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public class MyTextView extends View {
//字体颜色
private int MyTextColor = Color.parseColor("#000000");
//要绘制的文字
private String MyTextString = "";
//默认文字大小
private int MyTextSize = 15;
//画笔
private Paint mPaint;
public MyTextView(Context context) {
this(context, null);
}
public MyTextView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public MyTextView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
//获取自定义属性
TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.MyTextView);
MyTextColor = a.getColor(R.styleable.MyTextView_myTextColor, MyTextColor);
MyTextString = a.getString(R.styleable.MyTextView_myText);
MyTextSize = a.getDimensionPixelSize(R.styleable.MyTextView_myTextSize, sp2px(MyTextSize));
a.recycle();
//实例化画笔
mPaint = new Paint();
mPaint.setAntiAlias(true);
mPaint.setColor(MyTextColor);
mPaint.setTextSize(MyTextSize);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
//获取宽高模式
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
//获取宽高
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
if (widthMode == MeasureSpec.AT_MOST) {
//测量文字宽度
Rect rect = new Rect();
mPaint.getTextBounds(MyTextString, 0, MyTextString.length(), rect);
widthSize = rect.width() + getPaddingLeft() + getPaddingRight();
}
if (heightMode == MeasureSpec.AT_MOST) {
//测量文字的高度
Rect rect = new Rect();
mPaint.getTextBounds(MyTextString, 0, MyTextString.length(), rect);
heightSize = rect.height() + getPaddingTop() + getPaddingBottom();
}
setMeasuredDimension(widthSize, heightSize);
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
int x = getPaddingLeft();
//计算文字基线
Paint.FontMetricsInt metricsInt = mPaint.getFontMetricsInt();
Log.e("MyTextView-bottom",metricsInt.bottom+"");
Log.e("MyTextView-top",metricsInt.top+"");
//需要注意metricsInt.bottom是正值 metricsInt.top是负值
int dy = (metricsInt.bottom - metricsInt.top) / 2 - metricsInt.bottom;
int y = getHeight() / 2 + dy;
canvas.drawText(MyTextString, x, y, mPaint);
}
private int sp2px(int sp) {
return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_SP, sp, getResources().getDisplayMetrics());
}
}效果：
上面是一个简单的自定义textview效果，主要是说明下自定义view的大致实现流程，就是extend View，重写onMeasure、onDraw方法等，在onMeasure测试模式中介绍了系统的三种模式，在自定义view中如果给的AT_MOST模式，就需要通过Paint画笔进行测量，重新设置绘制宽高；在通过canvas.drawText()方法绘制文字时需要注意绘制文字的y，这里的y指的是文字的基线，而不是文字高度的一半；
需要注意的是metricsInt.bottom得到的是正值，metricsInt.top得到的是负值；这个是extend view实现的效果，其实有时候不注意的时候会extend ViewGroup，这个时候并没有出现想要的效果；这就涉及到view中的draw()方法，在onDraw中进行绘制，其实回去调用view中的draw方法；
//这里是View中draw方法的源码
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)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
// we're done...
return;
}
/*
* Here we do the full fledged routine...
* (this is an uncommon case where speed matters less,
* this is why we repeat some of the tests that have been
* done above)
*/
boolean drawTop = false;
boolean drawBottom = false;
boolean drawLeft = false;
boolean drawRight = false;
float topFadeStrength = 0.0f;
float bottomFadeStrength = 0.0f;
float leftFadeStrength = 0.0f;
float rightFadeStrength = 0.0f;
// Step 2, save the canvas' layers
int paddingLeft = mPaddingLeft;
final boolean offsetRequired = isPaddingOffsetRequired();
if (offsetRequired) {
paddingLeft += getLeftPaddingOffset();
}
int left = mScrollX + paddingLeft;
int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
int top = mScrollY + getFadeTop(offsetRequired);
int bottom = top + getFadeHeight(offsetRequired);
if (offsetRequired) {
right += getRightPaddingOffset();
bottom += getBottomPaddingOffset();
}
final ScrollabilityCache scrollabilityCache = mScrollCache;
final float fadeHeight = scrollabilityCache.fadingEdgeLength;
int length = (int) fadeHeight;
// clip the fade length if top and bottom fades overlap
// overlapping fades produce odd-looking artifacts
if (verticalEdges && (top + length > bottom - length)) {
length = (bottom - top) / 2;
}
// also clip horizontal fades if necessary
if (horizontalEdges && (left + length > right - length)) {
length = (right - left) / 2;
}
if (verticalEdges) {
topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
drawTop = topFadeStrength * fadeHeight > 1.0f;
bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
}
if (horizontalEdges) {
leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
drawLeft = leftFadeStrength * fadeHeight > 1.0f;
rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
drawRight = rightFadeStrength * fadeHeight > 1.0f;
}
saveCount = canvas.getSaveCount();
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);
}
// 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
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);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
}上面是View中draw的源码，其中采用了模板设计模式，在其中有这样一段代码：
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);很明显dirtyOpaque这个标识为false的时候才会去调用onDraw方法，extend ViewGroup没有效果，那就说明dirtyOpaque标识不为false而没有调用onDraw方法；
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;dirtyOpaque标识在已进入draw方法的时候通过privateFlags，最终又是通过mPrivateFlags来设置的，mPrivateFlags的话则是在View的构造函数中通过computeOpaqueFlags()方法来设置的；
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;
}
}这样的话，那就去看看是不是ViewGoup中通过方法将这一标识进行了修改；在ViewGroup的构造方法中有一个initViewGroup()方法；
private void initViewGroup() {
// ViewGroup doesn't draw by default
if (!debugDraw()) {
//这里调用的是View中的setFlags方法
setFlags(WILL_NOT_DRAW, DRAW_MASK);
}
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;
}在ViewGroup中的initViewGroup方法中调用了View中的setFlags方法将标识修改成了WILL_NOT_DRAW,就是不去调用onDraw方法；到这里差不多就明白了为什么extend ViewGroup会没有效果了，那就是说只要改变这一标识extend ViewGroup就会和extend View一样有效果咯；方法有很多种；
方法一：
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其实会发现在draw方法中的dispatchDraw()就没有标识来判断，就是不管extend ViewGroup还是extend View都会被调用，那就在extend ViewGroup的时候不重写onDraw方法而去重写dispatchDraw()方法，在dispatchDraw()方法中进行绘制；
方法二：
还是extend ViewGroup，重写的也是onDraw方法，但是给自定义view设置setBackground也会实现同样的效果；设置背景的时候回去调用View中的setBackground方法；
//view中setBackground源码
public void setBackground(Drawable background) {
//noinspection deprecation
setBackgroundDrawable(background);
}接着调用了setBackgroundDrawable()方法；一进入setBackgroundDrawable()方法就看到调用了computeOpaqueFlags()方法，在上面可以看到，computeOpaqueFlags()里面会对mPrivateFlags进行赋值，所以就看到效果了；
方法三：
既然ViewGroup可以通过setFlags方法设置不调用onDraw方法，那可以去找下是否可以通过调用方法来设置mPrivateFlags标识；
/**
* If this view doesn't do any drawing on its own, set this flag to
* allow further optimizations. By default, this flag is not set on
* View, but could be set on some View subclasses such as ViewGroup.
*
* Typically, if you override {@link #onDraw(android.graphics.Canvas)}
* you should clear this flag.
*
* @param willNotDraw whether or not this View draw on its own
*/
public void setWillNotDraw(boolean willNotDraw) {
setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);
}会看到这个方法，传入一个false就可以改变mPrivateFlags标识，从而可以实现效果，其实调用setFocusable(false)；等方法都可以实现；