    /**     * 颜色渐变系数     */    private float mColorGradientFactor = 1;    /**     * 状态切换时的动画时长     */    private long mAnimateDuration = 300L;    /**     * 开关未选中状态,即关闭状态时的背景颜色     */    private int mBackgroundColorUnchecked = 0xFFCCCCCC;    /**     * 开关选中状态,即打开状态时的背景颜色     */    private int mBackgroundColorChecked = 0xFF6495ED;    /**     * 开关指示器按钮的颜色     */    private int mButtonColor = 0xFFFFFFFF;    public SwitchButton(Context context) {        this(context, null);    }    public SwitchButton(Context context, AttributeSet attrs) {        this(context, attrs, 0);    }    public SwitchButton(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);        // 不显示 CheckBox 默认的 Button        setButtonDrawable(null);        // 不显示 CheckBox 默认的背景        setBackgroundResource(0);        // 默认 CheckBox 为关闭状态        setChecked(false);        mPaint = new Paint();        mPaint.setAntiAlias(true);        mRectF = new RectF();        // 点击时开始动画        setOnClickListener(new OnClickListener() {            @Override            public void onClick(View v) {                startAnimate();            }        });    }    @Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        int widthMode = MeasureSpec.getMode(widthMeasureSpec);        int widthSize = MeasureSpec.getSize(widthMeasureSpec);        int heightMode = MeasureSpec.getMode(heightMeasureSpec);        int heightSize = MeasureSpec.getSize(heightMeasureSpec);        int width;        int height;        if (widthMode == MeasureSpec.EXACTLY) {            width = widthSize;        } else {            width = (getPaddingLeft() + DEFAULT_WIDTH + getPaddingRight());        }        if (heightMode == MeasureSpec.EXACTLY) {            height = heightSize;        } else {            height = (getPaddingTop() + DEFAULT_HEIGHT + getPaddingBottom());        }        setMeasuredDimension(width, height);    }    @Override    protected void onDraw(Canvas canvas) {        super.onDraw(canvas);        // 设置画笔宽度为控件宽度的 1/40,准备绘制控件背景        mPaint.setStrokeWidth((float) getMeasuredWidth() / 40);        // 根据是否选中的状态设置画笔颜色        if (isChecked()) {            // 选中状态时,背景颜色由未选中状态的背景颜色逐渐过渡到选中状态的背景颜色            mPaint.setColor(getCurrentColor(mColorGradientFactor, mBackgroundColorUnchecked, mBackgroundColorChecked));        } else {            // 未选中状态时,背景颜色由选中状态的背景颜色逐渐过渡到未选中状态的背景颜色            mPaint.setColor(getCurrentColor(mColorGradientFactor, mBackgroundColorChecked, mBackgroundColorUnchecked));        }        // 设置背景的矩形范围        mRectF.set(mPaint.getStrokeWidth()                , mPaint.getStrokeWidth()                , getMeasuredWidth() - mPaint.getStrokeWidth()                , getMeasuredHeight() - mPaint.getStrokeWidth());        // 绘制圆角矩形作为背景        canvas.drawRoundRect(mRectF, getMeasuredHeight(), getMeasuredHeight(), mPaint);        // 设置画笔颜色,准备绘制开关按钮指示器        mPaint.setColor(mButtonColor);        /*         * 获取开关按钮指示器的半径         * 为了好看一点,开关按钮指示器在背景矩形中显示一点内边距,所以多减去两个画笔宽度         */        float radius = (getMeasuredHeight() - mPaint.getStrokeWidth() * 4) / 2;        float x;        float y;        // 根据是否选中的状态来决定开关按钮指示器圆心的 X 坐标        if (isChecked()) {//            // 选中状态时开关按钮指示器在右边//            x = getMeasuredWidth() - radius - mPaint.getStrokeWidth() - mPaint.getStrokeWidth();            // 选中状态时开关按钮指示器圆心的 X 坐标从左边逐渐移到右边            x = getMeasuredWidth() - radius - mPaint.getStrokeWidth() - mPaint.getStrokeWidth() - mButtonCenterXOffset;        } else {//            // 未选中状态时开关按钮指示器在左边//            x = radius + mPaint.getStrokeWidth() + mPaint.getStrokeWidth();            // 未选中状态时开关按钮指示器圆心的 X 坐标从右边逐渐移到左边            x = radius + mPaint.getStrokeWidth() + mPaint.getStrokeWidth() + mButtonCenterXOffset;        }        // Y 坐标就是控件高度的一半不变        y = (float) getMeasuredHeight() / 2;        canvas.drawCircle(x, y, radius, mPaint);    }    /**     * Author: QinHao     * Email:qinhao@jeejio.com     * Date:2019/6/3 9:45     * Description:开始开关按钮切换状态和背景颜色过渡的动画     */    private void startAnimate() {        // 计算开关指示器的半径        float radius = (getMeasuredHeight() - mPaint.getStrokeWidth() * 4) / 2;        // 计算开关指示器的 X 坐标的总偏移量        float centerXOffset = getMeasuredWidth() - mPaint.getStrokeWidth() - mPaint.getStrokeWidth() - radius                - (mPaint.getStrokeWidth() + mPaint.getStrokeWidth() + radius);        AnimatorSet animatorSet = new AnimatorSet();        // 偏移量逐渐变化到 0        ObjectAnimator objectAnimator = ObjectAnimator.ofFloat(this, "buttonCenterXOffset", centerXOffset, 0);        objectAnimator.setDuration(mAnimateDuration);        objectAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {            @Override            public void onAnimationUpdate(ValueAnimator animation) {                invalidate();            }        });        // 背景颜色过渡系数逐渐变化到 1        ObjectAnimator objectAnimator2 = ObjectAnimator.ofFloat(this, "colorGradientFactor", 0, 1);        objectAnimator2.setDuration(mAnimateDuration);        // 同时开始修改开关指示器 X 坐标偏移量的动画和修改背景颜色过渡系数的动画        animatorSet.play(objectAnimator).with(objectAnimator2);        animatorSet.start();    }    /**     * Author: QinHao     * Email:qinhao@jeejio.com     * Date:2019/6/3 9:04     * Description:获取一个过渡期中当前颜色,fraction 为过渡系数,取值范围 0f-1f,值越接近 1,颜色就越接近 endColor     *     * @param fraction   当前渐变系数     * @param startColor 过渡开始颜色     * @param endColor   过渡结束颜色     * @return 当前颜色     */    private int getCurrentColor(float fraction, int startColor, int endColor) {        int redStart = Color.red(startColor);        int blueStart = Color.blue(startColor);        int greenStart = Color.green(startColor);        int alphaStart = Color.alpha(startColor);        int redEnd = Color.red(endColor);        int blueEnd = Color.blue(endColor);        int greenEnd = Color.green(endColor);        int alphaEnd = Color.alpha(endColor);        int redDifference = redEnd - redStart;        int blueDifference = blueEnd - blueStart;        int greenDifference = greenEnd - greenStart;        int alphaDifference = alphaEnd - alphaStart;        int redCurrent = (int) (redStart + fraction * redDifference);        int blueCurrent = (int) (blueStart + fraction * blueDifference);        int greenCurrent = (int) (greenStart + fraction * greenDifference);        int alphaCurrent = (int) (alphaStart + fraction * alphaDifference);        return Color.argb(alphaCurrent, redCurrent, greenCurrent, blueCurrent);    }    public void setButtonCenterXOffset(float buttonCenterXOffset) {        mButtonCenterXOffset = buttonCenterXOffset;    }    public void setColorGradientFactor(float colorGradientFactor) {        mColorGradientFactor = colorGradientFactor;    }    public void setAnimateDuration(long animateDuration) {        mAnimateDuration = animateDuration;    }    public void setBackgroundColorUnchecked(int backgroundColorUnchecked) {        mBackgroundColorUnchecked = backgroundColorUnchecked;    }    public void setBackgroundColorChecked(int backgroundColorChecked) {        mBackgroundColorChecked = backgroundColorChecked;    }    public void setButtonColor(int buttonColor) {        mButtonColor = buttonColor;    }}

Android(安卓)自定义控件之 SwitchButton(仿 iOS 开关)