android View measure過程原始碼解析

一.view.java onmeasure

當activity啟動的時候，觸發初始化view過程的是由Window物件的DecorView呼叫View物件的 public final void measure(int widthMeasureSpec, int heightMeasureSpec)方法開始的，這個方法是final型別的，也就是所有的子類都不能繼承該方法，保證android初始化view的原理不變。

    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);
        }
        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {

            // first clears the measured dimension flag
            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
            resolveRtlPropertiesIfNeeded();
            // measure ourselves, this should set the measured dimension flag back
            onMeasure(widthMeasureSpec, heightMeasureSpec);

            // 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;
    }

widthMeasureSpec heightMeasureSpec這個兩個引數都是有父view傳遞過來的，也就是代表了父view的大小。其實說大小不太對，應該說是建議“規格”。他有兩部分組成，第一部分：高2位表示MODE，定義在MeasureSpec類中，有三種型別，MeasureSpec.EXACTLY：表示確定大小， MeasureSpec.AT_MOST：表示最大大小， MeasureSpec.UNSPECIFIED：不確定。第二部分：低30位表示size，既父view的大小，

View$MeasureSpec.java

private static final int MODE_SHIFT = 30;
 private static final int MODE_MASK  = 0x3 << MODE_SHIFT;       //c0 00 00 00
public static int makeMeasureSpec(int size, int mode) {
            if (sUseBrokenMakeMeasureSpec) {
                return size + mode;
            } else {
                return (size & ~MODE_MASK) | (mode & MODE_MASK);
            }
        }
 public static int getMode(int measureSpec) {
            return (measureSpec & MODE_MASK);
        }
 public static int getSize(int measureSpec) {
            return (measureSpec & ~MODE_MASK);
        }
  static int adjust(int measureSpec, int delta) {
            return makeMeasureSpec(getSize(measureSpec + delta), getMode(measureSpec));
        }

view.xml 

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        switch (specMode) {
        case MeasureSpec.UNSPECIFIED:
            result = size;
            break;
        case MeasureSpec.AT_MOST:
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
        }
        return result;
    }
//有背景圖的時候，取背景尺寸與view尺寸的最大值
 protected int getSuggestedMinimumHeight() {
        return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());

    }

 protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int opticalWidth  = insets.left + insets.right;
            int opticalHeight = insets.top  + insets.bottom;

            measuredWidth  += optical ? opticalWidth  : -opticalWidth;
            measuredHeight += optical ? opticalHeight : -opticalHeight;
        }
        mMeasuredWidth = measuredWidth;
        mMeasuredHeight = measuredHeight;
        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
    }

ViewGroup沒有重寫onmeasure方法，Linearlayout.java

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        if (mOrientation == VERTICAL) {
            measureVertical(widthMeasureSpec, heightMeasureSpec);
        } else {
            measureHorizontal(widthMeasureSpec, heightMeasureSpec);
        }

void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
        mTotalLength = 0;
        int maxWidth = 0;
        int childState = 0;
        int alternativeMaxWidth = 0;
        int weightedMaxWidth = 0;
        boolean allFillParent = true;
        float totalWeight = 0;

        final int count = getVirtualChildCount();
        
        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);

        boolean matchWidth = false;

        final int baselineChildIndex = mBaselineAlignedChildIndex;        
        final boolean useLargestChild = mUseLargestChild;

        int largestChildHeight = Integer.MIN_VALUE;

        // See how tall everyone is. Also remember max width.
        for (int i = 0; i < count; ++i) {
            final View child = getVirtualChildAt(i);

            if (child == null) {
                mTotalLength += measureNullChild(i);
                continue;
            }

            if (child.getVisibility() == View.GONE) {
               i += getChildrenSkipCount(child, i);
               continue;
            }

            if (hasDividerBeforeChildAt(i)) {
                mTotalLength += mDividerHeight;
            }

            LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();

            totalWeight += lp.weight;
            
            if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
                // Optimization: don't bother measuring children who are going to use
                // leftover space. These views will get measured again down below if
                // there is any leftover space.
                final int totalLength = mTotalLength;
                mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
            } else {
                int oldHeight = Integer.MIN_VALUE;

                if (lp.height == 0 && lp.weight > 0) {
                    // heightMode is either UNSPECIFIED or AT_MOST, and this
                    // child wanted to stretch to fill available space.
                    // Translate that to WRAP_CONTENT so that it does not end up
                    // with a height of 0
                    oldHeight = 0;
                    lp.height = LayoutParams.WRAP_CONTENT;
                }

                // Determine how big this child would like to be. If this or
                // previous children have given a weight, then we allow it to
                // use all available space (and we will shrink things later
                // if needed).
                measureChildBeforeLayout(
                       child, i, widthMeasureSpec, 0, heightMeasureSpec,
                       totalWeight == 0 ? mTotalLength : 0);

                if (oldHeight != Integer.MIN_VALUE) {
                   lp.height = oldHeight;
                }

                final int childHeight = child.getMeasuredHeight();
                final int totalLength = mTotalLength;
                mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
                       lp.bottomMargin + getNextLocationOffset(child));

                if (useLargestChild) {
                    largestChildHeight = Math.max(childHeight, largestChildHeight);
                }
            }

