Flutter life cycle

One 、Flutter The two main Widget

1. StatelessWidget（ Stateless component ）

Stateless components are immutable , This means that their properties cannot be changed , All values are final . It can be understood as transforming the external incoming data into the content displayed on the interface , It will only render once . For stateless component lifecycles, there are only build This process .

The construction method of stateless components is usually called in only three cases ： The widget is inserted into the tree for the first time , The parent component of the widget changes its configuration , And what it depends on InheritedWidget When something changes .

2. StatefulWidget（ Stateful component ）

Stateful components may hold states in Widget Change in the life cycle , Is to define interaction logic and business logic . It can be understood as having a dynamic and interactive content interface , It will render multiple times according to the change of data .

StatefulWidget The class itself is immutable , however State Class in Widget There is always in the life cycle .StatefulWidget Store its variable state in the createState Method to create State In the object , Or stored in the State In the subscribed object .

Two 、StatefulWidget Life cycle

createState：
The function of StatefulWidget Created in State Methods , When StatefulWidget It will be executed immediately when it is created createState.createState After the function is executed, it indicates that the current component is already in Widget In the tree , There is a very important attribute here mounted Be set to true.

initState：
The function of State Initialize call , Will only be called once , therefore , Usually do some one-time operations in this callback , Such as implementation State Initial assignment of each variable 、 Subscribe to event notifications for subtrees 、 Interact with the server , After receiving server data, call setState To set up State.

didChangeDependencies：
The function is The component depends on State Is called when it changes . here State by overall situation State, For example, system language Locale Or application theme, etc ,Flutter The framework will inform widget Call this callback . After this method is called , The state of the component changes to dirty, Call immediately build Method .

build：
Mainly Returns the image to render Widget, because build Will be called many times , Therefore, only returns can be made in this function Widget Related logic , Avoid state exceptions caused by multiple executions .
Use scenarios ：
a. Calling initState() after .
b. Calling didChangeDependencies() after .
c. Calling setState() after .
d. Calling didUpdateWidget() after .
e. stay State After the object is removed from a location in the tree （ Would call deactivate） Reinsert again Enter into After going to other places in the tree .

reassemble：
The main Use during development , stay debug In mode , This function is called for each hot overload , So in debug The stage can be increased during this period debug Code , To check for code problems . This callback stay release Mode will never be called .

didUpdateWidget：
This function is mainly used in Component rebuild , For example, thermal overload , The parent component build Under the circumstances , This method will only be called by the subcomponent , secondly After this method is called, it must call build Method .

deactivate：
After the node of the component is removed, it will be called , If the component is removed, the node , then When not inserted into other nodes , Will continue to call dispose Permanently removed .

dispose：
Permanently remove components , And release component resources . Call complete dispose after ,mounted Property is set to false, On behalf of The end of the component life cycle .


It can be roughly divided into four stages ：
（1） Initialization phase , Includes two lifecycle functions createState and initState;
（2） Component creation phase , Include didChangeDependencies and build;
（3） Trigger the component more than once build , This stage may be due to didChangeDependencies、 setState perhaps didUpdateWidget And the resulting component re build , It will be triggered many times during the operation of the component , This is also a key point in the optimization process ;
（4） The last stage is the component destruction stage ,deactivate and dispose.

This reference article is as follows ：
interviewer ： know Flutter Life cycle ？ Come on next week ！

State Life cycle