我正试图在主线性布局中放置一个滚动视图,在滚动视图内部有一个包含自定义视图的线性布局,我不太确定该如何使其工作。 我是Android Studio的新手,如果您能指出我的错误,我将不胜感激。谢谢! 代码在我的自定义视图类中看起来像这样
package com.example.myapplication;
import android.content.Context;
import android.content.res.Resources;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
public class mview extends View {
public int lines = 0;
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
public void addLine(){
postInvalidate();
lines++;
}
public mview(Context context) {
super(context);
init(null, 0);
}
public mview(Context context, AttributeSet attr) {
super(context, attr);
init(attr, 0);
}
public mview(Context context, AttributeSet attr, int def) {
super(context, attr, def);
init(attr, def);
}
private void init(AttributeSet set, int def) {
line1.setColor(Color.BLACK);
line1.setAntiAlias(true);
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
//draw grids
for (int i = 0; i < lines; i++) {
canvas.drawLine(...);
}
}
}
我的MainActivity类中的代码长这样:
package com.example.myapplication;
import android.widget.LinearLayout;
import android.widget.ScrollView;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
private void clicked() {
mview.addLine();
}
private mview mview;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mview = findViewById(R.id.mview);
setContentView(R.layout.activity_main);
ScrollView scrollView = findViewById(R.id.scrollview);
Button button = findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
clicked();
}
});
}
}
我的activity_main布局长这样:
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<ScrollView
android:id="@+id/scrollview"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="1">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical">
<com.example.myapplication.mview
android:id="@+id/mview"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
</ScrollView>
<Button
android:id="@+id/button"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="1"
android:text="Button" />
</LinearLayout>
</androidx.constraintlayout.widget.ConstraintLayout>
ava.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.myapplication.mview.addLine()' on a null object reference
,而它应该向mview的画布添加新行。 - Yuchen Huang