Android View的重绘过程之Measure

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View绘制的三部曲，  测量，布局，绘画
今天我们分析测量过程

view的测量是从ViewRootImpl发起的，View需要重绘，都是发送请求给ViewRootImpl，然后他组织重绘
在重绘的过程中，有一步就是测量，通过代码来分析测量过程

    private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,
            final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) {
        int childWidthMeasureSpec;
        int childHeightMeasureSpec;
        boolean windowSizeMayChange = false;

        if (DEBUG_ORIENTATION || DEBUG_LAYOUT) Log.v(TAG,
                "Measuring " + host + " in display " + desiredWindowWidth
                + "x" + desiredWindowHeight + "...");

        boolean goodMeasure = false;
        if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT) {
            // On large screens, we don't want to allow dialogs to just
            // stretch to fill the entire width of the screen to display
            // one line of text.  First try doing the layout at a smaller
            // size to see if it will fit.
            final DisplayMetrics packageMetrics = res.getDisplayMetrics();
            res.getValue(com.android.internal.R.dimen.config_prefDialogWidth, mTmpValue, true);
            int baseSize = 0;
            if (mTmpValue.type == TypedValue.TYPE_DIMENSION) {
                baseSize = (int)mTmpValue.getDimension(packageMetrics);
            }
            if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": baseSize=" + baseSize);
            if (baseSize != 0 && desiredWindowWidth > baseSize) {
                //获取测量的规格，是一个32位的二进制数值，前两位标识mode，后30位表示view的长/宽
                childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
                childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
                //向DecorView发起重绘
                performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("
                        + host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")");
                if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {
                    goodMeasure = true;
                } else {
                    // Didn't fit in that size... try expanding a bit.
                    baseSize = (baseSize+desiredWindowWidth)/2;
                    if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": next baseSize="
                            + baseSize);
                    childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
                    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                    if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("
                            + host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")");
                    if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {
                        if (DEBUG_DIALOG) Log.v(TAG, "Good!");
                        goodMeasure = true;
                    }
                }
            }
        }

        if (!goodMeasure) {
            childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
            childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
            performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
            if (mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight()) {
                windowSizeMayChange = true;
            }
        }

        if (DBG) {
            System.out.println("======================================");
            System.out.println("performTraversals -- after measure");
            host.debug();
        }

        return windowSizeMayChange;
    }

这个函数通过getRootMeasureSpec方法，获取测量规格，然后调用performMeasure方法开始分发给整个的view树。

    private static int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {

        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }

通过MeasureSpec的makeMeasureSpec方法来生成测量规格，先判断出布局是 match_parent 或者是 wrap_content，或者是确定的数值
 然后把windowSize传递下去。

    public static int makeMeasureSpec(int size, int mode) {
            if (sUseBrokenMakeMeasureSpec) {
                return size + mode;
            } else {
                return (size & ~MODE_MASK) | (mode & MODE_MASK);
            }
    }

API大于17的都会走else判断，这块我分析一下计算结果。有助于理解后边的运算

makeMeasureSpec的运算结果是一个32位的二进制数值，前2位表示测量的规格 EXACTLY/AT_most 后30位表示 windowSize，举个运算例子    
 size=320,mode=EXACTLY,换算成二进制就是下边的两串值
 size=0000 0000 0000 0000 0000 0001 0100 0000
 mode=0100 0000 0000 0000 0000 0000 0000 0000
 mask=1100 0000 0000 0000 0000 0000 0000 0000
 最后用与运算整合size和mode
 0000 0000 0000 0000 0000 0001 0100 0000 |
 0100 0000 0000 0000 0000 0000 0000 0000 =
 0100 0000 0000 0000 0000 0001 0100 0000

    private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
        try {
            mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

这个mView就是我们在window类中组合出来的DecorView，这个方法调用了view的measure方法，measure会调用OnMeasure方法，然后就实现了整个view树的测量工作

    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);
        }

        // 这块又把宽的测量规格和高的测量规格拼接在了一起，作为缓存中的key
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {

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

            resolveRtlPropertiesIfNeeded();
            //判断是否强制测量，如果强制就重新调用onMeasure，整个view树重新测量，否则就从缓存中得到上次的测量规格，
            //因为DecorView是FrameLayout的子类,所以onMeasure就是调用FrameLayout的onMeasure方法
            int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -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
                //调用完这个方法之后，getMeasuredWidth和getMeasuredHeight就可以取到值了
                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("View with id " + getId() + ": "
                        + getClass().getName() + "#onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }

            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }

        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;
        //在按照固定的格式，把本次的测量规格put到集合中
        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
    }

这个方法做的功能大概就是这样，先判断是否有缓存，没缓存就测量一下整个树，然后按照固定的格式在存入缓存中

从这里开始，我们的整个测量过程就开始跑起来了

首先第一站是我们的DecorView的onMeasure方法

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        final DisplayMetrics metrics = getContext().getResources().getDisplayMetrics();
        final boolean isPortrait = metrics.widthPixels < metrics.heightPixels;

        //正常情况下两个mode都是EXACTY,除非手动更改window的宽和高
        final int widthMode = getMode(widthMeasureSpec);
        final int heightMode = getMode(heightMeasureSpec);

        boolean fixedWidth = false;
        //如果不是采用精准测量的方式，就按照宽度计算出测量规格
        if (widthMode == AT_MOST) {
            final TypedValue tvw = isPortrait ? mFixedWidthMinor : mFixedWidthMajor;
            if (tvw != null && tvw.type != TypedValue.TYPE_NULL) {
                final int w;
                if (tvw.type == TypedValue.TYPE_DIMENSION) {
                    w = (int) tvw.getDimension(metrics);
                } else if (tvw.type == TypedValue.TYPE_FRACTION) {
                    w = (int) tvw.getFraction(metrics.widthPixels, metrics.widthPixels);
                } else {
                    w = 0;
                }

                if (w > 0) {
                    final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
                    widthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            Math.min(w, widthSize), EXACTLY);
                    fixedWidth = true;
                }
            }
        }

