绘制机制
绘制流程
- ViewRootImpl.performTraversals 起点
- Measure 决定view大小
- Layout 决定view的位置
- Draw 绘制
performTraversals
依次执行measure,layout,draw
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);
}
}
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
...
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
...
}
private void performDraw() {
...
}
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
...
}
measure
父容器传递measureSpec至子容器。子容器根据measureSpec，并结合自身的layoutParams，测量出大小。父容器最后根据子容器的测量值，再测量出自身规格.
- 元素visibility为GONE的不参与measure
- 当viewGroup的子类的属性有margin或padding时，子类会执行viewGroup中的measureChildWithMargins，最终测量的大小会specSize - padding。
- View中的源码
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
//获取的是android:minHeight属性的值或者View背景图片的大小
protected int getSuggestedMinimumHeight() {
return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());
}
//获取默认的大小,size为自身大小，measureSpec为父元素传下来的，可见一个view的大小是由父元素的measureSpec和view的layoutParams共通决定
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;
}
- 其它view的子类有各自的实现方式，不是直接拿measurespec和layoutParams进行测量
- ViewGroup中没有实现onMeasure,都由ViewGroup中的子类进行实现，如FrameLayout等，ViewGroup的子类在onMeasure过程中，递归所有子view，执行measureChildWithMargins、measure、onMeasure。测量完所有子View后，再通过用view中的setMeasuredDimension进行测量。
measureChildWithMargins
- 如果view有固定值，则view的measureSpec的mode为EXACTLY，size为该固定值。
- 在父元素为EXACTLY的情况下。子元素若为match_parent,则子元素的measureSpec，mode则为EXACTLY,size为父元素size。子元素若为wrap_content,则子元素的measureSpec，mode为AT_MOST,size为Math.max(0, specSize - padding)
- 在父元素为AT_MOST的情况下。子元素无论layoutParams是match_parent还是wrap_content,子元素的measureSpec，mode都为AT_MOST,size都为Math.max(0, specSize - padding)
- 在父元素为UNSPECIFIED时，子元素无论是match还是wrap，子元素的measureSpec,mode为UNSPECIFIED，size为View.sUseZeroUnspecifiedMeasureSpec ? 0 : size；
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
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 = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
layout
- 最先执行的是viewGroup中的layout,判断当前有没有正在执行的layoutTransition,没有就调用view中的layout，否则，就等transition执行完后，继续调用。
- view的layout中，有两种设置view位置的方法，一种是setOpticalFrame，另外一种是setFrame。调用哪个，取决于父对象是不是viewGroup，且是否可见，若全是，则调用setOpticalFrame,getOpticalInsets则是获取drawable的insets。
private boolean setOpticalFrame(int left, int top, int right, int bottom) {
Insets parentInsets = mParent instanceof View ?
((View) mParent).getOpticalInsets() : Insets.NONE;
Insets childInsets = getOpticalInsets();
return setFrame(
left + parentInsets.left - childInsets.left,
top + parentInsets.top - childInsets.top,
right + parentInsets.left + childInsets.right,
bottom + parentInsets.top + childInsets.bottom);
}
- setFrame结束后，还会执行OnLayoutChangeListener.onLayoutChange函数，若需要监听，layout的变化，可addOnLayoutChangeListener。
- layout的四个参数，默认是建立在measure的width和height基础上的，当也可以重写。并不依赖measure。如果重写ltrb，则getMeasuredWidth和getWidth取得的值不一致。
draw
ViewGroup中并没有重写draw，则执行view中draw
- 如果有设置背景，则绘制背景 Draw the background
- 保存canvas层 If necessary, save the canvas' layers to prepare for fading
- 绘制自身内容 Draw view's content
- 如果有子元素则绘制子元素 Draw children
- 绘制效果 If necessary, draw the fading edges and restore layers
- 绘制装饰品(scrollbars) Draw decorations (scrollbars for instance)
- canvas对象由ViewRootImpl的drawSoftware建立，传递至view中。
- 每当调用invalite时，都会将之前view区域设为dirty
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
...
canvas = mSurface.lockCanvas(dirty);
...
}
public Canvas lockCanvas(Rect inOutDirty)
throws Surface.OutOfResourcesException, IllegalArgumentException {
synchronized (mLock) {
checkNotReleasedLocked();
if (mLockedObject != 0) {
// Ideally, nativeLockCanvas() would throw in this situation and prevent the
// double-lock, but that won't happen if mNativeObject was updated. We can't
// abandon the old mLockedObject because it might still be in use, so instead
// we just refuse to re-lock the Surface.
throw new IllegalArgumentException("Surface was already locked");
}
mLockedObject = nativeLockCanvas(mNativeObject, mCanvas, inOutDirty);
return mCanvas;
}
}
surfaceView
measureSpec
- 一个measureSpec包含了父容器传递给子容器的布局要求。
- 包含了mode与size的组合int值，高两位为mode，后面是size。
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);
}
}
- 三种模式
-
- UPSPECIFIED : 父容器对于子容器没有任何限制,子容器想要多大就多大
-
- EXACTLY: 父容器已经为子容器设置了尺寸,子容器应当服从这些边界,不论子容器想要多大的空间。
-
- AT_MOST：子容器可以是声明大小内的任意大小
