[Android FrameWork 6.0源码学习] View的重绘过程 _Framework

天下之事常成于困约，而败于奢靡。这篇文章主要讲述[Android FrameWork 6.0源码学习] View的重绘过程相关的知识，希望能为你提供帮助。
【[Android FrameWork 6.0源码学习] View的重绘过程】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 = https://www.songbingjia.com/android/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

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); } } }