自定义View的基本方法

自定义View的最基本的三个方法分别是： onMeasure()、onLayout()、onDraw(); View在Activity中显示出来，要经 历测量、布局和绘制三个步骤，分别对应三个动作：measure、layout和draw。

测量：onMeasure()决定View的大小；

布局：onLayout()决定View在ViewGroup中的位置；

绘制：onDraw()决定绘制这个View。

自定义控件分类

自定义View: 只需重写onMeasure()和onDraw()

自定义ViewGroup：只需重写onMeasure()和onLayout()

测量：onMeasure()

自定义View，首先要测量宽高尺寸。在xml布局中定义了尺寸，我们的layout_width和layout_height参数可以不用写具体的尺寸，而是wrap_content或者是match_parent。

函数原型：

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)

参数widthMeasureSpec和HeightMeasureSpec为 MeasureSpec类型。

MeasureSpec介绍

MeasureSpec源码：

public static class MeasureSpec { private static final int MODE_SHIFT = 30; private static final int MODE_MASK = 0x3 << MODE_SHIFT; /** @hide */ @IntDef({UNSPECIFIED, EXACTLY, AT_MOST}) @Retention(RetentionPolicy.SOURCE) public @interface MeasureSpecMode {} /** * Measure specification mode: The parent has not imposed any constraint * on the child. It can be whatever size it wants. */ public static final int UNSPECIFIED = 0 << MODE_SHIFT; /** * Measure specification mode: The parent has determined an exact size * for the child. The child is going to be given those bounds regardless * of how big it wants to be. */ public static final int EXACTLY = 1 << MODE_SHIFT; /** * Measure specification mode: The child can be as large as it wants up * to the specified size. */ public static final int AT_MOST = 2 << MODE_SHIFT; /** * Creates a measure specification based on the supplied size and mode. * * The mode must always be one of the following: * <ul> * <li>{@link android.view.View.MeasureSpec#UNSPECIFIED}</li> * <li>{@link android.view.View.MeasureSpec#EXACTLY}</li> * <li>{@link android.view.View.MeasureSpec#AT_MOST}</li> * </ul> * * <p><strong>Note:</strong> On API level 17 and lower, makeMeasureSpec's * implementation was such that the order of arguments did not matter * and overflow in either value could impact the resulting MeasureSpec. * {@link android.widget.RelativeLayout} was affected by this bug. * Apps targeting API levels greater than 17 will get the fixed, more strict * behavior.</p> * * @param size the size of the measure specification * @param mode the mode of the measure specification * @return the measure specification based on size and mode */ public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size, @MeasureSpecMode int mode) { if (sUseBrokenMakeMeasureSpec) { return size + mode; } else { return (size & ~MODE_MASK) | (mode & MODE_MASK); } } /** * Like {@link #makeMeasureSpec(int, int)}, but any spec with a mode of UNSPECIFIED * will automatically get a size of 0. Older apps expect this. * * @hide internal use only for compatibility with system widgets and older apps */ public static int makeSafeMeasureSpec(int size, int mode) { if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) { return 0; } return makeMeasureSpec(size, mode); } /** * Extracts the mode from the supplied measure specification. * * @param measureSpec the measure specification to extract the mode from * @return {@link android.view.View.MeasureSpec#UNSPECIFIED}, * {@link android.view.View.MeasureSpec#AT_MOST} or * {@link android.view.View.MeasureSpec#EXACTLY} */ @MeasureSpecMode public static int getMode(int measureSpec) { //noinspection ResourceType return (measureSpec & MODE_MASK); } /** * Extracts the size from the supplied measure specification. * * @param measureSpec the measure specification to extract the size from * @return the size in pixels defined in the supplied measure specification */ public static int getSize(int measureSpec) { return (measureSpec & ~MODE_MASK); } static int adjust(int measureSpec, int delta) { final int mode = getMode(measureSpec); int size = getSize(measureSpec); if (mode == UNSPECIFIED) { // No need to adjust size for UNSPECIFIED mode. return makeMeasureSpec(size, UNSPECIFIED); } size += delta; if (size < 0) { Log.e(VIEW_LOG_TAG, "MeasureSpec.adjust: new size would be negative! (" + size + ") spec: " + toString(measureSpec) + " delta: " + delta); size = 0; } return makeMeasureSpec(size, mode); } /** * Returns a String representation of the specified measure * specification. * * @param measureSpec the measure specification to convert to a String * @return a String with the following format: "MeasureSpec: MODE SIZE" */ public static String toString(int measureSpec) { int mode = getMode(measureSpec); int size = getSize(measureSpec); StringBuilder sb = new StringBuilder("MeasureSpec: "); if (mode == UNSPECIFIED) sb.append("UNSPECIFIED "); else if (mode == EXACTLY) sb.append("EXACTLY "); else if (mode == AT_MOST) sb.append("AT_MOST "); else sb.append(mode).append(" "); sb.append(size); return sb.toString(); } }

