写在前面
自定义View的绘制流程：onMeasure() -> onLayout() ->onDraw(),在分析源码之前需要了解一下MeasureSpec类。
MeasureSpec代表一个32位int值，高2位代表SpecMode，低30位代表SpecSize，SpecMode是指测量模式，而specSize是指在某种测量模式下的规格大小。
其作用是测量一个视图View的宽/高。每个MeasureSpec代表一组宽高的测量规格。
查看MeasureSpec类源码
public static class MeasureSpec {
//进位大小为2的30次方
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
//测量模式specMode的三种类型：UNSPECIFIED ，EXACTLY，AT_MOST
//UNSPECIFIED的模式 0向左 进位30
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
public static final int EXACTLY = 1 << MODE_SHIFT;
public static final int AT_MOST = 2 << MODE_SHIFT;
//根据提供的size和mode创建测量规格
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);
}
}
//通过MeasureSpec获取测量模式mode
@MeasureSpecMode
public static int getMode(int measureSpec) {
//noinspection ResourceType
return (measureSpec & MODE_MASK);
}
//通过MeasureSpec获取测量大小size
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
//根据size或mode调整测量规格
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);
}
}
使用MeasureSpec的目的是减少对象内存分配
源码分析
1. 单一View的onMeasure()过程
measure： 计算View的宽/高
measure测量过程的入口为measure()
查看View.java$measure方法核心源码
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
......//代码省略
int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
if (cacheIndex < 0 || sIgnoreMeasureCache) {
//根据View的宽/高测量规格计算View的宽/高值
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
......//代码省略
}
该方法定义为final类，即子类中不能重写该方法
查看View.java$onMeasure方法源码
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
//继续查看setMeasuredDimension方法源码
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
//View是否使用了视觉边界布局
boolean optical = isLayoutModeOptical(this);
//mParent为ViewParent对象，为该View的父类
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;
}
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
//继续查看
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
// 将测量后View的宽 / 高值进行传递
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
//查看getDefaultSize方法源码
public static int getDefaultSize(int size, int measureSpec) {
//设置默认大小
int result = size;
// 获取View的宽/高测量规格模式mode，测量大小size
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
//模式为UNSPECIFIED时，使用默认大小
case MeasureSpec.UNSPECIFIED:
result = size;
break;
//模式为AT_MOST，EXACTLY时，使用View测量后的宽/高值
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
//默认大小size = getSuggestedMinimumWidth()/getSuggestedMinimumHeight()
//分析getSuggestedMinimumHeight()
protected int getSuggestedMinimumHeight() {
//若View无设置背景则View的高度为mMinHeight
//mMinHeight为android:minHeight属性值，若未设置则为0
//若设置了View的背景，把背景图高度与mMinHeight相比取最大值，
return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());
}
//继续查看Drawable.java$getMinimumHeight方法源码
public int getMinimumHeight() {
//获取背景图Drawable的原始高度
final int intrinsicHeight = getIntrinsicHeight();
return intrinsicHeight > 0 ? intrinsicHeight : 0;
}
以上是单一View的onMeasure测量过程，下面给出其流程图。
单一View的measure过程.jpg
2. 单一View的onLayout() 过程
layout过程： 计算本身View的的位置（left、right、top和bottom）
计算View的位置为Layout方法 查看View.java$layout方法核心源码
public void layout(int l, int t, int r, int b) {
......//代码省略
//当前视图View的四个顶点
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
//判断视图View大小和位置是否发生了变化
//islayoutModeOptical判断是否使用View的视图边界布局
//若使用则调用setOpticalFrame方法根据传入的值设置View的四个顶点位置
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
//若View大小和位置发生改变则调用该方法
//在单一View中因无子View该方法为空方法
onLayout(changed, l, t, r, b);
......//代码省略
}
//继续分析setFrame()方法核心源码
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
......//代码省略
//判断View位置是否发生改边
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
changed = true;
......//代码省略
//设置View的四个顶点位置
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
......//代码省略
}
return changed;
}
//查看setOpticalFrame方法源码
private boolean setOpticalFrame(int left, int top, int right, int bottom) {
Insets parentInsets = mParent instanceof View ?
