[安卓开发日记] 自定义View

一、自定义View三个步骤：onMeasure,onLayout,onDraw

onMeasure 一定要在onLayout之前，之后再onLayout
ViewGroup一般不用处理onDraw

二、onMeasure方法，测量布局

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}

MeasureSpec

MeasureSpec 是View的一个内部类，他可以用一个int同时表示一个宽度或者高度的mode和size
一个int的32位，高两位表示mode,剩余30位表示size

mode 包含 UNSPECIFIED, EXACTLY, AT_MOST，其值的大小如下

private static final int MODE_SHIFT = 30;
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
public static final int EXACTLY     = 1 << MODE_SHIFT;
public static final int AT_MOST     = 2 << MODE_SHIFT;

调用MeasureSpec.makeMeasureSpec(int size,int mode) 可以计算出合并后MeasureSpec值

三种模式的含义：UNSPECIFIED一般是类似ScrollView的高这种不确定值，EXACTLY一般是MatchParent和传入具体值的情况，AT_MOST一般是WRAP_CONTENT的情况，只有一个父布局限制他的最大值

实际情况会比上面复杂这里贴出关键代码

/**
  * specMode是自定义容器的mode
  * size是自定义容器的size
  * childDimension 是子ViewLayoutParams的宽或者高
  **/
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;
        }

Measure流程如下

1. DecorView 调用measure(int widthMeasureSpec, int heightMeasureSpec)方法
2. measure方法中，这里会处理measure缓存等问题，一般不需要覆写，默认会在这里调用onMeasure(int widthMeasureSpec, int heightMeasureSpec)
3. 如果是ViewGroup容器，则会先剔除Padding和Margin,然后计算出child的MeasureSpec并调用子View的measure(int widthMeasureSpec, int heightMeasureSpec)方法。子view又会走相同的流程形成递归
4. 计算测量结果后，在onMeasure中调用setMeasuredDimension(int measuredWidth, int measuredHeight),来设置测量后的宽高。

三、onLayout方法，定位View

非容器的View一般不用实现onLayout方法

@Override
protected void onLayout(int left, int top, int right, int bottom) {
    super.onLayout(left, top, right, bottom);
}

这里从getPaddingLeft和getPaddingTop开始，遍历子View逐一计算它应该放置的left,right,top,bottom
调用子View的onLayout,子View逐级调用onLayout。

getWidth和getHeight要在onLayout结束后才能获取到值

四、onDraw方法，绘制具体的显示逻辑

一般容器View不需要实现onDraw方法

@Override
protected void onDraw(Canvas canvas) {

}

为了避免在动效类自定义View频繁调用onDraw方法，导致频繁GC，要提前在构造函数中初始化Paint,Path等对象，path可以利用reset方法使用， 不能在onDraw方法重复创建

canvas.restore可以回到上次canvas.save的状态