聊聊自定义View那些事
聊聊自定义View那些事
- 为什么要自定义View
- 如何自定义View
- 为什么要重写onMeasure方法
- 如何重写onMeasure方法
- 总结
为什么要自定义View
- 为了实现某些炫酷的动画和效果。
- 为了优化应用性能。
- 系统控件满足不了需要
如何自定义View
- 继承系统View,实现相应的构造方法
public class CircleView extends View {
private Paint mPaint;
private float cx;
private float cy;
//这个是外部通过new 的方式调用
public CircleView(Context context) {
super(context);
}
//这个是外部通过xml 的方式调用
public CircleView(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
init();
}
public CircleView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
public CircleView(Context context, @Nullable AttributeSet attrs, int defStyleAttr, int defStyleRes) {
super(context, attrs, defStyleAttr, defStyleRes);
}
...
}
- 重写onDraw方法，因为这个方法是你把所有东西绘制上去，实现自定义View的基础。
private void init() {
mPaint = new Paint();
//抗锯齿
mPaint.setAntiAlias(true);
mPaint.setStyle(Paint.Style.STROKE);
mPaint.setStrokeWidth(5);
}
@Override
protected void onDraw(Canvas canvas) {
//super内部空实现
super.onDraw(canvas);
cx = getMeasuredWidth()/2;
cy = getMeasuredHeight()/2;
float radius = Math.min(cx,cy);
radius = radius-5;
//画了个圆
canvas.drawCircle(cx,cy,radius,mPaint);
}
为什么要重写onMeasure方法
- 配置到xml中，run一下，看下效果，你会发现这个圆出来了。但是超级大，那么系统是如何控制View的大小的呢？
<FrameLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">
<com.okay.myapplication.CircleView
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</FrameLayout>
- 我们重写onMeasure方法，看看父类究竟干了些什么工作？
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
Log.d("CircleView", MeasureSpec.getSize(widthMeasureSpec)+":"+ MeasureSpec.getSize(heightMeasureSpec));
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
这里先留下个问题，就是onMeasure方法走了两次，why?
先不管那个，我们看下源码是怎么帮我们测量view的宽高的。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
/**
* @return The suggested minimum width of the view.
* 得到最小的宽度，如果设置了Background，取最小宽度和背景的最小宽度之中的最大值
*/
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
/**
* Utility to return a default size. Uses the supplied size if the
* MeasureSpec imposed no constraints. Will get larger if allowed
* by the MeasureSpec.
*
* @param size Default size for this view
* @param measureSpec Constraints imposed by the parent
* @return The size this view should be.
*/
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
//不能确定子view的大小
case MeasureSpec.UNSPECIFIED:
result = size;
break;
//最大能给子孩子的大小尺寸，也就是父控件的最大尺寸specSize
case MeasureSpec.AT_MOST:
//子孩子在xml里写的尺寸是固定的，android:layout_width="50dp"
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
- 研究下widthMeasureSpec和heightMeasureSpec这两个参数是怎么来的？
经过追代码，发现ViewGroup里面measureChild方法里面调用了view的measure，传进来参数。
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
getChildMeasureSpec关键方法，负责childWidthMeasureSpec值的拼装，lp.width就是childDimension，也就是我们在xml里android:layout_width写的值。
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);
}
- widthMeasureSpec和heightMeasureSpec是父控件传给子控件的测量规则和大小，测量规则和大小是根据父控件测量规则和子孩子在xml写的属性值来共同决定的。
- 当View采用固定宽/高时（ 即设置固定的dp/px） ,不管父容器的MeasureSpec是什么，View的MeasureSpec都是EXACTLY模式，并且大小遵循我们设置的值。
- 当View的宽/高是 match_parent 时，View的MeasureSpec都是EXACTLY模式并且其大小等于父容器的剩余空间。
- 当View的宽/高是 wrap_content 时，View的MeasureSpec都是AT_MOST模式并且其大小不能超过父容器的剩余空间。
- 父容器的UNSPECIFIED模式，一般用于系统内部多次Measure时，表示一种测量的状态，一般来说我们不需要关注此模式。
- 因为我们自定义view的宽高可能需要一个最小值来显示，来绘制我们的图形，但是当我们调用者没在xml写具体数值，就会导致我们宽度和高度是父布局的大小，以至于显示异常。所以我们要重写onMeasure方法，来根据自己的需求，设置子view的宽高。
如何重写onMeasure方法
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
Log.d("CircleView", MeasureSpec.getSize(widthMeasureSpec)+":"+ MeasureSpec.getSize(heightMeasureSpec));
Log.d("CircleView", MeasureSpec.getMode(widthMeasureSpec)+":"+ MeasureSpec.getMode(heightMeasureSpec));
// super.onMeasure(widthMeasureSpec, heightMeasureSpec);
setMeasuredDimension(getSize(50,widthMeasureSpec),getSize(50,heightMeasureSpec));
Log.d("CircleView", getMeasuredWidth()+":"+ getMeasuredHeight());
}
public static int getSize(int minSize, int measureSpec) {
int result = minSize;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
result = Math.min(minSize,specSize);
break;
//Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
default:
result = specSize;
break;
}
return result;
}
总结
- 实践去吧，多写几个自定义view就会了。