Android-实例理解View measure过程

这里有两个自定义view，HorizontalScrollViewEx（作用类似水平方向的viewpager）,CircleView（简单的画一个圆）。

代码如下：

public class HorizontalScrollViewEx extends ViewGroup {
    private static final String TAG = "HorizontalScrollViewEx";

    private int mChildrenSize;
    private int mChildWidth;
    private int mChildIndex;

    //     分别记录上次滑动的坐标
    private int mLastX = 0;
    private int mLastY = 0;
    // 分别记录上次滑动的坐标(onInterceptTouchEvent)
    private int mLastXIntercept = 0;
    private int mLastYIntercept = 0;

    private Scroller mScroller;
    private VelocityTracker mVelocityTracker;

    public HorizontalScrollViewEx(Context context) {
        super(context);
        init();
    }

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

    public HorizontalScrollViewEx(Context context, AttributeSet attrs,
                                  int defStyle) {
        super(context, attrs, defStyle);
        init();
    }

    private void init() {
        mScroller = new Scroller(getContext());
        mVelocityTracker = VelocityTracker.obtain();
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {
        boolean intercepted = false;
        int x = (int) event.getX();
        int y = (int) event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN: {
                intercepted = false;
                if (!mScroller.isFinished()) {
                    mScroller.abortAnimation();
                    intercepted = true;
                }
                break;
            }
            case MotionEvent.ACTION_MOVE: {
                int deltaX = x - mLastXIntercept;
                int deltaY = y - mLastYIntercept;
                if (Math.abs(deltaX) > Math.abs(deltaY)) {
                    intercepted = true;
                } else {
                    intercepted = false;
                }
                break;
            }
            case MotionEvent.ACTION_UP: {
                intercepted = false;
                break;
            }
            default:
                break;
        }

        Log.d(TAG, "intercepted=" + intercepted);
        mLastX = x;
        mLastY = y;
        mLastXIntercept = x;
        mLastYIntercept = y;

        return intercepted;
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        mVelocityTracker.addMovement(event);
        int x = (int) event.getX();
        int y = (int) event.getY();
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN: {
                if (!mScroller.isFinished()) {
                    mScroller.abortAnimation();
                }
                break;
            }
            case MotionEvent.ACTION_MOVE: {
                int deltaX = x - mLastX;
                int deltaY = y - mLastY;
                scrollBy(-deltaX, 0);
                break;
            }
            case MotionEvent.ACTION_UP: {
                int scrollX = getScrollX();
                int scrollToChildIndex = scrollX / mChildWidth;
                mVelocityTracker.computeCurrentVelocity(1000);
                float xVelocity = mVelocityTracker.getXVelocity();
                if (Math.abs(xVelocity) >= 50) {
                    mChildIndex = xVelocity > 0 ? mChildIndex - 1 : mChildIndex + 1;
                } else {
                    mChildIndex = (scrollX + mChildWidth / 2) / mChildWidth;
                }
                mChildIndex = Math.max(0, Math.min(mChildIndex, mChildrenSize - 1));
                int dx = mChildIndex * mChildWidth - scrollX;
                smoothScrollBy(dx, 0);
                mVelocityTracker.clear();
                break;
            }
            default:
                break;
        }

        mLastX = x;
        mLastY = y;
        return true;
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int measuredWidth = 0;
        int measuredHeight = 0;
        final int childCount = getChildCount();
        Log.e(TAG, "" + MeasureSpec.getSize(widthMeasureSpec));
        //测量子view的大小
        measureChildren(widthMeasureSpec, heightMeasureSpec);

        int widthSpaceSize = MeasureSpec.getSize(widthMeasureSpec);
        int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        int heightSpaceSize = MeasureSpec.getSize(heightMeasureSpec);
        int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
        if (childCount == 0) {
            setMeasuredDimension(0, 0);
        } else if (heightSpecMode == MeasureSpec.AT_MOST) {
            Log.e(TAG, "heightMode:"+heightSpecMode);
            final View childView = getChildAt(0);

            //TODO layout_height为wrap_content ==> specMode为AT_MOST,将使用childView.getMeasuredHeight()作为其高度

            setMeasuredDimension(300, childView.getMeasuredHeight());
        } else if (widthSpecMode == MeasureSpec.AT_MOST) {
            Log.e(TAG, "widthMode:"+widthSpecMode);
            final View childView = getChildAt(0);
            measuredWidth = childView.getMeasuredWidth() * childCount;
            setMeasuredDimension(measuredWidth, heightSpaceSize);
        } else {
            Log.e(TAG, "heightMode:"+heightSpecMode);
            Log.e(TAG, "widthMode:"+widthSpecMode);
            final View childView = getChildAt(0);
            measuredWidth = childView.getMeasuredWidth() * childCount;
            measuredHeight = childView.getMeasuredHeight();
            setMeasuredDimension(measuredWidth, measuredHeight);
        }
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        int childLeft = 0;
        final int childCount = getChildCount();
        mChildrenSize = childCount;

        for (int i = 0; i < childCount; i++) {
            final View childView = getChildAt(i);
            if (childView.getVisibility() != View.GONE) {
                final int childWidth = childView.getMeasuredWidth();
                mChildWidth = childWidth;
                childView.layout(childLeft, 0, childLeft + childWidth,
                        childView.getMeasuredHeight());
                childLeft += childWidth;
            }
        }
    }

    private void smoothScrollBy(int dx, int dy) {
        mScroller.startScroll(getScrollX(), 0, dx, 0, 500);
        invalidate();
    }

