直接子类只有有一个，CompoundButton。它是一个抽象类，而实现这个类的控件正是CheckBox, RadioButton, Switch, ToggleButton这四个，所以先重点说一下它。源码如下：

  

/**
 * <p>
 * A button with two states, checked and unchecked. When the button is pressed
 * or clicked, the state changes automatically.
 * </p>
 *
 * <p><strong>XML attributes </strong></p>
 * <p>
 * See {@link android.R.styleable#CompoundButton
 * CompoundButton Attributes}, {@link android.R.styleable#Button Button
 * Attributes}, {@link android.R.styleable#TextView TextView Attributes}, {@link
 * android.R.styleable #View View Attributes}
 * </p>
 */
public abstract class CompoundButton extends Button implements Checkable {
    private boolean mChecked ;
    private int mButtonResource ;
    private boolean mBroadcasting ;
    private Drawable mButtonDrawable;
    private OnCheckedChangeListener mOnCheckedChangeListener;
    private OnCheckedChangeListener mOnCheckedChangeWidgetListener ;

    private static final int[] CHECKED_STATE_SET = {
        R.attr.state_checked
    };

    public CompoundButton(Context context) {
        this(context, null);
    }

    public CompoundButton(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public CompoundButton(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);

        TypedArray a =
                context.obtainStyledAttributes(
                        attrs, com.android.internal.R.styleable.CompoundButton, defStyle, 0);

        Drawable d = a.getDrawable(com.android.internal.R.styleable.CompoundButton_button);
        if (d != null ) {
            setButtonDrawable(d);
        }

        boolean checked = a
                .getBoolean(com.android.internal.R.styleable.CompoundButton_checked, false);
        setChecked(checked);

        a.recycle();
    }

    public void toggle() {
        setChecked(! mChecked);
    }

    @Override
    public boolean performClick() {
        /*
         * XXX: These are tiny, need some surrounding ‘expanded touch area‘,
         * which will need to be implemented in Button if we only override
         * performClick()
         */

        /* When clicked, toggle the state */
        toggle();
        return super .performClick();
    }

    @ViewDebug.ExportedProperty
    public boolean isChecked() {
        return mChecked ;
    }

    /**
     * <p>Changes the checked state of this button.</p>
     *
     * @param checked true to check the button, false to uncheck it
     */
    public void setChecked(boolean checked) {
        if (mChecked != checked) {
            mChecked = checked;
            refreshDrawableState();
            notifyViewAccessibilityStateChangedIfNeeded(
                    AccessibilityEvent.CONTENT_CHANGE_TYPE_UNDEFINED );

            // Avoid infinite recursions if setChecked() is called from a listener
            if (mBroadcasting ) {
                return;
            }

            mBroadcasting = true ;
            if (mOnCheckedChangeListener != null) {
                mOnCheckedChangeListener.onCheckedChanged(this, mChecked);
            }
            if (mOnCheckedChangeWidgetListener != null) {
                mOnCheckedChangeWidgetListener .onCheckedChanged(this, mChecked);
            }

            mBroadcasting = false ;           
        }
    }

    /**
     * Register a callback to be invoked when the checked state of this button
     * changes.
     *
     * @param listener the callback to call on checked state change
     */
    public void setOnCheckedChangeListener(OnCheckedChangeListener listener) {
        mOnCheckedChangeListener = listener;
    }

    /**
     * Register a callback to be invoked when the checked state of this button
     * changes. This callback is used for internal purpose only.
     *
     * @param listener the callback to call on checked state change
     * @hide
     */
    void setOnCheckedChangeWidgetListener(OnCheckedChangeListener listener) {
        mOnCheckedChangeWidgetListener = listener;
    }

    /**
     * Interface definition for a callback to be invoked when the checked state
     * of a compound button changed.
     */
    public static interface OnCheckedChangeListener {
        /**
         * Called when the checked state of a compound button has changed.
         *
         * @param buttonView The compound button view whose state has changed.
         * @param isChecked  The new checked state of buttonView.
         */
        void onCheckedChanged(CompoundButton buttonView, boolean isChecked);
    }

