OverView
官方文档中关于ImageView的介绍是：
Displays image resources, for example Bitmap or Drawable resources. ImageView is also commonly used to apply tints to an image and handle image scaling.
即用来展示图像资源的控件。
其继承层次为
image.png
ImageView.ScaleType
在ImageView中有一个内部枚举类ScaleType，用来控制图像和ImageView的尺寸不一致时的拉伸规则：
| 枚举值 | 含义 |
|---|---|
| CENTER | 不拉伸图像，居中置于ImageView中 |
| CENTER_CROP | 等比例拉伸图像，直至长和宽都等于或者大于ImageView的对应边，然后居中置于ImageView，（如果本来的长宽就都大于ImageView的对应边，则不拉伸） |
| CENTER_INSIDE | 等比例拉伸图像，直至长和宽都等于或者小于ImageView的对应边，然后居中置于ImageView(如果本来的长宽就都小于ImageView的对应边，则不拉伸) |
| FILL_XY | 图像的长和宽都拉伸到和ImageView一致，以充满ImageView，这种情况下会改变图像的比例 |
| FILL_CENTER | 等比例拉伸图片，直到某一边刚好和ImageView一致，然后居中置于ImageView |
| FILL_START | 等比例拉伸图片，直到某一边刚好和ImageView一致，然后靠开始位置置于ImageView |
| FILL_END | 等比例拉伸图片，直到某一边刚好和ImageView一致，然后靠结束位置置于ImageView |
| MATRIX | 从左上角开始平铺图像 |
构造函数
public ImageView(Context context) {
super(context);
initImageView();
}
public ImageView(Context context, @Nullable AttributeSet attrs) {
this(context, attrs, 0);
}
public ImageView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
this(context, attrs, defStyleAttr, 0);
}
public ImageView(Context context, @Nullable AttributeSet attrs, int defStyleAttr,
int defStyleRes) {
super(context, attrs, defStyleAttr, defStyleRes);
initImageView();
if (getImportantForAutofill() == IMPORTANT_FOR_AUTOFILL_AUTO) {
setImportantForAutofill(IMPORTANT_FOR_AUTOFILL_NO);
}
final TypedArray a = context.obtainStyledAttributes(
attrs, R.styleable.ImageView, defStyleAttr, defStyleRes);
final Drawable d = a.getDrawable(R.styleable.ImageView_src);
if (d != null) {
setImageDrawable(d);
}
mBaselineAlignBottom = a.getBoolean(R.styleable.ImageView_baselineAlignBottom, false);
mBaseline = a.getDimensionPixelSize(R.styleable.ImageView_baseline, -1);
setAdjustViewBounds(a.getBoolean(R.styleable.ImageView_adjustViewBounds, false));
setMaxWidth(a.getDimensionPixelSize(R.styleable.ImageView_maxWidth, Integer.MAX_VALUE));
setMaxHeight(a.getDimensionPixelSize(R.styleable.ImageView_maxHeight, Integer.MAX_VALUE));
final int index = a.getInt(R.styleable.ImageView_scaleType, -1);
if (index >= 0) {
setScaleType(sScaleTypeArray[index]);
}
if (a.hasValue(R.styleable.ImageView_tint)) {
mDrawableTintList = a.getColorStateList(R.styleable.ImageView_tint);
mHasDrawableTint = true;
mDrawableTintMode = PorterDuff.Mode.SRC_ATOP;
mHasDrawableTintMode = true;
}
if (a.hasValue(R.styleable.ImageView_tintMode)) {
mDrawableTintMode = Drawable.parseTintMode(a.getInt(
R.styleable.ImageView_tintMode, -1), mDrawableTintMode);
mHasDrawableTintMode = true;
}
applyImageTint();
final int alpha = a.getInt(R.styleable.ImageView_drawableAlpha, 255);
if (alpha != 255) {
setImageAlpha(alpha);
}
mCropToPadding = a.getBoolean(
R.styleable.ImageView_cropToPadding, false);
a.recycle();
}
