Flutter Widget In Detail: Container

Flutter Widget In Detail: Container

Detailed Explanation of Container Widget

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Container

  • Container , is one of the most common and popular widget available in Flutter.
  • You can think of a Container as a wrapper, which wraps other widgets.
  • After wrapping your widget around the Container you can give a color, padding, margin, etc. For example,
    Container(
      margin: const EdgeInsets.all(10.0),
      color: Colors.blue,
      width: 48.0,
      height: 48.0,
    )
    
  • Output : empty container.png

It's not required to have a child. We can create an empty container too.

  • Let's discuss all the properties which are provided by the Container.

1. child :

  • The child property of a Container allows us to place our own widget inside it so that we can decorate our widget easily. We can place any widget inside the child, for example Text, Row, Column , Stack etc.
    Container(
     child: Text("Hello Container")
    )
    
  • Output : Hello Container.png

2. alignment :

  • The alignment property will align the child of the container in 10 different ways.
  • topLeft, topCenter, topRight, centerLeft, center, centerRight, bottomLeft, bottomCenter, bottomRight
Container(
        alignment: Alignment.bottomRight,
        color: Colors.blue,
        width: 150.0,
        height: 100.0,
        child: Text("Hello Container")
)
  • Output :

align-3.png


3. color :

  • color is another most common property of a container that is used to give the container background color.
  • color takes color provided by the Colors class. There are mainly two Colors classes, Material and CupertinoColors class. We can use both of these classes to give color to the Container
  • Container(
          alignment: Alignment.bottomRight,
          color: CupertinoColors.activeGreen, // or Colors.green
          child: Text("Hello Container")
    )
    
  • Output : color.png

4. constraints

  • With constraints property we can tell the container to expand to fill a given sizes
  • constraints takes BoxConstraints class as an input. There are many constructors available inside the BoxModel. Some of them are tightForFinite, loose, tight, tightFor, etc.
  • Container(
          constraints: BoxConstraints.tightForFinite(width: 200.0,height:100.0), 
          alignment: Alignment.center,
          color: CupertinoColors.activeGreen,
          child: Text("Hello Container")
    )
    
  • The above code will create a box with constraints that require the given width or height. If no width or height were given then the constraints will be infinite, which means the box will take all the available space.
  • Output : constraints.png
  • The loose constraint is nothing but an axis with a minimum constraint of 0.0. It takes Size(width,height) as an input.
    Container(
          constraints: BoxConstraints.loose(Size(100.0,150.0)),
    )
    
  • constraints2.png

  • You can define maxWidth, maxHeight, minWidth, minHeight also in BoxConstraints constructor.

  • BoxConstraints(
              maxHeight: 100,
              maxWidth: 100,
              minHeight: 80,
              minWidth: 80
    ),
    

5. decoration :

  • decoration is a very cool property that provides tons of features for our box. Let's explore them.
  • decoration takes BoxDecoration constructor as an input. This constructor provides a variety of ways to draw a box.
  • Box can have, border, shadow , gradients, image, alignment etc.
  • Container(
          decoration: BoxDecoration(
            // Note: You can't have a property of "color" inside a container now as you've already defined in the decoration
            color:Colors.purple, 
            borderRadius: BorderRadius.circular(10.0),
            border:Border.all(color: Colors.red),
            boxShadow: [
              BoxShadow(
                        color: Colors.green,
                        blurRadius: 5.0,
                        spreadRadius: 5.0,
                 ),
            ]
    ),
    
  • decoration.png

  • Now there are 4 more properties remaining of BoxDecoration i.e, gradient and image, backgroundBlendMode and shape. Let's see how to use them

Image :

Container(
        decoration: BoxDecoration(
          image:DecorationImage(image: NetworkImage("https://url.com/flutter-logo.png")),
          border:Border.all(color:Colors.black),
          gradient: LinearGradient(
                 colors: [
                     Colors.pinkAccent,
                     Colors.blueAccent
                 ])
        ),
)
  • imagedecoration.png
  • Here in the image property you can pass other type of images like AssetImage FileImage, MemoryImage ,etc.

shape :

  • You can make your container circular,rectangular, or you can also specify thevalues` for custom shapes.
  • Example :
  • Container(
               decoration:BoxDecoration(
                    color:Colors.blueAccent, 
                    shape: BoxShape.circle // or BoxShape.rectangle
                ),
            height:80.0,
            width: 120.0,
               child: Center(child:Text("Circle")),
    ),
    
  • Output :
  • boxshape.png

backgroundBlendMode :

  • Blend modes are used to determine how two layers are blended with each other.
  • There are many blend modes available inside the BlendMode enum.
  • color, colorBurn, darken, difference, dst, hardLight, etc.
  • Example :
  • Container(
                  decoration:BoxDecoration(
                    backgroundBlendMode: BlendMode.difference, 
                    // or BlendMode.xor, BlendMode.darken etc
                    color:Colors.blueAccent, 
                    shape: BoxShape.circle
                  ),
                 height:80.0,
                 width: 120.0,
                 child: Center(
                   child: Text(
                     "xor",
                     style:TextStyle(color:Colors.white)
                   )
                 ),
            ),
    