    /**
     * Set the background to a given Drawable, identified by its resource id.
     *
     * @param resid the resource id of the drawable to use as the background
     */
    public void setButtonDrawable(int resid) {
        if (resid != 0 && resid == mButtonResource ) {
            return;
        }

        mButtonResource = resid;

        Drawable d = null;
        if (mButtonResource != 0) {
            d = getResources().getDrawable(mButtonResource );
        }
        setButtonDrawable(d);
    }

    /**
     * Set the background to a given Drawable
     *
     * @param d The Drawable to use as the background
     */
    public void setButtonDrawable(Drawable d) {
        if (d != null ) {
            if (mButtonDrawable != null) {
                mButtonDrawable.setCallback(null);
                unscheduleDrawable( mButtonDrawable);
            }
            d.setCallback( this);
            d.setVisible(getVisibility() == VISIBLE, false);
            mButtonDrawable = d;
            setMinHeight(mButtonDrawable .getIntrinsicHeight());
        }

        refreshDrawableState();
    }

    @Override
    public void onInitializeAccessibilityEvent(AccessibilityEvent event) {
        super.onInitializeAccessibilityEvent(event);
        event.setClassName(CompoundButton.class .getName());
        event.setChecked( mChecked);
    }

    @Override
    public void onInitializeAccessibilityNodeInfo(AccessibilityNodeInfo info) {
        super.onInitializeAccessibilityNodeInfo(info);
        info.setClassName(CompoundButton.class .getName());
        info.setCheckable( true);
        info.setChecked( mChecked);
    }

    @Override
    public int getCompoundPaddingLeft() {
        int padding = super.getCompoundPaddingLeft();
        if (!isLayoutRtl()) {
            final Drawable buttonDrawable = mButtonDrawable;
            if (buttonDrawable != null) {
                padding += buttonDrawable.getIntrinsicWidth();
            }
        }
        return padding;
    }

    @Override
    public int getCompoundPaddingRight() {
        int padding = super.getCompoundPaddingRight();
        if (isLayoutRtl()) {
            final Drawable buttonDrawable = mButtonDrawable;
            if (buttonDrawable != null) {
                padding += buttonDrawable.getIntrinsicWidth();
            }
        }
        return padding;
    }

    /**
     * @hide
     */
    @Override
    public int getHorizontalOffsetForDrawables() {
        final Drawable buttonDrawable = mButtonDrawable ;
        return (buttonDrawable != null) ? buttonDrawable.getIntrinsicWidth() : 0;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        final Drawable buttonDrawable = mButtonDrawable ;
        if (buttonDrawable != null) {
            final int verticalGravity = getGravity() & Gravity.VERTICAL_GRAVITY_MASK ;
            final int drawableHeight = buttonDrawable.getIntrinsicHeight();
            final int drawableWidth = buttonDrawable.getIntrinsicWidth();

            int top = 0;
            switch (verticalGravity) {
                case Gravity.BOTTOM :
                    top = getHeight() - drawableHeight;
                    break;
                case Gravity.CENTER_VERTICAL :
                    top = (getHeight() - drawableHeight) / 2;
                    break;
            }
            int bottom = top + drawableHeight;
            int left = isLayoutRtl() ? getWidth() - drawableWidth : 0;
            int right = isLayoutRtl() ? getWidth() : drawableWidth;

            buttonDrawable.setBounds(left, top, right, bottom);
            buttonDrawable.draw(canvas);
        }
    }

    @Override
    protected int[] onCreateDrawableState(int extraSpace) {
        final int [] drawableState = super.onCreateDrawableState(extraSpace + 1);
        if (isChecked()) {
            mergeDrawableStates(drawableState, CHECKED_STATE_SET);
        }
        return drawableState;
    }

    @Override
    protected void drawableStateChanged() {
        super.drawableStateChanged();
       
        if (mButtonDrawable != null) {
            int[] myDrawableState = getDrawableState();
           
            // Set the state of the Drawable
            mButtonDrawable.setState(myDrawableState);
           
            invalidate();
        }
    }

    @Override
    protected boolean verifyDrawable(Drawable who) {
        return super .verifyDrawable(who) || who == mButtonDrawable;
    }