View的构造函数的写法都比较模板化，4个构造函数，2参数和3参数最终调用到的是4参数的构造函数，4参数的构造函数从xml中解析出各个属性也和其他View的构造函数是相同套路
单参数和4参数的构造函数，主要的工作都是调用了initImageView()函数
看下initImageView()函数的实现：
private void initImageView() {
mMatrix = new Matrix();
mScaleType = ScaleType.FIT_CENTER;
if (!sCompatDone) {
final int targetSdkVersion = mContext.getApplicationInfo().targetSdkVersion;
sCompatAdjustViewBounds = targetSdkVersion <= Build.VERSION_CODES.JELLY_BEAN_MR1;
sCompatUseCorrectStreamDensity = targetSdkVersion > Build.VERSION_CODES.M;
sCompatDrawableVisibilityDispatch = targetSdkVersion < Build.VERSION_CODES.N;
sCompatDone = true;
}
}
initImageView()函数也很简单，初始化了一个mMatrix变量，将mScaleType设置成默认的ScaleType.FIT_CENTER（后面展开说下ScaleType这个枚举类）
接下来看下4参数中调用的setImageDrawable(Drawable d)的调用，因为在实际使用中，ImageView的最关键就在于把图像展示出来
public void setImageDrawable(@Nullable Drawable drawable) {
if (mDrawable != drawable) {
mResource = 0;
mUri = null;
final int oldWidth = mDrawableWidth;
final int oldHeight = mDrawableHeight;
updateDrawable(drawable);
if (oldWidth != mDrawableWidth || oldHeight != mDrawableHeight) {
requestLayout();
}
invalidate();
}
}
主要逻辑在updateDrawable(Drawable d)里面
private void updateDrawable(Drawable d) {
if (d != mRecycleableBitmapDrawable && mRecycleableBitmapDrawable != null) {
mRecycleableBitmapDrawable.setBitmap(null);
}
boolean sameDrawable = false;
if (mDrawable != null) {
sameDrawable = mDrawable == d;
mDrawable.setCallback(null);
unscheduleDrawable(mDrawable);
if (!sCompatDrawableVisibilityDispatch && !sameDrawable && isAttachedToWindow()) {
mDrawable.setVisible(false, false);
}
}
mDrawable = d;
if (d != null) {
d.setCallback(this);
d.setLayoutDirection(getLayoutDirection());
if (d.isStateful()) {
d.setState(getDrawableState());
}
if (!sameDrawable || sCompatDrawableVisibilityDispatch) {
final boolean visible = sCompatDrawableVisibilityDispatch
? getVisibility() == VISIBLE
: isAttachedToWindow() && getWindowVisibility() == VISIBLE && isShown();
d.setVisible(visible, true);
}
d.setLevel(mLevel);
mDrawableWidth = d.getIntrinsicWidth();
mDrawableHeight = d.getIntrinsicHeight();
applyImageTint();
applyColorMod();
configureBounds();
} else {
mDrawableWidth = mDrawableHeight = -1;
}
}
其中的applyImageTint()和applyColorMod()函数的作用是设置着色、颜色相关的属性，比较关键的函数调用是configureBounds()
private void configureBounds() {
if (mDrawable == null || !mHaveFrame) {
return;
}
final int dwidth = mDrawableWidth;
final int dheight = mDrawableHeight;
final int vwidth = getWidth() - mPaddingLeft - mPaddingRight;
final int vheight = getHeight() - mPaddingTop - mPaddingBottom;
final boolean fits = (dwidth < 0 || vwidth == dwidth)
&& (dheight < 0 || vheight == dheight);
if (dwidth <= 0 || dheight <= 0 || ScaleType.FIT_XY == mScaleType) {
/* If the drawable has no intrinsic size, or we're told to
scaletofit, then we just fill our entire view.
