自定义ImageView设置成正方形：

public class MyImageView extends android.support.v7.widget.AppCompatImageView {


    public MyImageView(Context context) {
        super(context);
    }

    public MyImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public MyImageView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(0,widthMeasureSpec),getDefaultSize(0,heightMeasureSpec));
        int childWidthSize = getMeasuredWidth();
        //高度和宽度一样
        heightMeasureSpec = widthMeasureSpec = MeasureSpec.makeMeasureSpec(childWidthSize, MeasureSpec.EXACTLY);

        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }
}

自定义ImageView为圆形或者圆角：

首先在attrs.xml添加配置文件：

<declare-styleable name="RoundImageView">
        <attr name="type" format="enum">
            <enum name="circle" value="1"/>
            <enum name="round" value="2"/>
        </attr>
        <attr name="radius" format="dimension"/>ssss
    </declare-styleable>

其次添加代码：

public class RoundImageView extends android.support.v7.widget.AppCompatImageView {
    /**
     * 圆形模式
     */
    private static final int MODE_CIRCLE = 1;
    /**
     * 普通模式
     */
    private static final int MODE_NONE = 0;
    /**
     * 圆角模式
     */
    private static final int MODE_ROUND = 2;
    private Paint mPaint;
    private int currMode = 0;
    /**
     * 圆角半径
     */
    private int currRound = dp2px(10);

    public RoundImageView(Context context) {
        super(context);
        initViews();
    }

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

    public RoundImageView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        obtainStyledAttrs(context, attrs, defStyleAttr);
        initViews();
    }

    private void obtainStyledAttrs(Context context, AttributeSet attrs, int defStyleAttr) {
        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.RoundImageView, defStyleAttr, 0);
        currMode = a.hasValue(R.styleable.RoundImageView_type) ? a.getInt(R.styleable.RoundImageView_type, MODE_NONE) : MODE_NONE;
        currRound = a.hasValue(R.styleable.RoundImageView_radius) ? a.getDimensionPixelSize(R.styleable.RoundImageView_radius, currRound) : currRound;
        a.recycle();
    }

    private void initViews() {
        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        /**
         * 当模式为圆形模式的时候，我们强制让宽高一致
         */
        if (currMode == MODE_CIRCLE ) {
            super.onMeasure(widthMeasureSpec, heightMeasureSpec);
            int result = Math.min(getMeasuredHeight(), getMeasuredWidth());
            setMeasuredDimension(result, result);
        } else {
            //设置正方形
            setMeasuredDimension(getDefaultSize(0,widthMeasureSpec),getDefaultSize(0,heightMeasureSpec));
            int childWidthSize = getMeasuredWidth();
            //高度和宽度一样
            heightMeasureSpec = widthMeasureSpec = MeasureSpec.makeMeasureSpec(childWidthSize, MeasureSpec.EXACTLY);
            super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        }
    }

    @Override
    protected void onDraw(Canvas canvas) {
        Drawable mDrawable = getDrawable();
        Matrix mDrawMatrix = getImageMatrix();
        if (mDrawable == null) {
            return; // couldn't resolve the URI
        }

        if (mDrawable.getIntrinsicWidth() == 0 || mDrawable.getIntrinsicHeight() == 0) {
            return;     // nothing to draw (empty bounds)
        }

        if (mDrawMatrix == null && getPaddingTop() == 0 && getPaddingLeft() == 0) {
            mDrawable.draw(canvas);
        } else {
            final int saveCount = canvas.getSaveCount();
            canvas.save();

            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN) {
                if (getCropToPadding()) {
                    final int scrollX = getScrollX();
                    final int scrollY = getScrollY();
                    canvas.clipRect(scrollX + getPaddingLeft(), scrollY + getPaddingTop(),
                            scrollX + getRight() - getLeft() - getPaddingRight(),
                            scrollY + getBottom() - getTop() - getPaddingBottom());
                }
            }
            canvas.translate(getPaddingLeft(), getPaddingTop());
            if (currMode == MODE_CIRCLE) {//当为圆形模式的时候
                Bitmap bitmap = drawable2Bitmap(mDrawable);
                mPaint.setShader(new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP));
                canvas.drawCircle(getWidth() / 2, getHeight() / 2, getWidth() / 2, mPaint);
            } else if (currMode == MODE_ROUND) {//当为圆角模式的时候
                Bitmap bitmap = drawable2Bitmap(mDrawable);
                mPaint.setShader(new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP));
                canvas.drawRoundRect(new RectF(getPaddingLeft(), getPaddingTop(), getWidth() - getPaddingRight(), getHeight() - getPaddingBottom()),
                        currRound, currRound, mPaint);
            } else {
                if (mDrawMatrix != null) {
                    canvas.concat(mDrawMatrix);
                }
                mDrawable.draw(canvas);
            }
            canvas.restoreToCount(saveCount);
        }
    }

    /**
     * drawable转换成bitmap
     */
    private Bitmap drawable2Bitmap(Drawable drawable) {
        if (drawable == null) {
            return null;
        }
        Bitmap bitmap = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bitmap);
        //根据传递的scaletype获取matrix对象，设置给bitmap
        Matrix matrix = getImageMatrix();
        if (matrix != null) {
            canvas.concat(matrix);
        }
        drawable.draw(canvas);
        return bitmap;
    }

    private int dp2px(float value) {
        return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, value, getResources().getDisplayMetrics());
    }
}