    @Override
    public void jumpDrawablesToCurrentState() {
        super.jumpDrawablesToCurrentState();
        if (mButtonDrawable != null) mButtonDrawable.jumpToCurrentState();
    }

    static class SavedState extends BaseSavedState {
        boolean checked ;

        /**
         * Constructor called from {@link CompoundButton#onSaveInstanceState()}
         */
        SavedState(Parcelable superState) {
            super(superState);
        }
       
        /**
         * Constructor called from {@link #CREATOR}
         */
        private SavedState(Parcel in) {
            super(in);
            checked = (Boolean)in.readValue( null);
        }

        @Override
        public void writeToParcel(Parcel out, int flags) {
            super.writeToParcel(out, flags);
            out.writeValue( checked);
        }

        @Override
        public String toString() {
            return "CompoundButton.SavedState{"
                    + Integer.toHexString(System.identityHashCode(this))
                    + " checked=" + checked + "}" ;
        }

        public static final Parcelable.Creator<SavedState> CREATOR
                = new Parcelable.Creator<SavedState>() {
            public SavedState createFromParcel(Parcel in) {
                return new SavedState(in);
            }

            public SavedState[] newArray(int size) {
                return new SavedState[size];
            }
        };
    }

    @Override
    public Parcelable onSaveInstanceState() {
        // Force our ancestor class to save its state
        setFreezesText( true);
        Parcelable superState = super.onSaveInstanceState();

        SavedState ss = new SavedState(superState);

        ss. checked = isChecked();
        return ss;
    }

    @Override
    public void onRestoreInstanceState(Parcelable state) {
        SavedState ss = (SavedState) state;
 
        super.onRestoreInstanceState(ss.getSuperState());
        setChecked(ss. checked);
        requestLayout();
    }
}

  先从构造方法开始，在构造方法中，

 public CompoundButton(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);

        TypedArray a =
                context.obtainStyledAttributes(
                        attrs, com.android.internal.R.styleable.CompoundButton, defStyle, 0);

        Drawable d = a.getDrawable(com.android.internal.R.styleable.CompoundButton_button);
        if (d != null ) {
            setButtonDrawable(d);
        }

        boolean checked = a
                .getBoolean(com.android.internal.R.styleable.CompoundButton_checked, false);
        setChecked(checked);

        a.recycle();
    }
  

  先是从attrs中读取定义的属性，一个是Drawable用于设置背景；一个是布尔类型变量用于判断是否check过。设置背景使用的是setButtonDrawable()方法，代码如下：

/**
     * Set the background to a given Drawable
     *
     * @param d The Drawable to use as the background
     */
    public void setButtonDrawable(Drawable d) {
        if (d != null ) {
            if (mButtonDrawable != null) {
                mButtonDrawable.setCallback(null);
                unscheduleDrawable( mButtonDrawable);
            }
            d.setCallback( this);
            d.setVisible(getVisibility() == VISIBLE, false);
            mButtonDrawable = d;
            setMinHeight(mButtonDrawable .getIntrinsicHeight());
        }

        refreshDrawableState();
    }

  这个方法写的就比较完善，可以作为一个学习的典范。首先判断传递过来的Drawable是否为空，如果不为空并且默认的Drawable也不为空，那么取消默认Drawable的callback，然后调用unscheduleDrawable方法。这个方法代码如下：

 /**
     * Unschedule any events associated with the given Drawable.  This can be
     * used when selecting a new Drawable into a view, so that the previous
     * one is completely unscheduled.
     *
     * @param who The Drawable to unschedule.
     *
     * @see #drawableStateChanged
     */
    public void unscheduleDrawable(Drawable who) {
        if (mAttachInfo != null && who != null) {
            mAttachInfo.mViewRootImpl .mChoreographer.removeCallbacks(
                    Choreographer.CALLBACK_ANIMATION, null, who);
        }
    }

  从方法注释中可以看出它的用途，正是更换Drawable时候使用的。接下来开始重新设置Drawable，包括回调、可见性、最小高度。最后调用refreshDrawableState()方法，这个是View类的方法，用于更新Drawable状态。