            /**
             * If applicable, compute the additional offset to the child's baseline
             * we'll need later when asked {@link #getBaseline}.
             */
            if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) {
               mBaselineChildTop = mTotalLength;
            }

            // if we are trying to use a child index for our baseline, the above
            // book keeping only works if there are no children above it with
            // weight.  fail fast to aid the developer.
            if (i < baselineChildIndex && lp.weight > 0) {
                throw new RuntimeException("A child of LinearLayout with index "
                        + "less than mBaselineAlignedChildIndex has weight > 0, which "
                        + "won't work.  Either remove the weight, or don't set "
                        + "mBaselineAlignedChildIndex.");
            }

            boolean matchWidthLocally = false;
            if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.MATCH_PARENT) {
                // The width of the linear layout will scale, and at least one
                // child said it wanted to match our width. Set a flag
                // indicating that we need to remeasure at least that view when
                // we know our width.
                matchWidth = true;
                matchWidthLocally = true;
            }

            final int margin = lp.leftMargin + lp.rightMargin;
            final int measuredWidth = child.getMeasuredWidth() + margin;
            maxWidth = Math.max(maxWidth, measuredWidth);
            childState = combineMeasuredStates(childState, child.getMeasuredState());

            allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;
            if (lp.weight > 0) {
                /*
                 * Widths of weighted Views are bogus if we end up
                 * remeasuring, so keep them separate.
                 */
                weightedMaxWidth = Math.max(weightedMaxWidth,
                        matchWidthLocally ? margin : measuredWidth);
            } else {
                alternativeMaxWidth = Math.max(alternativeMaxWidth,
                        matchWidthLocally ? margin : measuredWidth);
            }

            i += getChildrenSkipCount(child, i);
        }

        if (mTotalLength > 0 && hasDividerBeforeChildAt(count)) {
            mTotalLength += mDividerHeight;
        }

        if (useLargestChild &&
                (heightMode == MeasureSpec.AT_MOST || heightMode == MeasureSpec.UNSPECIFIED)) {
            mTotalLength = 0;

            for (int i = 0; i < count; ++i) {
                final View child = getVirtualChildAt(i);

                if (child == null) {
                    mTotalLength += measureNullChild(i);
                    continue;
                }

                if (child.getVisibility() == GONE) {
                    i += getChildrenSkipCount(child, i);
                    continue;
                }

                final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)
                        child.getLayoutParams();
                // Account for negative margins
                final int totalLength = mTotalLength;
                mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +
                        lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
            }
        }

        // Add in our padding
        mTotalLength += mPaddingTop + mPaddingBottom;

        int heightSize = mTotalLength;

        // Check against our minimum height
        heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
        
        // Reconcile our calculated size with the heightMeasureSpec
        int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
        heightSize = heightSizeAndState & MEASURED_SIZE_MASK;
        
        // Either expand children with weight to take up available space or
        // shrink them if they extend beyond our current bounds
        int delta = heightSize - mTotalLength;
        if (delta != 0 && totalWeight > 0.0f) {
            float weightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;

            mTotalLength = 0;

            for (int i = 0; i < count; ++i) {
                final View child = getVirtualChildAt(i);
                
                if (child.getVisibility() == View.GONE) {
                    continue;
                }
                
                LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
                
                float childExtra = lp.weight;
                if (childExtra > 0) {
                    // Child said it could absorb extra space -- give him his share
                    int share = (int) (childExtra * delta / weightSum);
                    weightSum -= childExtra;
                    delta -= share;

                    final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            mPaddingLeft + mPaddingRight +
                                    lp.leftMargin + lp.rightMargin, lp.width);

                    // TODO: Use a field like lp.isMeasured to figure out if this
                    // child has been previously measured
                    if ((lp.height != 0) || (heightMode != MeasureSpec.EXACTLY)) {
                        // child was measured once already above...
                        // base new measurement on stored values
                        int childHeight = child.getMeasuredHeight() + share;
                        if (childHeight < 0) {
                            childHeight = 0;
                        }
                        
                        child.measure(childWidthMeasureSpec,
                                MeasureSpec.makeMeasureSpec(childHeight, MeasureSpec.EXACTLY));
                    } else {
                        // child was skipped in the loop above.
                        // Measure for this first time here      
                        child.measure(childWidthMeasureSpec,
                                MeasureSpec.makeMeasureSpec(share > 0 ? share : 0,
                                        MeasureSpec.EXACTLY));
                    }