*/
mDrawable.setBounds(0, 0, vwidth, vheight);
mDrawMatrix = null;
} else {
// We need to do the scaling ourself, so have the drawable
// use its native size.
mDrawable.setBounds(0, 0, dwidth, dheight);
if (ScaleType.MATRIX == mScaleType) {
// Use the specified matrix as-is.
if (mMatrix.isIdentity()) {
mDrawMatrix = null;
} else {
mDrawMatrix = mMatrix;
}
} else if (fits) {
// The bitmap fits exactly, no transform needed.
mDrawMatrix = null;
} else if (ScaleType.CENTER == mScaleType) {
// Center bitmap in view, no scaling.
mDrawMatrix = mMatrix;
mDrawMatrix.setTranslate(Math.round((vwidth - dwidth) * 0.5f),
Math.round((vheight - dheight) * 0.5f));
} else if (ScaleType.CENTER_CROP == mScaleType) {
mDrawMatrix = mMatrix;
float scale;
float dx = 0, dy = 0;
if (dwidth * vheight > vwidth * dheight) {
scale = (float) vheight / (float) dheight;
dx = (vwidth - dwidth * scale) * 0.5f;
} else {
scale = (float) vwidth / (float) dwidth;
dy = (vheight - dheight * scale) * 0.5f;
}
mDrawMatrix.setScale(scale, scale);
mDrawMatrix.postTranslate(Math.round(dx), Math.round(dy));
} else if (ScaleType.CENTER_INSIDE == mScaleType) {
mDrawMatrix = mMatrix;
float scale;
float dx;
float dy;
if (dwidth <= vwidth && dheight <= vheight) {
scale = 1.0f;
} else {
scale = Math.min((float) vwidth / (float) dwidth,
(float) vheight / (float) dheight);
}
dx = Math.round((vwidth - dwidth * scale) * 0.5f);
dy = Math.round((vheight - dheight * scale) * 0.5f);
mDrawMatrix.setScale(scale, scale);
mDrawMatrix.postTranslate(dx, dy);
} else {
// Generate the required transform.
mTempSrc.set(0, 0, dwidth, dheight);
mTempDst.set(0, 0, vwidth, vheight);
mDrawMatrix = mMatrix;
mDrawMatrix.setRectToRect(mTempSrc, mTempDst, scaleTypeToScaleToFit(mScaleType));
}
}
}
函数的主要部分是一大堆的if条件对ScaleType的判断，对相应的ScaleType进行图像的矩阵Matrix处理
onMeasure()
看完构造函数，接下来开始看绘制相关的内容，首先是第一阶段的onMeasure()
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
resolveUri();
int w;
int h;
// Desired aspect ratio of the view's contents (not including padding)
float desiredAspect = 0.0f;
// We are allowed to change the view's width
boolean resizeWidth = false;
// We are allowed to change the view's height
boolean resizeHeight = false;
final int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
if (mDrawable == null) {
// If no drawable, its intrinsic size is 0.
mDrawableWidth = -1;
mDrawableHeight = -1;
w = h = 0;
} else {
w = mDrawableWidth;
h = mDrawableHeight;
if (w <= 0) w = 1;
if (h <= 0) h = 1;
// We are supposed to adjust view bounds to match the aspect
// ratio of our drawable. See if that is possible.
if (mAdjustViewBounds) {
resizeWidth = widthSpecMode != MeasureSpec.EXACTLY;
resizeHeight = heightSpecMode != MeasureSpec.EXACTLY;
desiredAspect = (float) w / (float) h;
}
}
final int pleft = mPaddingLeft;
final int pright = mPaddingRight;
final int ptop = mPaddingTop;
final int pbottom = mPaddingBottom;
int widthSize;
int heightSize;
if (resizeWidth || resizeHeight) {
/* If we get here, it means we want to resize to match the
drawables aspect ratio, and we have the freedom to change at
least one dimension.
