1 关于关于TextView的baseline计算。
baseline如图所示
image
英文显示baseline比中文的更明显一些。可以通过FontMetrics来top等获取属性值，FontMetrics类介绍如下
/**
* Class that describes the various metrics for a font at a given text size.
* Remember, Y values increase going down, so those values will be positive,
* and values that measure distances going up will be negative. This class
* is returned by getFontMetrics().
*/
public static class FontMetrics {
/**
* The maximum distance above the baseline for the tallest glyph in
* the font at a given text size.
*/
public float top;
/**
* The recommended distance above the baseline for singled spaced text.
*/
public float ascent;
/**
* The recommended distance below the baseline for singled spaced text.
*/
public float descent;
/**
* The maximum distance below the baseline for the lowest glyph in
* the font at a given text size.
*/
public float bottom;
/**
* The recommended additional space to add between lines of text.
*/
public float leading;
}
通过Paint画笔获取FontMetrics或FontMetricsInt。FontMetrics是float类型，FontMetricsInt是int类型的返回值
Paint.FontMetrics fontMetrics=mPaint.getFontMetrics();
Paint.FontMetricsInt fontMetricsInt= mPaint.getFontMetricsInt();
然后就可以计算baseline了，可以通过top和bottom计算，也可以通过ascent和descent来计算。
int dy=(fontMetricsInt.bottom-fontMetricsInt.top)/2-fontMetricsInt.bottom;
int dy=(fontMetricsInt.descent -fontMetricsInt.ascent) / 2 - fontMetricsInt.descent;
canvas的drawText方法如下：
/**
* Draw the text, with origin at (x,y), using the specified paint. The origin is interpreted
* based on the Align setting in the paint.
*
* @param text The text to be drawn
* @param x The x-coordinate of the origin of the text being drawn
* @param y The y-coordinate of the baseline of the text being drawn
* @param paint The paint used for the text (e.g. color, size, style)
*/
public void drawText(@NonNull String text, float x, float y, @NonNull Paint paint) {
super.drawText(text, x, y, paint);
}
我们要注意第三个参数y与画布的原点位置有关系，比如如果原点在画布左上角，文字要画在画布Y轴中心那么
y=容器高度/2+dy
多试验试验就好了。
2自定义View继承ViewGroup 默认不会调用onDraw方法 为什么？
通过查看了一些资料，知道了是通过ViewRootImpl的performTraversals（）方法依次调用performMeasure（），performLayout（），performDraw（）方法，进而实现了View的measure、layout、draw三大流程。View中调用顺序为measure（），layout（），draw（）；
ViewGroup继承自View ，View的draw方法如下
@CallSuper
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
/*
* Draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. Draw the background
* 2. If necessary, save the canvas' layers to prepare for fading
* 3. Draw view's content
* 4. Draw children
* 5. If necessary, draw the fading edges and restore layers
* 6. Draw decorations (scrollbars for instance)
*/
// Step 1, draw the background, if needed
int saveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
// skip step 2 & 5 if possible (common case)
..........................
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Step 5, draw the fade effect and restore layers
..........................
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
}
主要有6个过程。发现了只有dirtyOpaque是false 才会调用onDraw()方法。而dirtyOpaque这个变量是由 mPrivateFlags控制的。mPrivateFlags 到底是怎么赋值的 通过查找代码， 发现在View的构造函数中调用 computeOpaqueFlags()来设置这个变量
protected void computeOpaqueFlags() {
// Opaque if:
// - Has a background
// - Background is opaque
// - Doesn't have scrollbars or scrollbars overlay
if (mBackground != null && mBackground.getOpacity() == PixelFormat.OPAQUE) {
mPrivateFlags |= PFLAG_OPAQUE_BACKGROUND;
} else {
mPrivateFlags &= ~PFLAG_OPAQUE_BACKGROUND;
}
final int flags = mViewFlags;
if (((flags & SCROLLBARS_VERTICAL) == 0 && (flags & SCROLLBARS_HORIZONTAL) == 0) ||
(flags & SCROLLBARS_STYLE_MASK) == SCROLLBARS_INSIDE_OVERLAY ||
(flags & SCROLLBARS_STYLE_MASK) == SCROLLBARS_OUTSIDE_OVERLAY) {
mPrivateFlags |= PFLAG_OPAQUE_SCROLLBARS;
} else {
mPrivateFlags &= ~PFLAG_OPAQUE_SCROLLBARS;
}
}
ViewGroup 为什么出不来 initViewGroup
private void initViewGroup() {
// ViewGroup doesn't draw by default
if (!debugDraw()) {
setFlags(WILL_NOT_DRAW, DRAW_MASK);
}
导致 mPrivateFlags 会重新赋值
mGroupFlags |= FLAG_CLIP_CHILDREN;
mGroupFlags |= FLAG_CLIP_TO_PADDING;
mGroupFlags |= FLAG_ANIMATION_DONE;
mGroupFlags |= FLAG_ANIMATION_CACHE;
mGroupFlags |= FLAG_ALWAYS_DRAWN_WITH_CACHE;
if (mContext.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.HONEYCOMB) {
mGroupFlags |= FLAG_SPLIT_MOTION_EVENTS;
}
setDescendantFocusability(FOCUS_BEFORE_DESCENDANTS);
mChildren = new View[ARRAY_INITIAL_CAPACITY];
mChildrenCount = 0;
mPersistentDrawingCache = PERSISTENT_SCROLLING_CACHE;
}
