13问13答全面学习Android View绘制(2)_H5之家 - 中国HTML5教程资源分享第一站

//view的measure是final 方法我们子类无法修改的。 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); if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT || widthMeasureSpec != mOldWidthMeasureSpec || heightMeasureSpec != mOldHeightMeasureSpec) { // first clears the measured dimension flag mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET; resolveRtlPropertiesIfNeeded(); 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 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; mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 | (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension } //不过可以看到的是在measure方法里调用了onMeasure方法 //所以就能知道我们在自定义view的时候一定是重写这个方法！ protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); }

6.简要分析view的measure流程？

答：先回顾问题4，viewgroup 算出子view的spec以后会调用子view的measure方法，而子view的measure方法我们问题5也看过了实际上是调用的onMeasure方法

所以我们只要分析好onMeasure方法即可，注意onMeasure方法的参数正是他的父view算出来的那2个spec的值(这里view的measure方法会把这个spec里的specSize值做略微的修改这个部分不做分析因为measure方法修改specSize的部分很简单)。

//可以看出来这个就是setMeasuredDimension方法的调用这个方法看名字就知道就是确定view的测量宽高的 //所以我们分析的重点就是看这个getDefaultSize 方法是怎么确定view的测量宽高的 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); } //这个方法特别简单基本可以认为就是近似的返回spec中的specSize，除非你的specMode是UNSPECIFIED //UNSPECIFIED 这个一般都是系统内部测量才用的到，这种时候返回size 也就是getSuggestedMinimumWidth的返回值 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; } //跟view的背景相关这里不多做分析了 protected int getSuggestedMinimumWidth() { return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth()); }

7.自定义view中如果onMeasure方法没有对wrap_content 做处理会发生什么？为什么？怎么解决？