自定义view从0到1

不带源码的讲解都是耍流氓

package com.example.myapplication.ui;

import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.util.Log;
import android.view.TextureView;
import android.view.View;
import android.widget.TextView;

import androidx.annotation.Nullable;

@SuppressLint("AppCompatCustomView")
public class MyTextView extends TextView {
    public MyTextView(Context context) {
        super(context);
    }

    public MyTextView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public MyTextView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        Log.e("TAG", "onMeasure  widthMeasureSpec: "+widthMeasureSpec+"  heightMeasureSpec: "+heightMeasureSpec );
        Log.e("TAG", "onMeasure  View.MeasureSpec.getMode(widthMeasureSpec): "+View.MeasureSpec.getMode(widthMeasureSpec) );
        switch (View.MeasureSpec.getMode(widthMeasureSpec)){
            case MeasureSpec.AT_MOST:
                Log.e("TAG", "onMeasure AT_MOST: "+MeasureSpec.AT_MOST );
                break;
            case MeasureSpec.EXACTLY:
                Log.e("TAG", "onMeasure EXACTLY: "+MeasureSpec.EXACTLY );
                break;
            case MeasureSpec.UNSPECIFIED:
                Log.e("TAG", "onMeasure UNSPECIFIED: "+MeasureSpec.UNSPECIFIED );
                break;

        }

    }
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        Log.e("TAG", "onLayout  changed:  "+changed+" left:"+left+" top:"+top+" right: "+right+" bottom:"+bottom );
    }



    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Log.e("TAG", "onDraw canvas: "+canvas.toString() );
    }
}

运行日志显示：执行的先后顺序为onMeasure,onLayout,onDraw

1讲解onMeasure的三种模式：UNSPECIED=0<<30无限制，EXACTLY=1<<30精确地，AT_MOST=2<<30 最大是父视图

所谓的父视图就是应用到xml中的视图

 <com.example.myapplication.ui.MyTextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

换句话说：

当xml中父视图是ScrollView,子视图的heightMode都是UNSPECIFIED ，无论ScrollView的高是wrap还是match；
当xml中设置为match_content或是指定大小,测量模式为：EXACTLY；
当xml中设置为wrap_content,测量模式为：AT_MOST；

UNSPECIFIED ：未指定模式，也可以称为无限制模式。不受父视图的限制，比如ScrollView的childView的heightMode中
EXACTLY ：精确模式。父视图指定大小，通常在 xml 中指定大小或者设为 match_parent 时会收到 EXACTLY 模式
AT_MOST ：最大模式；不能超过父视图的范围，通常在 xml 中设为 wrap_content 时会收到 AT_MOST 模式。

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
    <ScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent">
        <com.example.myapplication.ui.MyTextView
        android:background="@color/cardview_shadow_start_color"
        android:layout_width="match_parent"
        android:layout_height="50dp"/>
    </ScrollView>

    
</LinearLayout>

package com.example.myapplication.ui;

import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.util.Log;
import android.view.TextureView;
import android.view.View;
import android.widget.TextView;

import androidx.annotation.Nullable;

@SuppressLint("AppCompatCustomView")
public class MyTextView extends TextView {
    public MyTextView(Context context) {
        super(context);
    }

    public MyTextView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public MyTextView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        Log.e("TAG", "onMeasure  widthMeasureSpec: "+widthMeasureSpec+"  heightMeasureSpec: "+heightMeasureSpec );
        Log.e("TAG", "onMeasure  View.MeasureSpec.getMode(widthMeasureSpec): "+View.MeasureSpec.getMode(widthMeasureSpec) );
        switch (View.MeasureSpec.getMode(heightMeasureSpec)){

            case MeasureSpec.UNSPECIFIED:
                Log.e("TAG", "onMeasure UNSPECIFIED: "+MeasureSpec.UNSPECIFIED );
                break;
            case MeasureSpec.EXACTLY:
                Log.e("TAG", "onMeasure EXACTLY: "+MeasureSpec.EXACTLY );
                break;
            case MeasureSpec.AT_MOST:
                Log.e("TAG", "onMeasure AT_MOST: "+MeasureSpec.AT_MOST );
                break;


        }

    }
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        Log.e("TAG", "onLayout  changed:  "+changed+" left:"+left+" top:"+top+" right: "+right+" bottom:"+bottom );
    }



    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Log.e("TAG", "onDraw canvas: "+canvas.toString() );
    }
}