/**
     * <p>Changes the checked state of this button.</p>
     *
     * @param checked true to check the button, false to uncheck it
     */
    public void setChecked( boolean checked) {
        if (mChecked != checked) {
            mChecked = checked;
            refreshDrawableState();
            notifyViewAccessibilityStateChangedIfNeeded(
                    AccessibilityEvent.CONTENT_CHANGE_TYPE_UNDEFINED );

            // Avoid infinite recursions if setChecked() is called from a listener
            if (mBroadcasting ) {
                return;
            }

            mBroadcasting = true ;
            if (mOnCheckedChangeListener != null) {
                mOnCheckedChangeListener.onCheckedChanged(this, mChecked);
            }
            if (mOnCheckedChangeWidgetListener != null) {
                mOnCheckedChangeWidgetListener .onCheckedChanged(this, mChecked);
            }
            mBroadcasting = false ;           
        }
    }

/**
     * Interface definition for a callback to be invoked when the checked state
     * of a compound button changed.
     */
    public static interface OnCheckedChangeListener {
        /**
         * Called when the checked state of a compound button has changed.
         *
         * @param buttonView The compound button view whose state has changed.
         * @param isChecked  The new checked state of buttonView.
         */
        void onCheckedChanged(CompoundButton buttonView, boolean isChecked);
    }

  @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        final Drawable buttonDrawable = mButtonDrawable ;
        if (buttonDrawable != null) {
            final int verticalGravity = getGravity() & Gravity.VERTICAL_GRAVITY_MASK ;
            final int drawableHeight = buttonDrawable.getIntrinsicHeight();
            final int drawableWidth = buttonDrawable.getIntrinsicWidth();

            int top = 0;
            switch (verticalGravity) {
                case Gravity.BOTTOM :
                    top = getHeight() - drawableHeight;
                    break;
                case Gravity.CENTER_VERTICAL :
                    top = (getHeight() - drawableHeight) / 2;
                    break;
            }
            int bottom = top + drawableHeight;
            int left = isLayoutRtl() ? getWidth() - drawableWidth : 0;
            int right = isLayoutRtl() ? getWidth() : drawableWidth;

            buttonDrawable.setBounds(left, top, right, bottom);
            buttonDrawable.draw(canvas);
        }
    }

 public class CheckBox extends CompoundButton {
    public CheckBox(Context context) {
        this(context, null);
    }
   
    public CheckBox(Context context, AttributeSet attrs) {
        this(context, attrs, com.android.internal.R.attr.checkboxStyle);
    }

    public CheckBox(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    public void onInitializeAccessibilityEvent(AccessibilityEvent event) {
        super.onInitializeAccessibilityEvent(event);
        event.setClassName(CheckBox. class.getName());
    }

    @Override
    public void onInitializeAccessibilityNodeInfo(AccessibilityNodeInfo info) {
        super.onInitializeAccessibilityNodeInfo(info);
        info.setClassName(CheckBox. class.getName());
    }
}

public class RadioButton extends CompoundButton {
   
    public RadioButton(Context context) {
        this(context, null);
    }
   
    public RadioButton(Context context, AttributeSet attrs) {
        this(context, attrs, com.android.internal.R.attr.radioButtonStyle);
    }

    public RadioButton(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    /**
     * {@inheritDoc}
     * <p>
     * If the radio button is already checked, this method will not toggle the radio button.
     */
    @Override
    public void toggle() {
        // we override to prevent toggle when the radio is already
        // checked (as opposed to check boxes widgets)
        if (!isChecked()) {
            super.toggle();
        }
    }

    @Override
    public void onInitializeAccessibilityEvent(AccessibilityEvent event) {
        super.onInitializeAccessibilityEvent(event);
        event.setClassName(RadioButton. class.getName());
    }

    @Override
    public void onInitializeAccessibilityNodeInfo(AccessibilityNodeInfo info) {
        super.onInitializeAccessibilityNodeInfo(info);
        info.setClassName(RadioButton. class.getName());
    }
}

  和CheckBox实现差不多，区别在于多重写了一个方法，用于防止按钮被重复点击。另外还有ToggleButton以及Switch，前者实现也比较简单，后者稍微麻烦了一些，感兴趣可以自己分析。