((View) mParent).getOpticalInsets() : Insets.NONE;
Insets childInsets = getOpticalInsets();
//内部调用的是setFrame方法
return setFrame(
left + parentInsets.left - childInsets.left,
top + parentInsets.top - childInsets.top,
right + parentInsets.left + childInsets.right,
bottom + parentInsets.top + childInsets.bottom);
}
//查看onLayout方法源码
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
//该方法为空方法，单一View中在layout()中已经对自身View进行了位置计算
}
layout的整个计算流程
单一View的layout过程.png
3. 单一View的onDraw过程
draw过程： 仅绘制视图View本身
View的绘制入口为draw()方法 查看View.java$draw方法核心源码
public void draw(Canvas canvas) {
......//代码省略
// 步骤1：绘制View自身背景
int saveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
......//代码省略
// 步骤2：保存画布图层
int solidColor = getSolidColor();
if (solidColor == 0) {
final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;
if (drawTop) {
canvas.saveLayer(left, top, right, top + length, null, flags);
}
if (drawBottom) {
canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
}
if (drawLeft) {
canvas.saveLayer(left, top, left + length, bottom, null, flags);
}
if (drawRight) {
canvas.saveLayer(right - length, top, right, bottom, null, flags);
}
} else {
scrollabilityCache.setFadeColor(solidColor);
}
// 步骤3：绘制View内容
if (!dirtyOpaque) onDraw(canvas);
// 步骤4：绘制子View视图
dispatchDraw(canvas);
// 步骤5：绘制淡入淡出效果并还原图层
final Paint p = scrollabilityCache.paint;
final Matrix matrix = scrollabilityCache.matrix;
final Shader fade = scrollabilityCache.shader;
if (drawTop) {
matrix.setScale(1, fadeHeight * topFadeStrength);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, right, top + length, p);
}
if (drawBottom) {
matrix.setScale(1, fadeHeight * bottomFadeStrength);
matrix.postRotate(180);
matrix.postTranslate(left, bottom);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, bottom - length, right, bottom, p);
}
if (drawLeft) {
matrix.setScale(1, fadeHeight * leftFadeStrength);
matrix.postRotate(-90);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, left + length, bottom, p);
}
if (drawRight) {
matrix.setScale(1, fadeHeight * rightFadeStrength);
matrix.postRotate(90);
matrix.postTranslate(right, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(right - length, top, right, bottom, p);
}
canvas.restoreToCount(saveCount);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// 步骤6：绘制装饰（比如前景色，滑动条）
onDrawForeground(canvas);
}
//查看步骤1 drawBackground方法源码
private void drawBackground(Canvas canvas) {
//获取背景图
final Drawable background = mBackground;
if (background == null) {
return;
}
//根据layout过程中得到的View的位置参数，来设置背景的边界
setBackgroundBounds();
......// 代码省略
final int scrollX = mScrollX;
final int scrollY = mScrollY;
if ((scrollX | scrollY) == 0) {
//调用draw方法绘制背景
background.draw(canvas);
} else {
//若scrollX或scrollY不为0 则对canvas的坐标进行旋转
canvas.translate(scrollX, scrollY);
background.draw(canvas);
canvas.translate(-scrollX, -scrollY);
}
}
//查看setBackgroundBounds方法
void setBackgroundBounds() {
if (mBackgroundSizeChanged && mBackground != null) {
//根据layout计算获取的View的顶点位置，对背景图设置边界
mBackground.setBounds(0, 0, mRight - mLeft, mBottom - mTop);
mBackgroundSizeChanged = false;
rebuildOutline();
}
}
//查看步骤3 onDraw方法源码
protected void onDraw(Canvas canvas) {
//该方法为空实现需要复写该方法 绘制View自身内容
}
//查看步骤4 dispatchDraw方法源码
protected void dispatchDraw(Canvas canvas) {
//由于单一View无子View所以该方法为空实现
//在ViewGroup中该方法为绘制子View
}
//查看步骤6 onDrawForeground方法源码
public void onDrawForeground(Canvas canvas) {
//绘制滑动bar
onDrawScrollIndicators(canvas);
onDrawScrollBars(canvas);
final Drawable foreground = mForegroundInfo != null ? mForegroundInfo.mDrawable : null;
if (foreground != null) {
......//代码省略
//绘制前景色
foreground.draw(canvas);
}
}
单一View的draw绘制过程
单一View的draw过程.png
考虑到分析ViewGroup源码会导致篇幅较长，View和ViewGroup绘制过程分开分析。
下一篇为ViewGroup绘制过程源码分析
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