自定义支持展开的TextView

参考开源项目ExpendableTextView，原项目使用LinearLayout包装TextView和Button的做法，需要保证id的一致，，文本内容需要在代码中设置，操作较为复杂，故修改继承自TextView，保留TextView特性，简化操作。

//TODO 转为gif

1.定义一些属性

• animDuration textview展开的动效时长
• animAlphaStart 透明度渐变初始值
• expandDrawable 指示可以展开的小箭头图标
• collapseDrawable 指示可以折叠的小箭头图标

• maxCollapsedLines 开始折叠的起始行数

  <declare-styleable name="ExpandableTextView">
         <attr name="maxCollapsedLines" format="integer"/>
         <attr name="animDuration" format="integer"/>
         <attr name="animAlphaStart" format="float"/>
         <attr name="expandDrawable" format="reference"/>
         <attr name="collapseDrawable" format="reference"/>
     </declare-styleable>

2.设置点击事件

点击时，行数不足，不响应点击事件，行数足够，开始展开／折叠 动画

@Override
    public void onClick(View v) {
        if(!needCollapse){
            return;//行数不足,不响应点击事件
        }
        mCollapsed = !mCollapsed;

        if (mCollapsedStatus != null) {
            mCollapsedStatus.put(mPosition, mCollapsed);
        }

        // 标志是否正在进行动画，动画是不重复进行
        mAnimating = true;

        Animation animation;
        if (mCollapsed) {
            animation = new ExpandCollapseAnimation(this, getHeight(), mCollapsedHeight);
        } else {
            animation = new ExpandCollapseAnimation(this, getHeight(),mTextHeightWithMaxLines);
        }

        animation.setFillAfter(true);
        animation.setAnimationListener(new Animation.AnimationListener() {
            @Override
            public void onAnimationStart(Animation animation) {
                applyAlphaAnimation(ExpandTextView.this, mAnimAlphaStart);
            }
            @Override
            public void onAnimationEnd(Animation animation) {
                // clear animation here to avoid repeated applyTransformation() calls
                clearAnimation();
                // clear the animation flag
                mAnimating = false;

                // notify the listener
                if (mListener != null) {
                    mListener.onExpandStateChanged(ExpandTextView.this, !mCollapsed);
                }
            }
            @Override
            public void onAnimationRepeat(Animation animation) { }
        });

        clearAnimation();
        startAnimation(animation);
    }

3.重写onMeasure()方法

• 先取消对TextView显示行数的限制，调用super.onMeasure()方法计算出TextView的总高度；
• 然后判断TextView的行数满不满足折叠条件，满足时致needCollapse为true；
• 使用setMaxLines()方法设置TextView能显示的最大行数，并重新使用super.onMeasure()方法计算出此时TextView的高度，以方便做动效

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        if(getVisibility()==GONE){
            super.onMeasure(widthMeasureSpec, heightMeasureSpec);
            return;
        }

        setMaxLines(Integer.MAX_VALUE);
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        System.out.println("行数:"+getLineCount());
        if (getLineCount() <= mMaxCollapsedLines) {
            //不需要折叠
            needCollapse = false;
            return;
        }

        needCollapse = true;

        mTextHeightWithMaxLines = getRealTextViewHeight(this);
        if (mCollapsed) {
            setMaxLines(mMaxCollapsedLines);
        }

        //设置完成后重新测量
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        if (mCollapsed) {
            mCollapsedHeight = getMeasuredHeight();
        }
    }

3.重写onDraw()方法，在合适的位置绘制展开／折叠 箭头

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

        if(needCollapse){
            if(mCollapsed){
                int right = getRight()-getTotalPaddingRight()-sp2dp(16);
                int left = right-mExpandDrawable.getIntrinsicWidth();
                int bottom = getBottom() - getTotalPaddingBottom()-sp2dp(16);
                int top = bottom - mExpandDrawable.getIntrinsicHeight();

                canvas.translate(left,top);
                mExpandDrawable.setBounds(0,0,mExpandDrawable.getIntrinsicWidth(),mExpandDrawable.getIntrinsicHeight());
                mExpandDrawable.draw(canvas);
            }else{
                int right = getRight()-getTotalPaddingRight()-sp2dp(16);
                int left = right-mCollapseDrawable.getIntrinsicWidth();
                int bottom = getBottom() - getTotalPaddingBottom()-sp2dp(16);
                int top = bottom - mCollapseDrawable.getIntrinsicHeight();

                canvas.translate(left,top);
                mCollapseDrawable.setBounds(0,0,mCollapseDrawable.getIntrinsicWidth(),mCollapseDrawable.getIntrinsicHeight());
                mCollapseDrawable.draw(canvas);
            }
        }
    }

4.动效

class ExpandCollapseAnimation extends Animation {
        private final View mTargetView;
        private final int mStartHeight;
        private final int mEndHeight;

        public ExpandCollapseAnimation(View view, int startHeight, int endHeight) {
            mTargetView = view;
            mStartHeight = startHeight;
            mEndHeight = endHeight;
            setDuration(mAnimationDuration);
        }

        @Override
        protected void applyTransformation(float interpolatedTime, Transformation t) {
            final int newHeight = (int)((mEndHeight - mStartHeight) * interpolatedTime + mStartHeight);
            setMaxHeight(newHeight);
            if (Float.compare(mAnimAlphaStart, 1.0f) != 0) {
                applyAlphaAnimation(ExpandTextView.this, mAnimAlphaStart + interpolatedTime * (1.0f - mAnimAlphaStart));
            }
            mTargetView.getLayoutParams().height = newHeight;
            mTargetView.requestLayout();
        }

        @Override
        public void initialize( int width, int height, int parentWidth, int parentHeight ) {
            super.initialize(width, height, parentWidth, parentHeight);
        }

        @Override
        public boolean willChangeBounds( ) {
            return true;
        }
    };

5.当前存在的bug

• onDraw()方法中，小箭头的绘制位置的计算还有问题，TextView的padding值的计算需要