自定义 View：resolveSizeAndState 方法

/**     * Utility to reconcile a desired size and state, with constraints imposed     * by a MeasureSpec. Will take the desired size, unless a different size     * is imposed by the constraints. The returned value is a compound integer,     * with the resolved size in the {@link #MEASURED_SIZE_MASK} bits and     * optionally the bit {@link #MEASURED_STATE_TOO_SMALL} set if the     * resulting size is smaller than the size the view wants to be.     *     * @param size How big the view wants to be.     * @param measureSpec Constraints imposed by the parent.     * @param childMeasuredState Size information bit mask for the view's     *                           children.     * @return Size information bit mask as defined by     *         {@link #MEASURED_SIZE_MASK} and     *         {@link #MEASURED_STATE_TOO_SMALL}.     */    public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {        final int specMode = MeasureSpec.getMode(measureSpec);        final int specSize = MeasureSpec.getSize(measureSpec);        final int result;        switch (specMode) {            case MeasureSpec.AT_MOST: // 如果mode=AT_MOST，则使用上限                if (specSize < size) { // 如果当前View期望尺寸大于父View允许尺寸，那么返回父View允许尺寸                    result = specSize | MEASURED_STATE_TOO_SMALL;                } else { // 否则返回当前View的期望尺寸                    result = size;                }                break;            case MeasureSpec.EXACTLY: // 如果mode=EXACTLY，则使用父View指定的尺寸                result = specSize;                break;            case MeasureSpec.UNSPECIFIED:            default:                result = size;        }        return result | (childMeasuredState & MEASURED_STATE_MASK);    }

public class TestView extends View {
    Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);    int RADIUS = 800;    int PADDING = 30;
    public TestView(Context context, @Nullable AttributeSet attrs) {        super(context, attrs);    }
    @Override    protected void onDraw(Canvas canvas) {        super.onDraw(canvas);
        paint.setColor(Color.GRAY);        canvas.drawCircle(PADDING+RADIUS, PADDING+RADIUS, RADIUS, paint);    }
    @Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int width = (PADDING+RADIUS)*2;        int height = (PADDING+RADIUS)*2;
        setMeasuredDimension(width, height);    }}