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87 lines
3.9 KiB
Go
87 lines
3.9 KiB
Go
package smartcontract
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import (
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"github.com/nspcc-dev/neo-go/pkg/io"
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"github.com/nspcc-dev/neo-go/pkg/smartcontract/callflag"
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"github.com/nspcc-dev/neo-go/pkg/util"
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"github.com/nspcc-dev/neo-go/pkg/vm/emit"
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"github.com/nspcc-dev/neo-go/pkg/vm/opcode"
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)
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// Builder is used to create arbitrary scripts from the set of methods it provides.
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// Each method emits some set of opcodes performing an action and (in most cases)
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// returning a result. These chunks of code can be composed together to perform
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// several actions in the same script (and therefore in the same transaction), but
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// the end result (in terms of state changes and/or resulting items) of the script
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// totally depends on what it contains and that's the responsibility of the Builder
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// user. Builder is mostly used to create transaction scripts (also known as
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// "entry scripts"), so the set of methods it exposes is tailored to this model
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// of use and any calls emitted don't limit flags in any way (always use
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// callflag.All).
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//
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// When using this API keep in mind that the resulting script can't be larger than
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// 64K (transaction.MaxScriptLength) to be used as a transaction entry script and
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// it can't have more than 2048 elements on the stack. Technically, this limits
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// the number of calls that can be made to a lesser value because invocations use
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// the same stack too (the exact number depends on methods and parameters).
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//
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// This API is not (and won't be) suitable to create complex scripts that use
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// returned values as parameters to other calls or perform loops or do any other
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// things that can be done in NeoVM. This hardly can be expressed in an API like
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// this, so if you need more than that and if you're ready to work with bare
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// NeoVM instructions please refer to [emit] and [opcode] packages.
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type Builder struct {
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bw *io.BufBinWriter
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}
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// NewBuilder creates a new Builder instance.
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func NewBuilder() *Builder {
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return &Builder{bw: io.NewBufBinWriter()}
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}
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// InvokeMethod is the most generic contract method invoker, the code it produces
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// packs all of the arguments given into an array and calls some method of the
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// contract. The correctness of this invocation (number and type of parameters) is
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// out of scope of this method, as well as return value, if contract's method returns
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// something this value just remains on the execution stack.
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func (b *Builder) InvokeMethod(contract util.Uint160, method string, params ...interface{}) {
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emit.AppCall(b.bw.BinWriter, contract, method, callflag.All, params...)
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}
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// Assert emits an ASSERT opcode that expects a Boolean value to be on the stack,
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// checks if it's true and aborts the transaction if it's not.
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func (b *Builder) Assert() {
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emit.Opcodes(b.bw.BinWriter, opcode.ASSERT)
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}
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// InvokeWithAssert emits an invocation of the method (see InvokeMethod) with
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// an ASSERT after the invocation. The presumption is that the method called
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// returns a Boolean value signalling the success or failure of the operation.
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// This pattern is pretty common, NEP-11 or NEP-17 'transfer' methods do exactly
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// that as well as NEO's 'vote'. The ASSERT then allow to simplify transaction
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// status checking, if action is successful then transaction is successful as
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// well, if it went wrong than whole transaction fails (ends with vmstate.FAULT).
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func (b *Builder) InvokeWithAssert(contract util.Uint160, method string, params ...interface{}) {
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b.InvokeMethod(contract, method, params...)
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b.Assert()
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}
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// Len returns the current length of the script. It's useful to perform script
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// length checks (wrt transaction.MaxScriptLength limit) while building the
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// script.
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func (b *Builder) Len() int {
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return b.bw.Len()
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}
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// Script return current script, you can't use Builder after invoking this method
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// unless you Reset it.
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func (b *Builder) Script() ([]byte, error) {
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err := b.bw.Err
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return b.bw.Bytes(), err
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}
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// Reset resets the Builder, allowing to reuse the same script buffer (but
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// previous script will be overwritten there).
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func (b *Builder) Reset() {
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b.bw.Reset()
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}
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