Question

My activity has a LinearLayout with a single child view. I want both to fill the screen, minus a 12 dp margin.

Unfortunately, the child view is drawn 12 dp too big and gets cut off. Apparently match_parent ignores the layout_margin attribute when calculating the size of the child view. What is the simplest way to fix this?

myActivity.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_margin="12dp"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical">
    <com.myapp.myView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
    />
</LinearLayout>

myActivity.java

package com.myapp;

import android.app.Activity;
import android.os.Bundle;

public class myActivity extends Activity {


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.myActivity);
    }
}

myView.java

package com.myapp;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;

public class myView extends View {

    private Paint paint = new Paint();

    public myView(Context context, AttributeSet attrs) {
        super(context, attrs);
        paint.setColor(0xFFFF0000); //red
        paint.setStyle(Paint.Style.STROKE); // for unfilled rectangles
        paint.setStrokeWidth(4);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        int size = canvas.getWidth(); // width = height (see onMeasure())
        canvas.drawRect(0, 0, size, size, paint);   
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, widthMeasureSpec);
        // This gives us a square canvas!
    }
}

Answer 1

Child views can have a margin around them, parent views (or view groups like layouts) can have a padding between their boundaries and their child views. In other words, margin is outside a view, padding is inside.

Also, see this excellent explanation: Difference between a View's Padding and Margin

Example with standard View and padding instead of margin:

I created a little example with a standard view instead of your custom one, and using padding for the LinearLayout as suggested above and it works perfectly (see screenshot):

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="12dp"
    android:orientation="vertical" >

    <View
        android:layout_width="match_parent"
        android:layout_height="100dp"
        android:background="#ff0000"/>
</LinearLayout>

Solution

Turns out, the problem was your using canvas.getWidth() in your custom view’s onDraw method. Using the view’s getWidth() instead, solved the problem. Finally 🙂