                    // Child may now not fit in vertical dimension.
                    childState = combineMeasuredStates(childState, child.getMeasuredState()
                            & (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));
                }

                final int margin =  lp.leftMargin + lp.rightMargin;
                final int measuredWidth = child.getMeasuredWidth() + margin;
                maxWidth = Math.max(maxWidth, measuredWidth);

                boolean matchWidthLocally = widthMode != MeasureSpec.EXACTLY &&
                        lp.width == LayoutParams.MATCH_PARENT;

                alternativeMaxWidth = Math.max(alternativeMaxWidth,
                        matchWidthLocally ? margin : measuredWidth);

                allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;

                final int totalLength = mTotalLength;
                mTotalLength = Math.max(totalLength, totalLength + child.getMeasuredHeight() +
                        lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
            }

            // Add in our padding
            mTotalLength += mPaddingTop + mPaddingBottom;
            // TODO: Should we recompute the heightSpec based on the new total length?
        } else {
            alternativeMaxWidth = Math.max(alternativeMaxWidth,
                                           weightedMaxWidth);


            // We have no limit, so make all weighted views as tall as the largest child.
            // Children will have already been measured once.
            if (useLargestChild && heightMode != MeasureSpec.EXACTLY) {
                for (int i = 0; i < count; i++) {
                    final View child = getVirtualChildAt(i);

                    if (child == null || child.getVisibility() == View.GONE) {
                        continue;
                    }

                    final LinearLayout.LayoutParams lp =
                            (LinearLayout.LayoutParams) child.getLayoutParams();

                    float childExtra = lp.weight;
                    if (childExtra > 0) {
                        child.measure(
                                MeasureSpec.makeMeasureSpec(child.getMeasuredWidth(),
                                        MeasureSpec.EXACTLY),
                                MeasureSpec.makeMeasureSpec(largestChildHeight,
                                        MeasureSpec.EXACTLY));
                    }
                }
            }
        }

        if (!allFillParent && widthMode != MeasureSpec.EXACTLY) {
            maxWidth = alternativeMaxWidth;
        }
        
        maxWidth += mPaddingLeft + mPaddingRight;

        // Check against our minimum width
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
        
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                heightSizeAndState);

        if (matchWidth) {
            forceUniformWidth(count, heightMeasureSpec);
        }
    }

不完整的圖：

1.如果在xml中指定了子view的具體大小，那麼計算結果不管父的measure是什麼，結果都是EXACITY+child_size，

所有的子view測量結束後，才開始對layout_weight計算，這樣我們可能想到，如果父view已經被佔滿了，那麼有可能layout_weight大於0的view物件是不會顯示的，而計算layout_weight的方法也很簡單，就是用總高度減去上面分析完mTotalLength的值，就是剩下，然後去平分給view物件，注意計算權重時優先去android:android:weightSum（LinearLayout的xml屬性）的值，如果不設定該值會計算和，所以該值既然設定了，就一定要子view的weight的總和相等，否則平分可能不能得到預期效果，分析重置排序，獲取所有的子view數量，對每個子view開始處理，如果子view是GONE的，則直接跳過

Linearlayout.java onmeasure中

measureChildBeforeLayout
forceUniformWidth
measureChildWithMargins

直接或間接呼叫了，viewGroup的measureChildWithMargins方法

  protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                        + widthUsed, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                        + heightUsed, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

/**
     * Ask all of the children of this view to measure themselves, taking into
     * account both the MeasureSpec requirements for this view and its padding.
     * We skip children that are in the GONE state The heavy lifting is done in
     * getChildMeasureSpec.
     *
     * @param widthMeasureSpec The width requirements for this view
     * @param heightMeasureSpec The height requirements for this view
     */
    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);
            }
        }
    }

    /**
     * Ask one of the children of this view to measure itself, taking into
     * account both the MeasureSpec requirements for this view and its padding.
     * The heavy lifting is done in getChildMeasureSpec.
     *
     * @param child The child to measure
     * @param parentWidthMeasureSpec The width requirements for this view
     * @param parentHeightMeasureSpec The height requirements for this view
     */
    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);
    }

measureChild  measureChildWithMargins 相比多考慮了margin 及已經使用的寬高

之後再呼叫子類的measure(final 不可修改) measure內呼叫自身的onmeasure 在onmeasure內呼叫setMeasuredDimension 設定自身的寬高具體值

/**
     * Does the hard part of measureChildren: figuring out the MeasureSpec to
     * pass to a particular child. This method figures out the right MeasureSpec
     * for one dimension (height or width) of one child view.
     *
     * The goal is to combine information from our MeasureSpec with the
     * LayoutParams of the child to get the best possible results. For example,
     * if the this view knows its size (because its MeasureSpec has a mode of
     * EXACTLY), and the child has indicated in its LayoutParams that it wants
     * to be the same size as the parent, the parent should ask the child to
     * layout given an exact size.
     *
     * @param spec The requirements for this view
     * @param padding The padding of this view for the current dimension and
     *        margins, if applicable
     * @param childDimension How big the child wants to be in the current
     *        dimension
     * @return a MeasureSpec integer for the child
     */
    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);
    }