自定义View——切换按钮

自定义View的步骤

今天来介绍下如何自定义类似iOS的按钮。

要想自定义View，需要了解自定义View的相关步骤，一般来说自定义View需要实现以下操作：

自定义切换按钮

1.自定义属性

根据自定义View中需要用户设置的属性，来构造自定义属性，直接在xml资源文件中的resources标签下声明：

<declare-styleable name="Switch">
    <attr name="checked" format="reference|boolean"/>
    <attr name="switchOnColor" format="reference|color"/>
    <attr name="switchOffColor" format="reference|color"/>
    <attr name="spotOnColor" format="reference|color"/>
    <attr name="spotOffColor" format="reference|color"/>
    <attr name="spotPadding" format="reference|dimension"/>
    <attr name="duration" format="reference|integer" />
</declare-styleable>

并在继承View类的Switch类中声明全局变量：

private int switchOnColor;
private int switchOffColor;
private int spotOnColor;
private int spotOffColor;
private int spotPadding;
private boolean mChecked;
private int duration;

2.初始化构造函数

一般来说，如果自定义View用于在布局文件声明，则必须初始化覆盖基类的两个参数的构造函数；其他带参构造函数，根据需求进行初始化。

在构造函数中通过obtainStyledAttributes()方法获取用户设置的属性值：

public Switch(Context context, AttributeSet attrs) {
        super(context, attrs);
        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.Switch);
        switchOnColor = a.getColor(R.styleable.Switch_switchOnColor, DEFAULT_SWITCH_ON_COLOR);
        switchOffColor = a.getColor(R.styleable.Switch_switchOffColor, DEFAULT_SWITCH_OFF_COLOR);
        spotOnColor = a.getColor(R.styleable.Switch_spotOnColor, DEFAULT_SPOT_ON_COLOR);
        spotOffColor = a.getColor(R.styleable.Switch_spotOffColor, DEFAULT_SPOT_OFF_COLOR);
        spotPadding = a.getDimensionPixelSize(R.styleable.Switch_spotPadding, dp2px(DEFAULT_SPOT_PADDING));
        duration = a.getInteger(R.styleable.Switch_duration, ANIMATION_DURATION);
        mChecked = a.getBoolean(R.styleable.Switch_checked, false);
        a.recycle();

        state = mChecked ? State.SWITCH_ON : State.SWITCH_OFF;
        setClickable(true);
}

3.测量View——onMeasure

onMeasure方法是决定自定义View布局大小的关键，可通过MeasureSpec获取父布局分配给子布局的大小及布局模式。

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
    int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
    int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
    int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);

    int width = dp2px(DEFAULT_WIDTH) + getPaddingLeft() + getPaddingRight();
    int height = dp2px(DEFAULT_HEIGHT) + getPaddingTop() + getPaddingBottom();

    if (widthSpecMode != MeasureSpec.AT_MOST) {
        width = Math.max(width, widthSpecSize);
    }

    if (heightSpecMode != MeasureSpec.AT_MOST) {
        height = Math.max(height, heightSpecSize);
    }

    setMeasuredDimension(width, height);
}

4.给定View高宽——setMeasuredDimension

通过setMeasuredDimension方法设置自定义布局想要的View大小。代码如上所示。

5.绘制View——onDraw

最核心的方法莫过于onDraw了，onDraw是绘制View的关键，结合ValueAnimator、ObjectAnimator等可实现View的动画效果。

private void animateToCheckedState() {
    valueAnimator = ValueAnimator.ofFloat(0, 1);
    valueAnimator.setDuration(duration);
    valueAnimator.setInterpolator(new AccelerateDecelerateInterpolator());
    valueAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
        @Override
        public void onAnimationUpdate(ValueAnimator animation) {
            currentPos = (float) animation.getAnimatedValue();
            invalidate();
        }
    });

    valueAnimator.addListener(new AnimatorListenerAdapter() {
        @Override
        public void onAnimationStart(Animator animation) {
            super.onAnimationStart(animation);
            isMoving = true;
        }

        @Override
        public void onAnimationEnd(Animator animation) {
            super.onAnimationEnd(animation);
            isMoving = false;
        }
    });

    if (!valueAnimator.isRunning()) {
        valueAnimator.start();
        currentPos = 0;
    }
}

6.回调监听

切换按钮的回调，主要体现在开关状态的变化：

public interface OnCheckedChangeListener {
    /**
     * Called when the checked state of a switch has changed.
     *
     * @param s The switch whose state has changed.
     * @param isChecked The new checked state of switch.
     */
    void onCheckedChanged(Switch s, boolean isChecked);
}

7.手势处理

本项目中并未使用onTouchEvent方法处理手势监听，而是覆盖了父类的performClick方法，同时保证了OnClickListener的生效。

@Override
public boolean performClick() {
    toggle();

    final boolean handled = super.performClick();
    if (!handled) {
        // View only makes a sound effect if the onClickListener was
        // called, so we'll need to make one here instead.
        playSoundEffect(SoundEffectConstants.CLICK);
    }

    return handled;
}

作者 @fynn

项目地址 github