背景是我们有需求，做类似ios中开关的按钮。github上有一些开源项目，比如 SwitchButton， 但是这个项目中提供了很多选项，并且实际使用中会出现一些奇怪的问题。

我调整了下代码，把无关的功能都给删了，保留核心的功能，大概这样。

package org.yeshen.widget;// 修改自：https://github.com/zcweng/SwitchButton
// 菜单中的类似iOS中开关的样式import static org.yeshen.widget.YsSwitchButton.ANIMATE.*;import android.animation.Animator;
import android.animation.ValueAnimator;
import android.annotation.SuppressLint;
import android.content.Context;
import android.content.res.Resources;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.RectF;
import android.os.Build;
import android.util.AttributeSet;
import android.util.TypedValue;
import android.view.MotionEvent;
import android.view.View;
import android.widget.Checkable;public final class YsSwitchButton extends View implements Checkable {private static final int DEFAULT_WIDTH = dp2pxInt(44);private static final int DEFAULT_HEIGHT = dp2pxInt(25);private static final int DEFAULT_BUTTON_PADDING = dp2pxInt(8);private final int uncheckColor = 0xFFFF0000;private final int checkedColor = 0xFF0000FF;private final int uncheckButtonColor = Color.WHITE;private float viewRadius;private float left;private float top;private float right;private float bottom;private float centerY;private float buttonMinX;private float buttonMaxX;private final Paint buttonPaint;private final Paint paint;private final ViewState viewState;private final ViewState beforeState;private final ViewState afterState;private final RectF rect = new RectF();private ANIMATE animateState = ANIMATE_STATE_NONE;private final ValueAnimator valueAnimator;private final android.animation.ArgbEvaluator argbEvaluator = new android.animation.ArgbEvaluator();private boolean isChecked = false;private boolean isTouchingDown = false;private boolean isUiInit = false;private boolean isEventBroadcast = false;private OnCheckedChangeListener onCheckedChangeListener;private long touchDownTime;private boolean switchByUser;public YsSwitchButton(Context context) {this(context, null);}public YsSwitchButton(Context context, AttributeSet attrs) {this(context, attrs, 0);}public YsSwitchButton(Context context, AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr);paint = new Paint(Paint.ANTI_ALIAS_FLAG);buttonPaint = new Paint(Paint.ANTI_ALIAS_FLAG);buttonPaint.setColor(uncheckButtonColor);viewState = new ViewState();beforeState = new ViewState();afterState = new ViewState();valueAnimator = ValueAnimator.ofFloat(0f, 1f);valueAnimator.setDuration(300);valueAnimator.setRepeatCount(0);ValueAnimator.AnimatorUpdateListener animatorUpdateListener = animation -> {float value = (Float) animation.getAnimatedValue();switch (animateState) {case ANIMATE_STATE_PENDING_SETTLE: {}case ANIMATE_STATE_PENDING_RESET: {}case ANIMATE_STATE_PENDING_DRAG: {if (animateState != ANIMATE_STATE_PENDING_DRAG) {viewState.buttonX = beforeState.buttonX + (afterState.buttonX - beforeState.buttonX) * value;}viewState.checkStateColor = (int) argbEvaluator.evaluate(value, beforeState.checkStateColor, afterState.checkStateColor);break;}case ANIMATE_STATE_SWITCH: {viewState.buttonX = beforeState.buttonX + (afterState.buttonX - beforeState.buttonX) * value;float fraction = (viewState.buttonX - buttonMinX) / (buttonMaxX - buttonMinX);viewState.checkStateColor = (int) argbEvaluator.evaluate(fraction, uncheckColor, checkedColor);break;}default:case ANIMATE_STATE_DRAGING: {}case ANIMATE_STATE_NONE: {break;}}postInvalidate();};valueAnimator.addUpdateListener(animatorUpdateListener);Animator.AnimatorListener animatorListener = new Animator.AnimatorListener() {@Overridepublic void onAnimationStart(Animator animation) {}@Overridepublic void onAnimationEnd(Animator animation) {switch (animateState) {case ANIMATE_STATE_PENDING_DRAG: {animateState = ANIMATE_STATE_DRAGING;postInvalidate();break;}case ANIMATE_STATE_PENDING_RESET: {animateState = ANIMATE_STATE_NONE;postInvalidate();break;}case ANIMATE_STATE_PENDING_SETTLE: {animateState = ANIMATE_STATE_NONE;postInvalidate();broadcastEvent(true);break;}case