下拉刷新动画研究

昨天公司提了一个需求。说是要有自己特色的下拉刷新上拉载入很多其它，随后就去万能的github上面看看自己之前存过的那些。偶然间看到这个Android-AnimatePullToRefreshListView，这玩意能够使用gif图片或者帧动画去运行headview动画载入，而且是基于chrisbanes的Android-PullToRefresh，这样还方便与我项目无缝衔接

上一张效果图

大体上简单说明下：
1.gif与帧动画播放採用的是android-gif-drawable
2.主题上是改动了RotateLoadingLayout类。看代码

//帧动画三张图
private int[] mGifRes = {
            R.drawable.dropdown_loading_00,
            R.drawable.dropdown_loading_01,
            R.drawable.dropdown_loading_02,
    };

//通过下拉的scale比例，去计算应该显示哪张图片(下拉距离/headview高度)
protected void onPullImpl(float scaleOfLayout) {
    int index = (int) (scaleOfLayout / 1f * 10);
    if (index == mPrevIndex) {
        return;
    } else {
        if (index > 10) {
            index = 10;
        }
        int res = getResources().getIdentifier(String.format("dropdown_anim_%02d",  index), "drawable", getContext().getPackageName());
        mHeaderImage.setImageResource(res);
        mPrevIndex = index;
    }
}

//正在刷新，就播放一下动画
@Override
protected void refreshingImpl() {
       if (mGifAnimation == null) {
           mGifAnimation = new GifAnimation(mHeaderImage, mGifRes);
       }
       mGifAnimation.start();
   }

//刷新结束，就停止动画播放
@Override
protected void resetImpl() {
       mHeaderImage.clearAnimation();
       if (mGifAnimation != null) {
           mGifAnimation.stop();
       }
}

在PullToRefreshBase中能够看到

/**
     * Actions a Pull Event
     *
     * @return true if the Event has been handled, false if there has been no
     * change
     */
    private void pullEvent() {
        final int newScrollValue;
        final int itemDimension;
        final float initialMotionValue, lastMotionValue;

        switch (getPullToRefreshScrollDirection()) {
            case HORIZONTAL:
                initialMotionValue = mInitialMotionX;
                lastMotionValue = mLastMotionX;
                break;
            case VERTICAL:
            default:
                initialMotionValue = mInitialMotionY;
                lastMotionValue = mLastMotionY;
                break;
        }
        //得到对应的高度
        switch (mCurrentMode) {
            case PULL_FROM_END:
                newScrollValue = Math.round(Math.max(initialMotionValue
                        - lastMotionValue, 0)
                        / FRICTION);
                itemDimension = getFooterSize();
                break;
            case PULL_FROM_START:
            default:
                newScrollValue = Math.round(Math.min(initialMotionValue
                        - lastMotionValue, 0)
                        / FRICTION);
                itemDimension = getHeaderSize();
                break;
        }

        setHeaderScroll(newScrollValue);

        if (newScrollValue != 0 && !isRefreshing()) {
            //通过当前移动的比例跟view自身的高度，计算出scale
            float scale = Math.abs(newScrollValue) / (float) itemDimension;
            //不管你设置的是何种mode。都会有对应的回调
            switch (mCurrentMode) {
                case PULL_FROM_END:
                    mFooterLayout.onPull(scale);
                    break;
                case PULL_FROM_START:
                default:
                    mHeaderLayout.onPull(scale);
                    break;
            }
            //前台接口回调
            if (mState != State.PULL_TO_REFRESH
                    && itemDimension >= Math.abs(newScrollValue)) {
                setState(State.PULL_TO_REFRESH);
            } else if (mState == State.PULL_TO_REFRESH
                    && itemDimension < Math.abs(newScrollValue)) {
                setState(State.RELEASE_TO_REFRESH);
            }
        }
    }

只是这边有个情况要考虑下，就是之前我们能够直接使用ptrDrawable、ptrDrawableStart、ptrDrawableEnd去替换上拉下拉不同的图片

<!-- Drawable to use as Loading Indicator. Changes both Header and Footer. -->
        <attr name="ptrDrawable" format="reference" />

        <!-- Drawable to use as Loading Indicator in the Header View. Overrides value set in ptrDrawable. -->
        <attr name="ptrDrawableStart" format="reference" />

        <!-- Drawable to use as Loading Indicator in the Footer View. Overrides value set in ptrDrawable. -->
        <attr name="ptrDrawableEnd" format="reference" />

可是通过如今代码去构建view的话。这些自己定义属性是不具备这个能力，所以要自行替换foot和head

// We need to create now layouts now
        mHeaderLayout = createLoadingLayout(context, Mode.PULL_FROM_START, a);
        mFooterLayout = createLoadingLayout(context, Mode.PULL_FROM_END, a);

//创建视图的方法
protected LoadingLayout createLoadingLayout(Context context, Mode mode, TypedArray attrs) {
        LoadingLayout layout = mLoadingAnimationStyle.createLoadingLayout(
                context, mode, getPullToRefreshScrollDirection(), attrs);
        layout.setVisibility(View.INVISIBLE);
        return layout;
    }

//能够改造这个枚举。返回须要的layout
public static enum AnimationStyle {
    /**
     * This is the default for Android-PullToRefresh. Allows you to use any
     * drawable, which is automatically rotated and used as a Progress Bar.
     */
    ROTATE,

    /**
     * This is the old default, and what is commonly used on iOS. Uses an
     * arrow image which flips depending on where the user has scrolled.
     */
    FLIP,

    GIF;

    static AnimationStyle getDefault() {
        return ROTATE;
    }

    /**
     * Maps an int to a specific mode. This is needed when saving state, or
     * inflating the view from XML where the mode is given through a attr
     * int.
     *
     * @param modeInt - int to map a Mode to
     * @return Mode that modeInt maps to, or ROTATE by default.
     */
    static AnimationStyle mapIntToValue(int modeInt) {
        switch (modeInt) {
            case 0x0:
            default:
                return ROTATE;
            case 0x1:
                return FLIP;
            case 0x10:
                return GIF;
        }
    }

    LoadingLayout createLoadingLayout(Context context, Mode mode,
                                      Orientation scrollDirection, TypedArray attrs) {
        switch (this) {
            case ROTATE:
            default:
                return new RotateLoadingLayout(context, mode, scrollDirection,
                        attrs);
            case FLIP:
                return new FlipLoadingLayout(context, mode, scrollDirection,
                        attrs);
            case GIF:
                return new GifLoadingLayout(context, mode, scrollDirection,
                        attrs);
        }
    }
}

仅仅有这样才干创建出不同的head与foot

OK，今天就说这么多，有疑问大家一起交流