MeasureSpec解析：

MeasureSpec是View中的内部类，基本都是二进制运算。由于int是32位的，用高两位表示mode，低30位表示size，MODE_SHIFT = 30的作用是移位

MODE_MASK = 0x3 << MODE_SHIFT，其中0x3是十六进制，它的二进制是11，向左移位30，结果是11000.....0000（一共30个0），1与任何数“&运算”均为任何数，0与任何数“&运算”均为0，故它的作用是获取measureSpec的高两位的mode，或者低30位的size（MODE_MASK取返，00111...111）

UNSPECIFIED = 0 << MODE_SHIFT，十进制0的二进制是00，左移30位是00000...000（00+30个0）

EXACTLY = 1 << MODE_SHIFT，十进制1的二进制是01，左移30位是00000...000（01+30个0）

AT_MOST = 2 << MODE_SHIFT，十进制2的二进制是10，左移30位是00000...000（10+30个0）

测量模式表示意思UNSPECIFIED父容器没有对当前View有任何限制，当前View可以任意取尺寸EXACTLY当前的尺寸就是当前View应该取的尺寸AT_MOST当前尺寸是当前View能取的最大尺寸 普通View的MeasureSpec创建规则标题 ViewGroup类public static int getChildMeasureSpec(int spec, int padding, int childDimension)源码 /** * 目标是将父控件的测量规格和child view的布局参数LayoutParams相结合，得到一个 * 最可能符合条件的child view的测量规格。 * * @param spec 父控件的测量规格 * @param padding 父控件里已经占用的大小 * @param childDimension child view布局LayoutParams里的尺寸 * @return child view 的测量规格 */ public static int getChildMeasureSpec(int spec, int padding, int childDimension) { //父控件的测量模式 int specMode = MeasureSpec.getMode(spec); //父控件的测量大小 int specSize = MeasureSpec.getSize(spec); int size = Math.max(0, specSize - padding); int resultSize = 0; int resultMode = 0; switch (specMode) { // 当父控件的测量模式 是 精确模式，也就是有精确的尺寸了 case MeasureSpec.EXACTLY: //如果child的布局参数有固定值，比如"layout_width" = "100dp" //那么显然child的测量规格也可以确定下来了，测量大小就是100dp，测量模式也是EXACTLY if (childDimension >= 0) { resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } //如果child的布局参数是"match_parent"，也就是想要占满父控件 //而此时父控件是精确模式，也就是能确定自己的尺寸了，那child也能确定自己大小了 else if (childDimension == LayoutParams.MATCH_PARENT) { resultSize = size; resultMode = MeasureSpec.EXACTLY; } //如果child的布局参数是"wrap_content"，也就是想要根据自己的逻辑决定自己大小， //比如TextView根据设置的字符串大小来决定自己的大小 //那就自己决定呗，不过你的大小肯定不能大于父控件的大小嘛 //所以测量模式就是AT_MOST，测量大小就是父控件的size else if (childDimension == LayoutParams.WRAP_CONTENT) { resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // 当父控件的测量模式 是 最大模式，也就是说父控件自己还不知道自己的尺寸，但是大小不能超过size case MeasureSpec.AT_MOST: //同样的，既然child能确定自己大小，尽管父控件自己还不知道自己大小，也优先满足孩子的需求？？ if (childDimension >= 0) { resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } //child想要和父控件一样大，但父控件自己也不确定自己大小，所以child也无法确定自己大小 //但同样的，child的尺寸上限也是父控件的尺寸上限size else if (childDimension == LayoutParams.MATCH_PARENT) { resultSize = size; resultMode = MeasureSpec.AT_MOST; } //child想要根据自己逻辑决定大小，那就自己决定呗 else if (childDimension == LayoutParams.WRAP_CONTENT) { 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); }

MeasureSpec方法：  

int widthMode = MeasureSpec.getMode(widthMeasureSpec); int widthSize = MeasureSpec.getSize(widthMeasureSpec);

布局：onLayout()

代码示例：

@Override protected void onLayout(boolean changed, int l, int t, int r, int b) { int lineCount = allLines.size(); int curL = getPaddingLeft(); int curT = getPaddingTop(); for (int i = 0; i < lineCount; i++) { List<View> lineViews = allLines.get(i); int lineHeight = lineHeights.get(i); for (int j = 0; j < lineViews.size(); j++) { View view = lineViews.get(j); int left = curL; int top = curT; int right = left + view.getMeasuredWidth(); int bottom = top + view.getMeasuredHeight(); view.layout(left, top, right, bottom); curL = right + mHorizontalSpacing; } curT = curT + lineHeight + mVerticalSpacing; curL = getPaddingLeft(); } }

计算ViewGroup子View的 left 、top、right、bottom坐标，调用View.layout方法在ViewGroup上布局View

View.Layout()函数：public void layout(int l, int t, int r, int b)

boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);