ANIMATE_STATE_SWITCH: {isChecked = !isChecked;animateState = ANIMATE_STATE_NONE;postInvalidate();broadcastEvent(switchByUser);break;}case ANIMATE_STATE_DRAGING:case ANIMATE_STATE_NONE:default: {break;}}}@Overridepublic void onAnimationCancel(Animator animation) {}@Overridepublic void onAnimationRepeat(Animator animation) {}};valueAnimator.addListener(animatorListener);super.setClickable(true);this.setPadding(0, 0, 0, 0);setLayerType(LAYER_TYPE_SOFTWARE, null);}@Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {final int widthMode = MeasureSpec.getMode(widthMeasureSpec);final int heightMode = MeasureSpec.getMode(heightMeasureSpec);if (widthMode == MeasureSpec.UNSPECIFIED || widthMode == MeasureSpec.AT_MOST) {widthMeasureSpec = MeasureSpec.makeMeasureSpec(DEFAULT_WIDTH, MeasureSpec.EXACTLY);}if (heightMode == MeasureSpec.UNSPECIFIED || heightMode == MeasureSpec.AT_MOST) {heightMeasureSpec = MeasureSpec.makeMeasureSpec(DEFAULT_HEIGHT, MeasureSpec.EXACTLY);}super.onMeasure(widthMeasureSpec, heightMeasureSpec);}@Overrideprotected void onSizeChanged(int w, int h, int oldw, int oldh) {super.onSizeChanged(w, h, oldw, oldh);float viewPadding = 0;float height = h - viewPadding - viewPadding;viewRadius = height * .5f;left = viewPadding;top = viewPadding;right = w - viewPadding;bottom = h - viewPadding;centerY = (top + bottom) * .5f;buttonMinX = left + viewRadius;buttonMaxX = right - viewRadius;if (isChecked()) {setCheckedViewState(viewState);} else {setUncheckViewState(viewState);}isUiInit = true;postInvalidate();}private void setUncheckViewState(ViewState viewState) {viewState.checkStateColor = uncheckColor;viewState.buttonX = buttonMinX;buttonPaint.setColor(uncheckButtonColor);}private void setCheckedViewState(ViewState viewState) {viewState.checkStateColor = checkedColor;viewState.buttonX = buttonMaxX;int checkedButtonColor = Color.WHITE;buttonPaint.setColor(checkedButtonColor);}@Overrideprotected void onDraw(Canvas canvas) {super.onDraw(canvas);// background colorpaint.setColor(uncheckColor);drawRoundRect(canvas, left, top, right, bottom, viewRadius, paint);// select colorpaint.setColor(viewState.checkStateColor);drawRoundRect(canvas, left, top, right, bottom, viewRadius, paint);// buttoncanvas.drawCircle(viewState.buttonX, centerY,viewRadius - DEFAULT_BUTTON_PADDING / 2F, buttonPaint);}@SuppressLint("ObsoleteSdkInt")private void drawRoundRect(Canvas canvas, float left, float top, float right,float bottom, float backgroundRadius, Paint paint) {if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {canvas.drawRoundRect(left, top, right, bottom, backgroundRadius, backgroundRadius, paint);} else {rect.set(left, top, right, bottom);canvas.drawRoundRect(rect, backgroundRadius, backgroundRadius, paint);}}@Overridepublic void setChecked(boolean checked) {if (checked == isChecked()) {postInvalidate();return;}toggle(true, false);}@Overridepublic boolean isChecked() {return isChecked;}@Overridepublic void toggle() {toggle(true);}public void toggle(boolean animate) {toggle(animate, true);}private void toggle(boolean animate, boolean broadcast) {toggle(animate, broadcast, false);}private void toggle(boolean animate, boolean broadcast, boolean byUser) {if (!isEnabled()) {return;}if (isEventBroadcast) {throw new RuntimeException("should NOT switch the state in method: [onCheckedChanged]!");}if (!isUiInit) {isChecked = !isChecked;if (broadcast) {broadcastEvent(byUser);}return;}if (valueAnimator.isRunning()) {valueAnimator.cancel();}if (!animate) {isChecked = !isChecked;if (isChecked()) {setCheckedViewState(viewState);} else {setUncheckViewState(viewState);}postInvalidate();if (broadcast) {broadcastEvent(byUser);}return;}animateState = ANIMATE_STATE_SWITCH;switchByUser = byUser;beforeState.copy(viewState);if (isChecked()) {setUncheckViewState(afterState);} else {setCheckedViewState(afterState);}valueAnimator.start();}private void broadcastEvent(boolean byUser) {if (onCheckedChangeListener != null) {isEventBroadcast = true;onCheckedChangeListener.onCheckedChanged(this, isChecked(), byUser);}isEventBroadcast = false;}@SuppressLint("ClickableViewAccessibility")@Overridepublic boolean onTouchEvent(MotionEvent event) {if (!isEnabled()) {return false;}int actionMasked = event.getActionMasked();switch (actionMasked) {case MotionEvent.ACTION_DOWN: {isTouchingDown = true;touchDownTime = System.currentTimeMillis();removeCallbacks(postPendingDrag);postDelayed(postPendingDrag, 100);break;}case MotionEvent.ACTION_MOVE: {float eventX = event.getX();if (isPendingDragState()) {float fraction = eventX / getWidth();fraction = Math.max(0f, Math.min(1f, fraction));viewState.buttonX = buttonMinX + (buttonMaxX - buttonMinX) * fraction;} else if (isDragState()) {float fraction = eventX / getWidth();fraction = Math.max(0f, Math.min(1f, fraction));viewState.buttonX = buttonMinX + (buttonMaxX - buttonMinX) * fraction;viewState.checkStateColor = (int) argbEvaluator.evaluate(fraction, uncheckColor, checkedColor);postInvalidate();}break;}case MotionEvent.ACTION_UP: {isTouchingDown = false;removeCallbacks(postPendingDrag);if (System.currentTimeMillis() - touchDownTime <= 300) {toggle(true, true, true);} else if (isDragState()) {float eventX = event.getX();float fraction = eventX / getWidth();fraction = Math.max(0f, Math.min(1f, fraction));boolean newCheck = fraction > .5f;if (newCheck == isChecked()) {pendingCancelDragState();} else {isChecked = newCheck;pendingSettleState();}} else if (isPendingDragState()) {pendingCancelDragState();}break;}case MotionEvent.ACTION_CANCEL: {isTouchingDown = false;removeCallbacks(postPendingDrag);if (isPendingDragState() || isDragState()) {pendingCancelDragState();}break;}}return true;}private final Runnable postPendingDrag = () -> {if (!isInAnimating()) {pendingDragState();}};private boolean isInAnimating() {return animateState != ANIMATE_STATE_NONE;}private boolean isPendingDragState() {return animateState == ANIMATE_STATE_PENDING_DRAG || animateState == ANIMATE_STATE_PENDING_RESET;}private boolean isDragState() {return animateState == ANIMATE_STATE_DRAGING;}private void pendingDragState() {if (isInAnimating()) {return;}if (!isTouchingDown) {return;}if (valueAnimator.isRunning()) {valueAnimator.cancel();}animateState = ANIMATE_STATE_PENDING_DRAG;beforeState.copy(viewState);afterState.copy(viewState);if (isChecked()) {afterState.checkStateColor = checkedColor;afterState.buttonX = buttonMaxX;} else {afterState.checkStateColor = uncheckColor;afterState.buttonX = buttonMinX;}valueAnimator.start();}private void pendingCancelDragState() {if (isDragState() || isPendingDragState()) {if (valueAnimator.isRunning()) {valueAnimator.cancel();}animateState = ANIMATE_STATE_PENDING_RESET;beforeState.copy(viewState);if (isChecked()) {setCheckedViewState(afterState);} else {setUncheckViewState(afterState);}valueAnimator.start();}}private void pendingSettleState() {if (valueAnimator.isRunning()) {valueAnimator.cancel();}animateState = ANIMATE_STATE_PENDING_SETTLE;beforeState.copy(viewState);if (isChecked()) {setCheckedViewState(afterState);} else {setUncheckViewState(afterState);}valueAnimator.start();}@SuppressWarnings("unused")public void setOnCheckedChangeListener(OnCheckedChangeListener l) {onCheckedChangeListener = l;}public interface OnCheckedChangeListener {void onCheckedChanged(YsSwitchButton view, boolean isChecked, boolean byUser);}private static float dp2px(float dp) {Resources r = Resources.getSystem();return TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, r.getDisplayMetrics());}private static int dp2pxInt(float dp) {return (int) dp2px(dp);}private static class ViewState {float buttonX;int checkStateColor;private void copy(ViewState source) {this.buttonX = source.buttonX;this.checkStateColor = source.checkStateColor;}}enum ANIMATE {ANIMATE_STATE_NONE, ANIMATE_STATE_PENDING_DRAG, ANIMATE_STATE_DRAGING, ANIMATE_STATE_PENDING_RESET, ANIMATE_STATE_PENDING_SETTLE, ANIMATE_STATE_SWITCH;}
}