    @Override
    public void computeScroll() {
        if (mScroller.computeScrollOffset()) {
            scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
            postInvalidate();
        }
    }

    @Override
    protected void onDetachedFromWindow() {
        mVelocityTracker.recycle();
        super.onDetachedFromWindow();
    }
}

public class CircleView extends View {

    private Paint mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
    private final int mCustomWidth = 400;
    private final int mCustomHeight = 400;

    public CircleView(Context context) {
        super(context);

        mPaint.setColor(Color.RED);
    }

    public CircleView(Context context, AttributeSet attrs) {
        super(context, attrs);

        mPaint.setColor(Color.RED);
    }

    public CircleView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        mPaint.setColor(Color.RED);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
        int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);

        //宽高都是AT_MOST
        if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST)
        {
            setMeasuredDimension(mCustomWidth, mCustomHeight);
        }
        else if (widthSpecMode == MeasureSpec.AT_MOST)
        {
            setMeasuredDimension(mCustomWidth, heightSpecSize);
        }
        else if (heightSpecMode == MeasureSpec.AT_MOST)
        {
            setMeasuredDimension(widthSpecSize, mCustomHeight);
        }

    }

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

        int width = getWidth();
        int height = getHeight();

        int radius = Math.min(width, height) / 2;

        canvas.drawCircle(width / 2, height / 2, radius, mPaint);
    }
}

布局文件：activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <com.dhn.customview.HorizontalScrollViewEx
        android:layout_width="wrap_content"
        android:layout_height="wrap_content">

        <com.dhn.customview.CircleView
            android:layout_width="100dp"
            android:layout_height="match_parent" />

        <TextView
            android:layout_width="100dp"
            android:layout_height="match_parent"
            android:background="#AAff0000"
            android:text="t1" />

        <TextView
            android:layout_width="100dp"
            android:layout_height="match_parent"
            android:background="#AAff00"
            android:text="t1" />

    </com.dhn.customview.HorizontalScrollViewEx>

</RelativeLayout>

问题：

布局文件中父View的高为wrap_content，子View为match_parent。这时它们的高度该如何确定呢？

因为HorizontalScrollViewEx的高设置为wrap_content所以它的高的SpecMode为AT_MOST（它的parent view和自身的LayoutParam在getChildMeasureSpec中计算得出），我们在自定义的onMeasure（）方法中将它的高设置为第一个子View的测量高度(第146行)，那么第一个子View的childView.getMeasuredHeight()为多少呢。子View的测量在132行measureChildren(widthMeasureSpec, heightMeasureSpec)中进行，进入该方法：

   protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
        final int size = mChildrenCount;
        final View[] children = mChildren;
        for (int i = 0; i < size; ++i) {
            final View child = children[i];
            if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
            }
        }
    }

第7行对第一个子View调用measureChild(child, widthMeasureSpec, heightMeasureSpec)

    protected void measureChild(View child, int parentWidthMeasureSpec,
            int parentHeightMeasureSpec) {
        final LayoutParams lp = child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

这里的getChildMeasureSpec(parentWidthMeasureSpec,mPaddingLeft + mPaddingRight, lp.width);返回第一个子View的MeasureSpec，该方法如下：

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(0, specSize - padding);

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

子View的MeasureSpec由父View的MeasureSpec和自身的LayoutParams 共同决定。这里父view高的MeasureSpec.Mode为AT_MOST，子View高的LayoutParams 为math_parent所以子View高的MeasureSpec也为AT_MOST。

接下来第10行调用第一个子View的measure()方法：

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int oWidth  = insets.left + insets.right;
            int oHeight = insets.top  + insets.bottom;
            widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
            heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
        }

        // Suppress sign extension for the low bytes
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);

        final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
        final boolean isExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY &&
                MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
        final boolean matchingSize = isExactly &&
                getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec) &&
                getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
        if (forceLayout || !matchingSize &&
                (widthMeasureSpec != mOldWidthMeasureSpec ||
                        heightMeasureSpec != mOldHeightMeasureSpec)) {

            // first clears the measured dimension flag
            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

            resolveRtlPropertiesIfNeeded();

            int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                // measure ourselves, this should set the measured dimension flag back
                onMeasure(widthMeasureSpec, heightMeasureSpec);
                mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            } else {
                long value = mMeasureCache.valueAt(cacheIndex);
                // Casting a long to int drops the high 32 bits, no mask needed
                setMeasuredDimensionRaw((int) (value >> 32), (int) value);
                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            }

            // flag not set, setMeasuredDimension() was not invoked, we raise
            // an exception to warn the developer
            if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }

            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }

        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;

        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
    }

第33行调用onMeasure(widthMeasureSpec, heightMeasureSpec);对于CircleView我们自定义了该方法：

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
        int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);

        //宽高都是AT_MOST
        if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST)
        {
            setMeasuredDimension(mCustomWidth, mCustomHeight);
        }
        else if (widthSpecMode == MeasureSpec.AT_MOST)
        {
            setMeasuredDimension(mCustomWidth, heightSpecSize);
        }
        else if (heightSpecMode == MeasureSpec.AT_MOST)
        {
            setMeasuredDimension(widthSpecSize, mCustomHeight);
        }

    }

在这里为高设置了我们自定义的值mCustomHeight=400,最终circleView的测量高度为400。

这样HorizontalScrollViewEx的高度也就确定为400。

结论：

自定义Veiw是需要在onMeasure方法中处理AT_MOST这种情况。像TextView的onMeasrue实现中，都对AT_MOST进行了处理。这样当出现parent view为wrap_content，child view为math_parent时才可以确定大小。