        //和宽度类似，也是计算工作
        if (heightMode == AT_MOST) {
            final TypedValue tvh = isPortrait ? mFixedHeightMajor : mFixedHeightMinor;
            if (tvh != null && tvh.type != TypedValue.TYPE_NULL) {
                final int h;
                if (tvh.type == TypedValue.TYPE_DIMENSION) {
                    h = (int) tvh.getDimension(metrics);
                } else if (tvh.type == TypedValue.TYPE_FRACTION) {
                    h = (int) tvh.getFraction(metrics.heightPixels, metrics.heightPixels);
                } else {
                    h = 0;
                }
                if (h > 0) {
                    final int heightSize = MeasureSpec.getSize(heightMeasureSpec);
                    heightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            Math.min(h, heightSize), EXACTLY);
                }
            }
        }

        getOutsets(mOutsets);
        if (mOutsets.top > 0 || mOutsets.bottom > 0) {
            int mode = MeasureSpec.getMode(heightMeasureSpec);
            if (mode != MeasureSpec.UNSPECIFIED) {
                int height = MeasureSpec.getSize(heightMeasureSpec);
                heightMeasureSpec = MeasureSpec.makeMeasureSpec(
                        height + mOutsets.top + mOutsets.bottom, mode);
            }
        }
        if (mOutsets.left > 0 || mOutsets.right > 0) {
            int mode = MeasureSpec.getMode(widthMeasureSpec);
            if (mode != MeasureSpec.UNSPECIFIED) {
                int width = MeasureSpec.getSize(widthMeasureSpec);
                widthMeasureSpec = MeasureSpec.makeMeasureSpec(
                        width + mOutsets.left + mOutsets.right, mode);
            }
        }
        //调用FrameLayout的onMeasure方法
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        int width = getMeasuredWidth();
        boolean measure = false;

        widthMeasureSpec = MeasureSpec.makeMeasureSpec(width, EXACTLY);

        if (!fixedWidth && widthMode == AT_MOST) {
            final TypedValue tv = isPortrait ? mMinWidthMinor : mMinWidthMajor;
            if (tv.type != TypedValue.TYPE_NULL) {
                final int min;
                if (tv.type == TypedValue.TYPE_DIMENSION) {
                    min = (int)tv.getDimension(metrics);
                } else if (tv.type == TypedValue.TYPE_FRACTION) {
                    min = (int)tv.getFraction(metrics.widthPixels, metrics.widthPixels);
                } else {
                    min = 0;
                }

                if (width < min) {
                    widthMeasureSpec = MeasureSpec.makeMeasureSpec(min, EXACTLY);
                    measure = true;
                }
            }
        }

        // TODO: Support height?

        if (measure) {
            super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        }
    }

这个函数是判断一下测量的模式，最后的测量，还是采用super的onMeasure来完成的，下一站就是FrameLayout中的onMeasure了

     protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();
        //只有match_parent和固定宽高 的mode才是EXACTLY，wrap_content是AT_MOST
        final boolean measureMatchParentChildren =
                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
        mMatchParentChildren.clear();

        int maxHeight = 0;  //包含padding的高度
        int maxWidth = 0;   //包含width的宽度
        int childState = 0; //前16位是宽的测量mode，后16位是高的测量mode
        //轮寻测量所有的child
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            //只要是不为GONE，就会进入测量范围
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                //测量child的时候，减去padding加上margin的距离，得到child的规格后回调用child的measure，然后在重新走上边分析的流程，调用child的onMeasure
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                //赋值maxWidth和maxHeight，之后就可以用getHeight和getWidth方法了
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                maxWidth = Math.max(maxWidth,
                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
                maxHeight = Math.max(maxHeight,
                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
                childState = combineMeasuredStates(childState, child.getMeasuredState());
                if (measureMatchParentChildren) {
                    if (lp.width == LayoutParams.MATCH_PARENT ||
                            lp.height == LayoutParams.MATCH_PARENT) {
                        mMatchParentChildren.add(child);
                    }
                }
            }
        }

        // Account for padding too
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

        // Check against our minimum height and width
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        // Check against our foreground's minimum height and width
        final Drawable drawable = getForeground();
        if (drawable != null) {
            maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
            maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
        }
        //设置子空间的测量结果
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));
        //开始测量宽高都是MATCH_PARENT的布局
        count = mMatchParentChildren.size();
        if (count > 1) {
            for (int i = 0; i < count; i++) {
                final View child = mMatchParentChildren.get(i);
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

                final int childWidthMeasureSpec;
                if (lp.width == LayoutParams.MATCH_PARENT) {
                    final int width = Math.max(0, getMeasuredWidth()
                            - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                            - lp.leftMargin - lp.rightMargin);
                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            width, MeasureSpec.EXACTLY);
                } else {
                    childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                            lp.leftMargin + lp.rightMargin,
                            lp.width);
                }

                final int childHeightMeasureSpec;
                if (lp.height == LayoutParams.MATCH_PARENT) {
                    final int height = Math.max(0, getMeasuredHeight()
                            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                            - lp.topMargin - lp.bottomMargin);
                    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            height, MeasureSpec.EXACTLY);
                } else {
                    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                            lp.topMargin + lp.bottomMargin,
                            lp.height);
                }

                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }
    }