  • Output :
  • Here I've displayed some of the blendMode. Take a look.
  • blendmode.png

gradient:

  • There are several types of gradients, represented by the various constructors in this class. LinearGradient, SweepGradient, RadialGradient,

LinearGradient() :

  • colors: List of colors
  • begin: Offset at which the gradient is placed.
  • end: Offset at which the gradient stops placed.
  • stops: A list of values from 0.0 to 1.0 that denote fractions along the gradient
  • Example: as explained above

RadialGradient() :

  • Same as LinearGradient, But this also takes properties like radius, focal, tileMode,
  • colors: List of colors
  • stops: A list of values from 0.0 to 1.0 that denote fractions along the gradient.
  • focal: The focal point of the gradient. If specified, the gradient will appear to be focused along the vector from center to focal.
  • focalRadius: The radius of the focal point of the gradient.
  • radius: The radius of the gradient
  • Example :
    Container(
          decoration: BoxDecoration(
            border: Border.all(color:Colors.black),
            gradient: RadialGradient(
              colors: [Colors.green, Colors.blue, Colors.orange, Colors.pink],
              stops: [0.2, 0.5, 0.7, 1],
              focal: Alignment(-0.1, 0.2),
              focalRadius: 1,
              radius: 0.1
          ),
    ),
    
  • radialg.png

SweepGradient() :

  • Similar to Linear and Radial Gradient, however, takes two different properties i.e, startAngle, endAngle
  • startAngle: The angle in radians at which stop 0.0 of the gradient is placed.
  • endAngle: The angle in radians at which stop 1.0 of the gradient is placed.
  • Example :
    Container(
          decoration: BoxDecoration(
          border: Border.all(color:Colors.black),
          gradient: SweepGradient(
          colors: [Colors.blue, Colors.green, Colors.yellow, Colors.red, Colors.blue],
          stops: [0.0, 0.25, 0.5, 0.75, 1],
               ),
          ),
    )
    
    sweep.png
  • After applying startAngle and endAngle
  • sweep2.png

6. margin:

  • Margin is used to create space around the widget
    Container(
          color:Colors.teal,
          alignment: Alignment.center,
          width: 150.0,
          height: 100.0,
          margin: EdgeInsets.only(left:10.0)
    )
    
  • margin.jpeg

7. padding:

  • Padding is used to create space from the inside of the widget
    Container(
          color:Colors.teal,
          alignment: Alignment.center,
          width: 150.0,
          height: 100.0,
          padding: EdgeInsets.all(10.0)
    )
    
  • padding.jpeg
  • margin and padding takes EdgeInsets class as an input.
  • The EdgeInsets has many constructor which is helpful to give margin/padding from different sides, some of them are:
  • EdgeInsets.all(): To gives space from all the sides.
  • EdgeInsets.only(): To give space from perticular side i.e left, right, top, bottom.
  • EdgeInsets.symmetric(): To give space horizontally and vertically

8. transform:

  • This is another cool property, which helps us to apply matrix transformation to paint our container/box as per our requirement.
  • transform takes Matrix4 class, which has many useful constructor like, rotationX(), rotationY(), rotationZ(), skew(), translation(), etc
  • Example :
  • Container(
          transform: Matrix4.rotationZ(0.1),
          width:250.0,
          height:150.0,
          color:Colors.teal,
          child:Center(
          child:Text("Hello Container")
          )
    )
    
  • Output :
  • transform.png
  • Similarly you can apply rotationX(), rotationY(), rotationZ(), skew(), translation() properties to see cool changes

9. transformAlignment:

  • This is an alignment of origin.
  • Note that this alignment is relative to the size of the container.
  • It will only work if transform is specified.
  • The alignments are topLeft, topCenter, topRight, centerLeft, center, centerRight, bottomLeft, bottomCenter, bottomRight.
  • Container(
            height:80.0,
            width: 120.0,
            color:Colors.green,
            child: Center(child:Text("topCenter")),
            transform: Matrix4.rotationZ(0.5),
            transformAlignment: Alignment.topCenter,
    )
    
  • Below I've displayed all the possible alignment.
  • transformalignment.png

10. foregroundDecoration:

  • This property will decorate the box from in front of the child.
  • The decoration property decorates/paints the box from behind the child.
  • Let's understand what do I meant by painting from the front of the child
  • If we take the same snippet used in the decoration example, but instead of the decoration I'm putting foregroundDecoration.
    Container(
          foregroundDecoration: BoxDecoration(
           color:Colors.purple, 
            borderRadius: BorderRadius.circular(10.0),
            border:Border.all(color: Colors.red),
            boxShadow: [
              BoxShadow(
                        color: Colors.green,
                        blurRadius: 5.0,
                        spreadRadius: 5.0,
                 ),
            ]
        ),
      child : //....
    ),
    
  • Output :
  • foregrounddecoration.png
  • As we can see the Text is not visible. It is because the foregroundDecoration has painted the whole thing in front of our child widget.

- That's It. That's all you need to know about the Container.

- Thank you reading.

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