*/
// Get the max possible width given our constraints
widthSize = resolveAdjustedSize(w + pleft + pright, mMaxWidth, widthMeasureSpec);
// Get the max possible height given our constraints
heightSize = resolveAdjustedSize(h + ptop + pbottom, mMaxHeight, heightMeasureSpec);
if (desiredAspect != 0.0f) {
// See what our actual aspect ratio is
final float actualAspect = (float)(widthSize - pleft - pright) /
(heightSize - ptop - pbottom);
if (Math.abs(actualAspect - desiredAspect) > 0.0000001) {
boolean done = false;
// Try adjusting width to be proportional to height
if (resizeWidth) {
int newWidth = (int)(desiredAspect * (heightSize - ptop - pbottom)) +
pleft + pright;
// Allow the width to outgrow its original estimate if height is fixed.
if (!resizeHeight && !sCompatAdjustViewBounds) {
widthSize = resolveAdjustedSize(newWidth, mMaxWidth, widthMeasureSpec);
}
if (newWidth <= widthSize) {
widthSize = newWidth;
done = true;
}
}
// Try adjusting height to be proportional to width
if (!done && resizeHeight) {
int newHeight = (int)((widthSize - pleft - pright) / desiredAspect) +
ptop + pbottom;
// Allow the height to outgrow its original estimate if width is fixed.
if (!resizeWidth && !sCompatAdjustViewBounds) {
heightSize = resolveAdjustedSize(newHeight, mMaxHeight,
heightMeasureSpec);
}
if (newHeight <= heightSize) {
heightSize = newHeight;
}
}
}
}
} else {
/* We are either don't want to preserve the drawables aspect ratio,
or we are not allowed to change view dimensions. Just measure in
the normal way.
*/
w += pleft + pright;
h += ptop + pbottom;
w = Math.max(w, getSuggestedMinimumWidth());
h = Math.max(h, getSuggestedMinimumHeight());
widthSize = resolveSizeAndState(w, widthMeasureSpec, 0);
heightSize = resolveSizeAndState(h, heightMeasureSpec, 0);
}
setMeasuredDimension(widthSize, heightSize);
}
- 首先会调用一次 resolveUri()，这个函数的作用是，确保在进行onMeasure()时已经有mDrawble，resolveUri()会在未设置mDrawble的情况下通过Uri来获取Drawable
- 通过mDrawable的宽高来设置w和h
- 默认情况下resizeWidth/resizeHeight取值为false，这样只会走最下面的一段else分支，w/h加上对应的padding，然后通过resolveSizeAndState产生相应的
MeasureSpec - 假如resizeWidth/resizeHeight为true，会走到中间的if代码块中，这个代码块的作用是，根据比例规则重算宽高
onLayout()
因为ImageView只是普通View的子类，因此没有重写onLayout()
onDraw()
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
if (mDrawable == null) {
return; // couldn't resolve the URI
}
if (mDrawableWidth == 0 || mDrawableHeight == 0) {
return; // nothing to draw (empty bounds)
}
if (mDrawMatrix == null && mPaddingTop == 0 && mPaddingLeft == 0) {
mDrawable.draw(canvas);
} else {
final int saveCount = canvas.getSaveCount();
canvas.save();
if (mCropToPadding) {
final int scrollX = mScrollX;
final int scrollY = mScrollY;
canvas.clipRect(scrollX + mPaddingLeft, scrollY + mPaddingTop,
scrollX + mRight - mLeft - mPaddingRight,
scrollY + mBottom - mTop - mPaddingBottom);
}
canvas.translate(mPaddingLeft, mPaddingTop);
if (mDrawMatrix != null) {
canvas.concat(mDrawMatrix);
}
mDrawable.draw(canvas);
canvas.restoreToCount(saveCount);
}
}
在onDraw()的源码可以看出，ImageView的绘制，最终是通过Drawable的draw(Canvas canvas)方法完成的。
