android自定义View初探，一秒画一个小川普

方法

使用ImageView可以有同样效果，现在我们来看看自定义的TrumpView如何实现
首先新建一个App项目，默认有MainActivity和activity_main.xml
然后将xml文件修改如下

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <com.example.makeview.TrumpView
        android:id="@+id/circle_view"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        tools:ignore="MissingConstraints" />

</androidx.constraintlayout.widget.ConstraintLayout>

当然，此时你的TrumpView会是红色的，
注意

新建一个Trump.java类继承View ，定义一个构造函数如下（全部代码放在文末）

 public TrumpView(Context context, AttributeSet attributeSet) {
        super(context,attributeSet);
        mBitmap=BitmapFactory.decodeResource(context.getResources(),R.drawable.trump);
        init();
  }

这里将 trump.png文件通过bitmap工厂创建到mBitmap中，这里的init()我只做了简单的处理，设置画笔颜色（如果我们画的图形），但是我们显示图片的话，画笔的颜色不重要
接下来是将图片画到指定区域

@Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Rect src = new Rect(0, 0, mBitmap.getWidth(), mBitmap.getHeight());
        Rect dst = new Rect(0, 0, mBitmap.getWidth(), mBitmap.getHeight());
        canvas.drawBitmap(mBitmap,src,dst,mPaint);
    }

src 表示从图片上取出得到区域，dst表示在控件中放置画的区域，都选择bitmap的宽高

最后进行尺寸测量，因为自定的View默认是无法显示

   android:layout_width="wrap_content"
   android:layout_height="wrap_content"

他会将wrap_content直接按match_content处理，所以我们在重写measure的时候，需要设置默认的wrap_content值，这里设置为800

 @Override
    protected void onMeasure(int widthMeasureSpec,int heightMeasureSpec){
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
        if (widthSpecMode == MeasureSpec.AT_MOST && heightMeasureSpec == MeasureSpec.AT_MOST) {
            setMeasuredDimension(800, 800); 
        } else if (widthSpecMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(800, heightSpecSize);
        } else if (heightMeasureSpec == MeasureSpec.AT_MOST) {
            setMeasuredDimension(widthSpecSize,800);
        }
    }

这样所有代码都大功告成了！

效果如图

在这里插入图片描述

以下是源代码

package com.example.makeview;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;



public class TrumpView extends View {
    final String TAG = "CircleView";
    private int mColor =Color.RED;
    private Paint mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
    Bitmap mBitmap;
    public TrumpView(Context context) {
        super(context);
        Log.d(TAG, "TrumpView: First");
        init();
    }

    public TrumpView(Context context, AttributeSet attributeSet) {
        super(context,attributeSet);
        Log.d(TAG, "TrumpView: Second");
        mBitmap=BitmapFactory.decodeResource(context.getResources(),R.drawable.trump);
        init();
    }

    public TrumpView(Context context, AttributeSet attributeSet, int defStyleAttr) {
        super(context, attributeSet, defStyleAttr);
        Log.d(TAG, "TrumpView: Third");
        init();
    }

    private void init() {
        mPaint.setColor(mColor);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Rect src = new Rect(0, 0, mBitmap.getWidth(), mBitmap.getHeight());
        Rect dst = new Rect(0, 0, mBitmap.getWidth(), mBitmap.getHeight());
        canvas.drawBitmap(mBitmap,src,dst,mPaint);
    }
    @Override
    protected void onMeasure(int widthMeasureSpec,int heightMeasureSpec){
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
        if (widthSpecMode == MeasureSpec.AT_MOST && heightMeasureSpec == MeasureSpec.AT_MOST) {
            setMeasuredDimension(800, 800);
        } else if (widthSpecMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(800, heightSpecSize);
        } else if (heightMeasureSpec == MeasureSpec.AT_MOST) {
            setMeasuredDimension(widthSpecSize,800);
        }
    }

}